Project P917-GI
BOBAN – Building and Operating Broadband Access
Network
Deliverable 10
DWDM technologies for access networks
Volume 1 of 4: Main Report
Draft waiting for Board of Governors' Approval
Suggested readers:
Managers, strategic planners, researchers and consultants involved in planning the evolution
towards broadband access networks, and in the work on the associated standards.
EDIN
0049-0917
Project
P917
For full publication
November 2000
EURESCOM PARTICIPANTS in Project P917-GI are:

Finnet Group

British Telecommunications plc.

Swisscom AG

Cyprus Telecommunications Authority

Deutsche Telekom AG

France Télécom

Matáv Hungarian Telecommunications Company Ltd.

Telecom Italia S.p.A.

Koninklijke KPN N.V.

Telenor AS

Hellenic Telecommunications Organisation S.A

Portugal Telecom S.A.

eircom plc
This document contains material which is the copyright of certain EURESCOM
PARTICIPANTS, and may not be reproduced or copied without permission.
All PARTICIPANTS have agreed to full publication of this document.
The commercial use of any information contained in this document may require a
license from the proprietor of that information.
Neither the PARTICIPANTS nor EURESCOM warrant that the information contained
in the report is capable of use, or that use of the information is free from risk, and
accept no liability for loss or damage suffered by any person using this information.
This document has been approved by EURESCOM Board of Governors for
distribution to all EURESCOM Shareholders.
 2000 EURESCOM Participants in P917-GI
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WDM technologies for access networks
Preface
Broadband access network introduction has been discussed extensively during the last
ten years within different research programmes in Europe. The search for a common
and ultimate strategy, guidelines and set of technologies somehow hampered real field
deployment.
In any case the number of users who can really enjoy broadband services are not in
proportion with the efforts put into standardisation over the years and V.34 modems
still represent the gate to the info highway for most of us.
On the other hand, as a result of the extensive research efforts, a number of
technologies are ready today or on the verge to be effectively exploited in
implementing the broadband access infrastructure. Unfortunately, and despite of the
standardisation efforts, the variety of technologies is made even more complex by the
differences among vendor specific implementations.
After the successful experience of EURESCOM Project P614 “Implementation
strategies for advanced access networks”, that addressed introduction scenarios and
technology appraisal, the BOBAN Project takes on board to investigate a wide range
of issues insufficiently covered so far, such as testing, installation and operation of
broadband access networks.
The quick evolution of the involved technologies and the major changes under way in
the telecommunication industry warrant a short study period to review the state-of-theart, after which the laboratory trials and demonstrator implementations can start.
It is also understood and incorporated into the BOBAN approach, that the access
networks will be based on a variety of technologies, and a major challenge will be to
assure consistent operation across different systems.
BOBAN also aims to reflect the ideas raised and proposals discussed during the
EURESCOM Senior Managers Conference (4th June 1998).
The main objectives of BOBAN are to:
 Obtain experience with the operation and management implications of some
broadband access systems;
 Develop methodology and experiment with procedures for access network
monitoring and supervision;
 Demonstrate low-cost fibre based access systems;
 Test and evaluate commercial and pre-commercial low cost DSL systems
(particularly ADSL lite);
 Understand the viability and the performances of power-line modems;
 Provide a comprehensive and reliable assessment of equipment and systems
available or under development;
 Understand the opportunities and the challenges of new powering solutions;
 Develop requirements and demonstrate a prototype of broadband access cabinet
including powering and mechanical parts;
 Identify possible application scenarios and evaluate WDM systems for the access
network;
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 Elaborate the specification for a common broadband access network planning tool;
 Identify guidelines for optical access systems deployment;
 Define scenarios and guidelines for a cost-effective migration of FTTH in the
access network.
It is expected that the findings of the BOBAN Project will significantly contribute to
the Shareholders strategic decisions on how to upgrade their access networks to
accommodate broadband services.
This Deliverable is one out of a series of Deliverables BOBAN is producing to
summarise its findings in the different aspects it is studying. This Deliverable reports
on the results of the investigations done regarding the use of DWDM technology in
the access network. It consists of a public main part (volume 1), and 3 EURESCOM
confidential Annexes published as separate volumes.
The results summarised in this Deliverable are particularly relevant for managers,
strategic planners, researchers and consultants involved in planning the evolution of
access networks.
Adam Kapovits, Project Supervisor (EURESCOM)
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Executive Summary
This report is about the introduction of DWDM technology in the access network. It
evaluates the potential of the technology and how it can add value to the telecom
business. The study was conducted to find answers to questions such as:

What services does DWDM technology enable?

What are the benefits of deploying DWDM in the access network for a telecom
operator?

What are the limitations of DWDM applicability in the access network?
The Benefits for Your Company
Although current demand for broadband services does not necessarily justify the
deployment of DWDM in the access network today, demand forecasts indicate the
need for DWDM technology in the near future. This Deliverable provides up to date
and operator oriented information about the status and the potential of DWDM
technology for deployment in the access network.
Main results
In our study we confirm that large business customers are typically served today by
optical fibre access rings using STM-1 (155 Mbit/s) connections. However, the overall
penetration of fibre in the access remains low and most of the business customers are
still served via copper.
Looking at the residential customer market we found that digital TV and video has not
taken off yet, and high speed Internet access at present is limited by the core network,
therefore providing more bandwidth in the access would not result in a better service
experience of the customers. However, the situation concerning the backbone is
rapidly changing; a number of large-scale projects are underway to increase the
bandwidth available. Clearly the next step is to provide more capacity in the access,
especially, because demand forecasts predict an exponential growth of the traffic.
Such a growth in the demand cannot be met by any other means than deploying
DWDM in the metro networks.
A range of application fields of DWDM in the access is defined based on real or
foreseen needs. It is important to note that some of these applications could be
implemented in some forms either today using available DWDM technology or in the
very near future (probably in 2001) taking advantage of emerging more flexible
network elements such as configurable OADMs.
The following key applications fields have been identified:

Wavelength leasing

Wavelength on demand

Intra-Enterprise Connectivity / Optical VPN
From the operator point of view DWDM technology is an option to introduce
flexibility and add new features to their networks that represent value to their
customers.
DWDM provides the following advantages:
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
Simultaneous access to several service optimised networks from one or different
operators.

Virtual private networks on shared fibres through wavelength dedicated optical
ADM with enhanced security.

Cost efficient upgrade of PON systems.

Increased network flexibility that allows rapid provisioning of bandwidth to
satisfy unexpected demands and dynamic configuration and assignment of QoS.

Higher fibre plant efficiency through fibre sharing, allowing the mixing of
different technologies in the same area (different protocols and transmission
formats, analogue and digital with different transmission codes).
Technology maturity is another key factor of deployment. Our study shows that for the
time being the immaturity of DWDM technology poses a serious limitation to the
introduction of DWDM in the access network. Although DWDM offers huge
transmission capacities and supports a wide variety of input signal formats, it still
suffers from limited flexibility and networking functionality compared to its full
potential. For example available transponders emit at pre-selected wavelengths and
only fixed OADMs are available, that cannot be dynamically configured. Moreover,
optical cross-connects that would enable the implementation of all-optical fully
configurable networks are still in the development stage.
Ultimately, cost will determine the use of DWDM in the access network. Here quick
and inexpensive provision of services is essential.
The responses to the RFI we have circulated gave very little information regarding
hardware/system costs. One aspect, however, that should be taken into account is that
almost all vendors offer “metro-oriented” WDM solutions that should allow the
development of WDM access network at a lower cost.
Our investment study revealed that today the deployment of DWDM technology in the
access cannot be justified purely on a cost basis.
We also verified, that the commercially available DWDM access/metro systems are
fully compatible with the legacy SDH systems.
Furthermore, in the framework of a laboratory trial we demonstrated the feasibility of
the PON upgrade using commercially available DWDM components.
We have studied a special aspect of deploying DWDM in the access, the convergence
with the core network. Ultimately this can lead to an all-optical, end-to-end DWDM
network. This technology convergence supports the blurring and disappearance of the
traditional demarcation of access and core. Our study shows that the critical point of
DWDM convergence is the access node that interconnects the metropolitan and access
network domains. The significance of the access node stems from the fact that it
interfaces various technologies and architectures, is cost sensitive, must be flexible to
accommodate new services / demands and has a large impact on OAM.
To enable DWDM deployment in the access network management systems should
also evolve from the presently available simple network element monitoring to
wavelength channel monitoring and provide all the management functionalities in the
optical layer that the SDH layer offers in today's networks.
DWDM convergence is envisaged to have a large impact on network structure and
planning, from the optical networking point. DWDM convergence is mandatory for
applications, such as:
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
Wavelength VPNs capable of supporting customers’ native signal formats,

Wavelength leasing and

Wavelength on demand.
Convergence should be expected when the technology becomes mature enough and
other options for capacity increase are exhausted, allowing DWDM to migrate from
core to metro and then to the access network. However, when these barriers are
eliminated DWDM convergence will constitute a real paradigm shift in optical
networking, changing the perception we presently have of optical networks.
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List of Authors
Marcelino Pousa
PT Inovação
Teresa Almeida
PT Inovação
Erik Weis
DT -T Nova
Mirko Adamy
DT -T Nova
Piero Bradley
IT - OTC
Lucas Soldano
IT - CSELT
Fabrice Bougart
FT- France Telecom R&D
Serge Mottet
FT- France Telecom R&D
Anjali Bhatnagar Riise
NT – TELENOR R&D
André-Fossen Mlonyeni
NT – TELENOR R&D
Risto Riihimaki
AF – ELISA Communications
Marco Smit
NL – KPN Research
Zacharias Ioannidis
OTE
Thomas Sphicopoulos
OTE
Klio Kallimani
OTE
Makis Pagiatakis
OTE
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Table of Contents
Preface ......................................................................................................................................... 2
Executive Summary .................................................................................................................... 4
The Benefits for Your Company ............................................................................................. 4
Main results ............................................................................................................................. 4
List of Authors............................................................................................................................. 7
Table of Contents ........................................................................................................................ 8
Abbreviations ............................................................................................................................ 10
1
Introduction ....................................................................................................................... 13
2
DWDM in the access network ........................................................................................... 14
2.1
General Network structure ......................................................................................... 14
2.1.1
Metro networks .................................................................................................. 14
2.1.1.1 Structure ........................................................................................................ 14
2.1.1.2 Requirements ................................................................................................. 15
2.1.1.3 Present state ................................................................................................... 15
2.1.1.4 DWDM Solutions .......................................................................................... 15
2.1.2
Access networks ................................................................................................ 16
3
DWDM solutions for business customers ......................................................................... 17
3.1
General trends in telecommunications ....................................................................... 17
3.2
WDM based applications in the business environment ............................................. 17
3.2.1
Wavelength leasing............................................................................................ 17
3.2.2
Wavelength on demand ..................................................................................... 17
3.2.3
Storage Area Networks ...................................................................................... 18
3.2.4
Intra-Enterprise Connectivity / Optical VPN ..................................................... 18
3.3
State-of-the-art in the DWDM systems for business applications ............................. 18
3.3.1
Definitions ......................................................................................................... 19
3.3.2
Evaluation of Available Systems ....................................................................... 19
3.3.3
System and architectures ................................................................................... 21
3.4
DWDM architectures for business customers............................................................ 21
3.4.1
Existing access network solutions for business customers ................................ 21
3.4.2
DWDM installation driving forces .................................................................... 22
3.4.2.1 Interest for a service perspective ................................................................... 22
3.4.2.2 Interest from a networking perspective.......................................................... 23
3.4.3
DWDM introduction constraints ....................................................................... 23
3.5
DWDM network topologies applicability.................................................................. 24
3.5.1
Point to point links ............................................................................................. 24
3.5.2
Point to multipoint links .................................................................................... 25
3.5.3
Ring structures ................................................................................................... 25
3.5.3.1 Upgrade concept with DWDM ...................................................................... 25
3.6
Network evolution paths towards DWDM networks for business customers ........... 27
3.6.1
Enhanced star structured copper networks (xDSL) ........................................... 27
3.6.2
Enhanced Point to Point optical links (SDH) .................................................... 28
3.6.3
Point to multipoint links (ITU-T G.983 ATM – PON) ...................................... 29
3.6.4
Enhanced cable Ring architectures .................................................................... 29
3.6.4.1 Particular issues regarding CR configurations ............................................... 29
3.6.4.2 Proposed ring configuration .......................................................................... 30
3.7
Conclusions ............................................................................................................... 30
4
DWDM Upgrade of existing PON .................................................................................... 32
4.1
Introduction ............................................................................................................... 32
4.2
Passive optical Networks (PON) ............................................................................... 32
4.2.1
General PON architecture .................................................................................. 32
4.2.2
Limitations of Power splitting PON .................................................................. 33
4.3
Drivers for the WDM upgrade ................................................................................... 33
4.3.1
Further requirements on future networks: ......................................................... 34
4.3.2
Basic Upgrade concepts ..................................................................................... 35
4.3.3
Upgrade constraints, restrictions and limitations ............................................... 38
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4.3.3.1 System constraints ......................................................................................... 38
4.3.3.2 Technological constraints .............................................................................. 38
4.3.3.3 Economic Constraints .................................................................................... 38
4.3.3.4 Operator Constraints ...................................................................................... 39
4.4
WDM Upgrade solutions ........................................................................................... 39
4.4.1
Solutions for business customers ....................................................................... 39
4.4.1.1 Downstream dedicated links in a single-fibre PON ....................................... 40
4.4.1.2 Bi-directional dedicated links ........................................................................ 41
4.4.1.2.1 Bi-directional upgrade based on discrete components ............................. 41
4.4.1.2.2 Bi-directional upgrade based on AWG wavelength routers..................... 41
4.4.1.2 Conclusions.................................................................................................... 42
4.4.2
Solutions for increased demands at single ONUs .............................................. 43
4.4.3
Solutions for increased gross Downstream bit rate ............................................ 43
4.4.4
Wavelength per Service: .................................................................................... 43
4.4.4.1 Wavelength per ONU: ................................................................................... 43
4.4.5
Solution for increased bi-directional capacity demand ...................................... 44
4.4.6
Future networks ................................................................................................. 44
4.4.7
Component issues .............................................................................................. 45
4.4.8
Management and protection aspects .................................................................. 46
4.5
Evaluation of the upgrade solutions and migration path ............................................ 46
4.6
Guidelines for Network planners ............................................................................... 49
4.7
Conclusions ............................................................................................................... 50
5
Convergence of DWDM Networks ................................................................................... 52
5.1
Reference Optical Metropolitan Access Architecture................................................ 52
5.2
Functional description of DWDM access node ......................................................... 53
5.2.1
Use of OXC instead of OADM .......................................................................... 54
5.2.1.1 Optical-electrical-optical conversion ............................................................. 54
5.2.2
Network level interfaces .................................................................................... 54
5.2.2.1 Ring-Ring interconnection ............................................................................. 54
5.2.2.1.1 Protection strategies ................................................................................. 55
5.2.3
Possible functional/physical implementation of ring interconnections .............. 56
5.2.3.1 OXC/Configurable OADM functionality ....................................................... 56
5.2.3.2 OADM/EADM(EXC)/OADM ......................................................................... 56
5.2.4
Feeder/SDH access ring interconnection ........................................................... 57
5.2.5
Feeder/FSAN PON interconnection .................................................................. 57
5.2.5.1 Distributive service PON ............................................................................... 57
5.2.5.2 Interactive services PON ............................................................................... 58
5.2.6
Wavelength routing policy................................................................................. 59
5.2.7
Aggregation of traffic in the distribution network for WDM applications ........ 61
5.2.7.1 Non-transparent solution for traffic aggregation in the distribution network,
Example : Wavelength per customer ............................................................................. 61
5.2.7.2 Transparent solution for aggregating traffic in the distribution network,
Example: Wavelength per service ................................................................................. 62
5.3
Use of DWDM for the reduction and concentration of network switches ................. 62
5.4
Network migration concepts towards all optical networks ........................................ 64
5.4.1
Migration Phases ............................................................................................... 64
5.4.1.1 Advantages .................................................................................................... 65
5.4.1.2 Disadvantages ................................................................................................ 65
5.5
Time frame for the DWDM application .................................................................... 66
5.5.1
DWDM network element maturity .................................................................... 66
5.5.2
Network Management Systems maturity. .......................................................... 67
6
Conclusions ....................................................................................................................... 68
7
References ......................................................................................................................... 72
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Abbreviations
ACTS
Advanced Communications Technologies and Services
ADSL
Asymmetric Digital Subscriber Line
APS
Automatic protection switching
AN
Access Network
ATM
Asynchronous Transfer Mode
AWG
Arrayed Waveguide Grating
BER
Bit Error Rate
BW
Bandwidth
DEMUX
Demultiplexer
DWDM
Dense Wavelength Division Multiplexing
E/O
Electrical to Optical Conversion
ESCON
Enterprise System Connection
EUR
Euro (€)
EURESCOM
EUropean institute for RESearch and strategic studies in
teleCOMmunications
FCF
Fused cascaded fibre technology
FDDI
Fibre Distributed Data Interface
FGR
Fibre Grating Router
FICON
Fibre Connectivity Channel
FSAN
Full Service Access Network
FSR
Free Spectral Range
FTTB
Fibre To The Building
FTTCab
Fibre To The Cabinet
FTTEx
Fibre To The Exchange
FTTH
Fibre To The Home
FTTC
Fiber To The Curb
FTTx
Fiber To The x
HHI
Heinrich Hertz Institute
InP
Indium Phosphide
IP
Internet Protocol
LAN
Local Area Network
LED
Light Emitting Diode
IP
Internet Protocol
ITU-T
International Telecommunications Union – Telecommunications
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MAC
Medium Access Protocol
MFL
Multi Frequency Laser
N-ISDN
Narrowband ISDN
NTE
Network Termination Equipment
OADM
Optical add-drop Multiplexer
OAM
Operation, Administration and Maintenance
O&M
Operations and Maintenance
OAN
Optical Access Network
OCH
Optical Channel Layer network
ODF
Optical Distribution Frame
ODN
Optical Distribution Network
OLT
Optical Line Terminal
ONT
Optical Network Termination
ONU
Optical Network Unit
OMS
Optical Multiplex Section Layer network
OSA
Optical Spectrum Analyser
OTS
Optical Transmission Layer network
PDL
Polarisation Dependence Loss
PLOAM
physical layer operation administration and maintenance
PON
Passive Optical Network
POTS
Plain Old Telephone Service
PS
Power Splitter
PSPON
Power Splitting Passive Optical Network
PSTN
Public Switched Telephone Network
QoS
Quality of Service
RFI
Request For Information
SDH
Synchronous Digital Hierarchy
SLS
Shared Mulitchannel Light Source
SM
Single Mode
SME
Small / Medium Enterprises
SNPM
Simple Network Protocol Management
SOHO
Small Office / Home Office
TDM
Time Division Multiplexing
TDMA
Time Division Multiple Access
UBR
Unknown Bit Rate
VDSL
Very high bit rate Digital Subscriber Line
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VBR
Variable Bit Rate
VoD
Video on Demand
WIC
Wavelength Insensitive Coupler
WDM
Wavelength Division Multiplexing /-er
WGR
Waveguide-Grating Router
WL
Wavelength
WR
Wavelength Router
WSF
Wavelength Selective Filter
xDSL
generic Digital Subscriber Line
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1
WDM technologies for access networks
Introduction
This Deliverable presents the results of the evaluation of the opportunities DWDM
technology offers in upgrading the telecommunications access networks. Besides
evaluating the maturity of the technology, the drivers and the constraints for its
introduction are also presented.
Today DWDM is widely deployed in the core network, because due to the high
distances and the existence of fibre the economics is in favour of it. Regarding the
access network the use of DWDM is still an issue of the future, because the technoeconomic analysis shows that the conventional solutions are still favourable.
In this Deliverable we summarise the existing situation in the Large Business
Networks and assess the potential use of DWDM technology. A special attention is
paid to the maturity of DWDM technology, the possible network architectures and to
the identification of the possible migration paths towards a full optical network.
Drivers and constrains are identified and a simplified investment study is presented,
that compares the use of DWDM with the traditional space division multiplexing.
In some countries PON systems are used to provide the narrow band services to
residential and SOHO customers. The Deliverable looks at how such PONs can be
upgraded using DWDM.
We also take a look at the issue of interfacing the access and core networks. Here we
consider both opaque and transparent networks, with the focus being on the
transparent networks.
Finally, we provide guidelines on how to migrate towards an end-to-end all optical
network based on DWDM technology (the so-called DWDM convergence
phenomena). We elaborate the required interfaces and time frame for this
development.
For who has interest in a deep study on these topics the annexes I, II and III give all
information that support this document, these annexes are EURESCOM confidential.
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2
DWDM in the access network
2.1
General Network structure
In general the whole network can be subdivided in the three domains: Backbone
network, Metro network and Access network. Figure 1 shows schematically these
three network domains.
Backbone-Network
Metro-Network
Access-Network
Figure 1 - General network domains
The requirements and functions of the above network parts are quite different from
each other. The Metro network serves as connection between the backbone (transport
network) and the access network. Therefore, at each "border", it has to fulfil the
requirements of the connected networks. The core network will not be defined because
it is out of the scope of this work and was studied in other EURESCOM projects
(P709, P918)
2.1.1
Metro networks
2.1.1.1
Structure
In the present document special attention was devoted to the metro networks because
they are in fact the support infrastructure for the large business users. The term "Metro
network" is not very well defined and several interpretations can be found in literature.
But it is most common to consider the Metro network as a segment in the access
network, with traffic aggregation functionalities, in the study we will also use the term
feeder network to refer it. But in the view of the convergence aspect, the metro access
network will grow from the core network towards the last drop. In the first
implementation,
it
might
connect
large
business
customers
and
concentration/aggregation points residing in the access network such as access
multiplexers, access routers, OLTs etc. Based on the high requirements for such
connections, one can assume that, for this segment, ring and mesh structures are
promising solutions, though different network topologies are also possible. In the
presence of cost-effective fibre based access solutions, the metro access network will
be expanded to connect the large segment of small and medium enterprises customers.
Therefore the metro access network will grow in relation with the move of the fibre
deeper to the customer.
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2.1.1.2
DWDM technologies for access networks
Requirements
The deployment of DWDM and increased transmission rates in the backbone network
has a strong impact on the metro networks. But the metro network is confronted with a
large variety of services, clients with different protocols and bit rates and customer
requirements delivered by the access / customer premises networks. The changing
paradigm from switched circuits to packet routing and from voice to data centric
traffic enhances the pressure on the metro network. Figure 2 shows the complexity of
requirements on the metro network.
Backbone network
(transport )
Survivability
Managability
Dynamic
provisioring
Capacity
Scalability
Metro
Low
first- cost
Service
transparency
Data
centricity
Access / Customer premises network
Figure 2 - Requirements on Metro networks
2.1.1.3
Present state
Today’s metro networks are generally based on SDH rings and do not employ DWDM
technology. Typically these networks are installed to connect LEX, CO, ISPs and
POPs. These SDH networks are confronted with the described requirements and have
to response on the growing requirements. Some of the main problems are:
2.1.1.4

Scalability: versus fixed Multiplex hierarchy

Transparency - ”native” data signals: versus SDH frame

Flexibility - Set up of new high bit rate connections: versus adding new SDH
connection and/or rebuilding of SDH equipment

Emerging new customer clients: versus supported by SDH - fitting or not
efficient in the SDH hierarchy

Capacity upgrade - reusing fibre infrastructure and equipment: versus installing
new fibre rings and SDH equipment
DWDM Solutions
DWDM technology offers the possibility to satisfy the enhanced requirements on
metro networks and overcomes the limits of a pure SDH solution. The availability of
multiple different wavelength channels offer additional degrees of freedom such as
high capacity, flexibility, scalability and transparency.
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Access networks
In today’s access networks ( also referred here as distribution), there exists a great
variety of access technologies. The main part of access networks in Europe are
determined by copper solutions. However, the growing demand for broadband
services as well as competition is pushing broadband solutions, apart from xDSL
systems, to broadband radio systems, PONs and AONs etc.. In general, such
broadband systems contain a multiplexing function, which multiplexes the different
customer bit rates to a higher bit rate. In some systems, traffic separation, for instance
IP/ ATM, is also implemented. Also, the existence of a variety of systems results in a
variety of interfaces, which can be either standardised or proprietary. Figure 3 shows
a variety of client formats which have to be transported over the access network and
integrated into the metro network.
Figure 3 - Variety of customer client formats
To cope with the multiple protocols and the considerably different bit rates of different
client formats DWDM seems a promising solution due to its transparency, high
capacity and the possibility to run different services and serve different clients on the
same (existing) fibre.
Our study shows that the DWDM technology in the access is in disadvantage when
compared with the conventional TDM or space division multiplexing as far as the cost
is regarded, it has been stated that the price of DWDM equipment should come down
by some 40% before this technology is worth deploying in the urban environment.
However even if carriers cannot afford it at present, they cannot ignore the fact that
DWDM could be the key to the grail of carrier services: just-in-time provisioning of
bandwidth and transparency. It is capable of handling different types of signals (SDH,
ATM, IP) simultaneously, by using the independent characteristics of the multiwavelength network. This transparency in protocol and signal format/bit rate is one of
the major advantages of the optical network concept.
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DWDM technologies for access networks
3
DWDM solutions for business customers
3.1
General trends in telecommunications
The amount of data traffic has been increasing significantly since the beginning of the
introduction of IP-protocol and Internet-services. This trend is going to be continued
also in the future. Access bandwidth demand of companies increases as the bandwidth
demand of various business applications grows. As an example it can be stated that
some SOHO companies will reach the access bandwidth demand of a current large
enterprise. This trend is shown in Figure 4.
Figure 4 - Development of access bandwidth demand
Due to the fact that the access bandwidth demand is increasing companies are moving
to faster connection types as the number of 2 – 34 Mbit/s connection slowly decreases
and contraversary number of faster connection types, e.g.155 Mbit/s and 622 Mbit/s,
increase. This increase in bandwidth demand is a signal indicating the need of high
bandwidth networks (DWDM networks) as a mean to overcome the existing fibre
systems.
3.2
WDM based applications in the business environment
The range of applications of DWDM in the metro/access network depends on a variety
of issues including technology maturity, capacity demand and need for new services.
Since many components of the above issues are still vague and uncertain, it is difficult
to predict with accuracy the actual range of applications. However, a list of application
fields can readily be defined based on real or foreseen needs [11].
3.2.1
Wavelength leasing
Resembles to dark fibre leasing but adds the advantage of “carrier manageability” of
the optical channel. The carrier should provide the customer with optical channel
performance monitoring, alternate path routing of signals, failure recovery and
isolation. Thus the value of a leased channel is being increased.
3.2.2
Wavelength on demand
A service in which an end-user has access to wavelength channels on demand. This
requires fairly sophisticated optical systems and management systems, but has been
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envisioned by a number of vendors as an application that may be useful to many large
organisations or smaller operators. Wavelengths on demand could be particularly
useful to customers who require vastly different amounts of bandwidth throughout the
year or throughout the day, and have the capability of bringing up and tearing down
optical channels from their own management systems. This service will not likely be
available for at least two years.
3.2.3
Storage Area Networks
Storage area networks (SANs) have emerged in the past few years in major
corporations and institutions requiring massive levels of data storage or data
"mirroring." These SANs are high-speed networks which establish direct connections
between storage elements and servers and perform various functions:
Disk Mirroring
Disaster recovery application that creates a second disk image of
critical data. Can be recovered instantaneously.
Backup and
Restore
Disaster recovery application that creates copies of data or tape
to protect against human error, equipment failure or catastrophic
event.
Archival and
Retrieval
Store of critical data on low-cost media. This can be
accomplished internally or through an arrangement with a data
warehousing provider.
Data migration
Moving data from one storage system to another
Shared Storage
Open and legacy access to common physical storage resources
Data Sharing
Extraction, movement and loading of data between
environments
Source: ADVA, CNT, 1999
The play for DWDM in the SAN market is through interconnection. SANs that exist
within an enterprise tend to be interconnected via ESCON, SCSI or Fibre Channel
today. When SANs need to be interconnected, however, dark fibre per link is typically
required, because these protocols do not map onto a TDM format well.
3.2.4
Intra-Enterprise Connectivity / Optical VPN
Intra-enterprise connectivity has already been discussed earlier in this report and refers
primarily to the demand for interconnection of buildings within a campus or a
metropolitan area of a large enterprise. Microsoft is an example of an early adopter of
DWDM as a means of increasing its capacity on its corporate-wide network.
Metropolitan access WDM solutions can provide a low-cost means of transporting the
various data protocols common to an enterprise network across a common fibre optic
infrastructure (Optical VPN), sometimes eliminating the need for a SDH intermediary
multiplexing stage, greatly simplifying enterprise networks.
3.3
State-of-the-art in the DWDM systems for business applications
The chapter presents definitions for the different components involved in the DWDM
infrastructures, as well an overview of the existing systems on the market, data
obtained with the RFI for DWDM large business solutions carried out by the project.
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3.3.1
DWDM technologies for access networks
Definitions
Functional breakdown to be built with a set among:
Transponder: network element performing wavelength converter and DWDM
adaptation
(Optical) Amplifier: network element performing signal amplification
Multiplexer/demultiplexer: network element performing merger/separation of
signals into/from one stream.
OADM: network element performing wavelength add/drop function
Optical switch: network element enabling to connect any of its input/output port to
any other, according to a switching signalling.
Optical router: network element similar to the switch but acting dynamically after
analysis of the incoming information routing label.
Optical cross connect: network element enabling to connect any of its upstream input
to any of its downstream output. These connections are fixed at first installation and
can be modified by upgrade. Passive devices may enter this type of NE enabling the
sharing (broadcast) of data or combine the data flows from N port/channel onto M
others and vice versa.
Filter: Passive elements able to isolate one or a group of wavelength
Fibre: As far as long distance transmission is concerned, there is little uncertainty,
with the physical medium chosen, as most parts of the networks have been built with
G.652 single mode fibres. Nevertheless the new development G.652 still demonstrate
a good balance between the dimension of the mode propagation area and the
dispersion, providing the best balance between non linearities and dispersion accuracy,
being in this sense the most promising option for the access network to carry DWDM
signals.
3.3.2
Evaluation of Available Systems
Comparing the received information in the RFI, with information provided by some
alternative sources referring manufacturers that didn’t answer to our RFI, the results
shows that, despite, the small number of the answers received, BOBAN’s market
research gives a fairly representative view of the current status of DWDM technology.
The most relevant facts about the described DWDM systems are summarised below:
Maximum capacity, in most systems, ranges from 40 to 80 Gb/s. This includes up to
16 to 32 individual signals of 2,5 Gb/s each (wavelength separation 100 – 200 GHz).
Further increase of this capacity is to be expected and some DWDM manufacturers
either have already announced the multiplexing of 80 STM-16 channels and 40 STM64 implemented in near-future versions. In any case, the 40 – 80 (or 160) Gb/s
capacity offered by available systems can be considered sufficient for the current as
well as the future demands for transmission bandwidth.
Electro-optical transponders are used at the entrance of all described DWDM
systems (some manufacturers also offer integrated SDH/DWDM solutions employing
coloured SDH terminals, at a lower cost). These accept a wide variety of tributary
signals (PDH, SDH, ATM, IP etc.) and have optical outputs that generally conform to
ITU-T Rec. G.692. Mainly owing to the use of transponders, described systems have
SDH compliant and open interfaces these are to a large extend bit-rate and signalformat transparent. Scalability in the transmitted information is accomplished by using
the appropriate number of transponder cards.
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Optical Add-Drop Multiplexers (OADMs) are offered by all manufacturers to
enhance the connectivity of their DWDM systems. Though, in the products offered,
the number of add-dropped channels ranges from 1 to 16, all OADMs are similar in
that the above number is fixed as well as in that dynamic reconfiguration of adddropped channels is not yet possible.
Optical Cross-Connect (OXC) products are not yet available although most
manufacturers have stated that are in the process of developing such products.
The following table presents a summary of the actual status of the DWDM technology
according our RFI results.
Table 1 - Key network elements for DWDM
DWDM Network
Elements
Status of technology
Time frame
Line systems:
Up to 32 multiplexed
wavelengths.
80 wavelengths are
expected by more
manufacturers by the
end of 2000.
Number of
wavelengths
Some manufacturers offer 64
wavelengths.
128 wavelengths may
be available in 2001.
Optical add-drop
multiplexers
Fixed OADMs available, up to 32
wavelengths.
(OADMs)
No configurable OADMs
available yet, under development
by some manufacturers.
Optical crossconnects
No product available yet.
Configurable OADM
technology is still
immature, possibly no
product available
before 2001.
Wavelength crossconnects based on
transponders are
expected in 2001.
(OXCs)
Under development by some
manufacturers.
Optical switches
No product available yet
Immature technology,
no safe forecast
Wavelength routers
No product available yet
Immature technology,
no safe forecast
Optical amplifiers
(Wideband)
1) 64 x 2.5 Gbps
1) currently available
2) 32 x 10 Gbps (ERICSSON)
2) 2001
3) 80 x 10 Gbps (SIEMENS)
3) 2000
(C+L band, 40 channels /
band)
Laser modules
DFB lasers used in transponders.
Fixed-wavelength output (1550
nm window).
Transponders
Electrooptical, mostly with fixedwavelength DFB lasers.
Input signals: either 1310 or 1550
nm window, output signals: 1550
nm window.
page 20 (73)
Tuneable lasers have
already been
announced
Transponders with
tuneable lasers have
already been
announced.
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DWDM technologies for access networks
DWDM Network
Elements
Status of technology
Time frame
Wavelength
converters
No product available yet.
Immature technology,
no safe forecast.
Filters
100GHz spacing available.
50 GHz spacing is
expected by more
manufacturers in 2001
50 GHz (64 channels) claimed by
at least one manufacturer
(OSICOM)
3.3.3
System and architectures
All available systems support point-to-point and ring topologies. Protection mainly
refers to the aggregate transmission path though some products also anticipate
protection at the transponder or multiplexer/demultiplexer levels.
System supervision is performed by means of a dedicated Optical Supervisory
Channel, outside or inside the wavelength zone used for the transmission of the
DWDM aggregate signal (at 1310, 1485, 1510 or 1625 nm). However, due to the lack
of configurability of modules, management is usually limited to fault reporting. All
offered management systems have “open” interfaces to the TMN.
In view of the above, the following remark can be made. Though DWDM offers huge
transmission capacities and supports a wide variety of input signal formats (conditions
that are essential for the migration of DWDM to the access part of the network),
DWDM technology still provides limited configurability and networking
functionality.
Another important issue regarding the migration of DWDM to the access network is
that of cost. In the access area, where usually, there is high demand for quick and
inexpensive provision of services by means of mature and standardized technologies,
the cost issue is much more complex and renders necessary the techno-economical
comparison between DWDM and non-DWDM solutions (such has xDSL and
SDH/FITL). In the answers to the RFI, there is very little information on product
costs, however one should take into account that almost all vendors offer “metrooriented” DDWDM solutions that are simplified versions of a DWDM core solution.
3.4
DWDM architectures for business customers
3.4.1
Existing access network solutions for business customers
European operators appear to have adopted similar solutions to serve Business
Customers (BCs). Nearly all of them make use of optical access networks based on a
physical fibre ring topology and SDH transmission. BCs are connected through pointto-point or ring logical connections. This creates a rather common “present situation”
from which future networks, such as DWDM networks, will evolve.
In spite of the commonly claimed “booming” increase of BC capacity demand, most
BC connections still appear to be in the under-STM-1 range. This means that
considerable further increase of capacity demand may be needed to boost DWDM
deployment in AN.
It is common practice among operators to develop optical fibre infrastructure in
physical ring topology, thus future optical AN will have to make use of ring legacy
infrastructure. However, It should be noted that penetration of optical access lines - as
percentage of total access lines - is presently well below 1% in the majority of
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WDM technologies for access networks
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operators. This means that new fibre will have to be installed in any case leaving space
for new network concepts.
Large divergence on the availability of services exists among operators (claimed
availability ranges from 99.9% - 99.9999%), although most of them use the same
basic architecture and type of optical AN. This means that availability of services
depends largely on other parameters, such as operations efficiency. Therefore, to
retain or enhance availability when introducing DWDM, OAM issues need careful
examination.
3.4.2
DWDM installation driving forces
It has already thoroughly described that the services perspective and the need for
bandwidth upgrade is for sure the major reason for DWDM introduction.
Nevertheless, DWDM may feature some other functions that are to be investigated as
switching for network flexibility and service differentiation. For this purpose, the
benefits for all actors will be envisioned.
3.4.2.1
Interest for a service perspective
Since the bit rates required to deliver digital video to the residential client are quite
high, it must be quoted that the bit rate differentiation between business and
residential, is no more so clear. So that the main difference remains on the asymmetry
of the bit rates since the residential where mainly supposed to be downloading
information rather than issuing some.
So where’s the difference then? Business clients are more quality and service
availability sensitive than residential customers, they also have telecom engineers that
are able to have a very accurate service value appreciation and express an extremely
focused cost sensitivity. Therefore, the access network operator has to provide a range
of solutions according to the customer’s specifications.
To answer to some possible customer demand, the following applications have been
identified:

Service capability enhancement (N channels per fibre) higher bandwidth: When
the aggregate bandwidth exceeds the electrical channel capacity, DWDM is an
option. (The alternative solutions are the electrical upgrade from STMx to STMx*2
or the fibre enrichment of the link).

Simultaneous or selectable access to several operators: the second one would be
to be able for any service to send out an RFP to operators and select the best
offer, without any change in the access network infrastructure. It must be kept in
mind that the latter aim also matches somehow the regulators willingness to
introduce competition.

Simultaneous access to several service optimised networks: This solution aims at
segregating the services in order to link the service multiplexers to a telecom
network, the IP router to an IP access network thus enabling an per
service/technology throughput and translation optimisation, and thus minder the
clients need to upgrade the existing CPE.

Virtual private networks on shared fibres through wavelength dedicated optical
channel.

Enhanced security: While the passive optical wavelength extractors OADM, are
located in the Telco’s network. The single client signal would only be available
on the fibre drop which is likely to be very short thus less easily accessible.
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3.4.2.2
DWDM technologies for access networks
Interest from a networking perspective
Though the aim for the operator is to provide services to its clients, some benefits
from new technologies can be taken for internal purposes, such as operational cost
savings. But this implies either the absolute transparency of technology to the end
user, or the sharing of the benefits through new features or cheaper services. The
major features of DWDM for the operator are the following:
Minor cost PON system evolution.

From the infrastructure point of view, unless extra fibres have been laid from the
start, the lowest bandwidth upgrade possible for the clients is to skip from a
FSAN 155/155Mbps system to a 622/155Mbps which are both included in the
G.983 standard, then move to the 622/622Mbps which is likely to get studied
within the next months. Beyond, no plans have been drawn on the electrical
upgrade.
Higher fibre plant efficiency through fibre sharing:

The operator’s interest is to provide a maximum of services through its existing
infrastructure, due to the cost of new fibre laying and the fee per civil work meter
introduced in some countries.
Individual service management on a shared infrastructure:

While in TPON or APON systems, the bandwidth and the user management is
strongly correlated through the sharing, in DWDM the bandwidth are nearly
independent from one user to another, at least it isn’t anymore physical layer
dependent. Each wavelength channel’s bandwidth may evolve at its own pace, the
bottleneck is likely to become the operator’s first electrical access NE.
Mixing of technologies in the same area:

3.4.3
Depending on the market, ATM or IP or DTM NEs will be the cheapest ones
either for the Access Network operator and the end user, if he’s the DWDM
modem owner. So it will be of major importance to be able to accommodate the
latest and cheapest technology with minor infrastructure changes. It is also
important to have a technology which will fit into most of the FTTx (X= O, H, B,
C, R, ) architectures.
DWDM introduction constraints
It is clear that unless some strong access network issues are solved, the DWDM in the
access benefits listed before won’t convince any operator to take all risks involved.
Management of a set of fixed wavelength network terminals: If the DWDM used is
a point to multipoint or point to point scheme with a passive wavelength splitting
device, each drop will then be coloured. That does mean that the end user NE should
match the wavelength or wavelength range allocated. If the NE wavelength is fixed,
then it implies to the operator to have a full set of NE  to manage, which induces
important costs.
OAM procedures for DWDM networks monitoring: the introduction of DWDM
technologies for business clients are possible only if the operator is given the tools to
monitor each channel’s status, detect faults, identify and localise the default. This is of
importance for point to point links, but even more for DWDM fibre sharing schemes.
Wavelength allocation management in the infrastructure (fixed, dynamic):
DWDM introduction in the access, introduces a new parameter to be managed into the
OAM procedures, the wavelengths. This induces an upgrade of all supervisory
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software, which might be very light in case of point to point channel upgrade or
complex for point to multipoint schemes especially if these allocations are dynamic.
3.5
DWDM network topologies applicability
3.5.1
Point to point links
All solutions based on DWDMmux and DWDMdemux can be classified under this
category. They all deal with one user and the central office, it is independent whether
the physical fibre link uses a physical ring or a straight path between user and LEX
building. If linked through a ring, these links can be secured with exactly the same
mechanisms as in SDH: G783 defined K1, K2 Automated protection switching.
All suppliers have a solution for point-to-point topologies, since it implies no
intermediate passive plant feature and is the lowest grade DWDM. Any upgrade in
this scheme requires the replacement of the NE on both sides of the fibre.
One very special issue, is whether the outcoming modems developed for the LAN
world will satisfy the access network operators or not, since they most will probably
offer few monitoring functions and quality monitoring but at the same time be the
ones at the lowest costs. These lowest costs will be due both to the low functions
available and to the higher market than the raw access network.
The cost has a major impact on the future deployment of DWDM in the access
network . From our studies, considering two different scenarios for a point to point
architecture and the two options presented in the following pictures: duplication of
systems (Space Division Multiplexing) or the increase of bite rate in the electrical
equipment, was tested.
N x 1Fibre
1 Fibre
Fiber optic terminal
1
1
10
Gbit/s
1
1 Fibre
Fiber optic terminals
ouo
equivalente
Fiber optic terminals
1
Increase
of
bndwidth
2.5
Gbit/s
Fiber optic terminal

3
1
1
W
D
M
1 Fibre
4
Fiber optic terminal
a) WDM versus SDM

1
W
D 
3
M

3
4
4
F iber optic terminal
Fiber optic terminal
W
D
M
1 Fibre
1
2.5
Gbit/s
Fiber optic terminal
1
10
Gbit/s
Fiber optic terminal
1

W
D 
3
M
4
Fiber optic terminal
b) WDM versus bite-rate increase
Figure 5 - Point to point and ring architecture used today for business customers
In the case a) DWDM versus SDM we considered 3, 6 and 9 terminals using the same
cable over the average distance of 8 Km, and STM-1 lines for each customer. The
figure shows that for this scenario DWDM is not a competitive solution even
considering the cost of the fibre in the SDM case.
In case b) DWDM versus increase of bite rate, we considered the case with 3, 6 and 9
circuits ( STM-1) per node and average distance of 8 Km. the results shows that from
a economic point of view the DWDM solution never is advisable. The case with 3
circuits show the same value for both solutions, this is due to the fact that we are overdimensioning the equipment, use a ADM STM-16 with protection to support only 3
circuits, so the investment curve for this case has a very slight increase.
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3.5.2
DWDM technologies for access networks
Point to multipoint links
Point-to-multipoint solutions are adapted to residential and SoHo markets, where the
demand for broadband services (bandwidth) are not developed, so the presence of this
type of systems is yet residual, only some operators are using it for narrow band
applications. The identification of limitations regarding the downstream capacity, and
the importance of this system for the fibre deployment in the access justified our
decision to devote a dedicated section to this field of PON upgrade.
3.5.3
Ring structures
Among the various components the quality of service expected by business customers,
transmission quality, availability and confidentiality are the most basic requirements.
Due this presently, taking advantage of the SDH ADM availability, ring topology is
broadly used as a cost effective survivable network architecture allowing bandwidth
sharing and improved survivability. SDH ring architectures are widely installed in the
transmission network and in the upper level of the access network (metro network).
A techno-economical evaluation made in EURESCOM P413 Task 2 project shows
SDH STM-1 ring solution has lower cost, for large customers, than point to point SDH
STM-1 links.
3.5.3.1
Upgrade concept with DWDM
The concept of a Coloured Node Ring (CNR) architecture which is a logical star
configuration implemented on a physical ring (A Hamel et al "Coloured Node
architecture for business customers", OAN’97) is the most favourable option. The
driving idea of the CNR architecture for business customer application is to assign a
dedicated wavelength to each customer or node. This solution combines the
advantages of point to point links (confidentiality, security) and the advantages of
SDH (end to end performance monitoring, protection and restoration capacities, civil
work shared between several users). This architecture is potentially cost effective
because the Optical Add Drop Multiplexer (OADM) are passive components, and
simplified terminal SDH equipment can be used instead of more expensive SDH
ADMs, while using the same fibre pair as in a conventional SDH ring. This
architecture can be upgraded with the introduction of ATM enabling statistical
multiplexing.
Transmission Network
ADM
Network
Manager
1, 2, 3, 4
Acces Network
4
STM1
customer 4
protection fibre
3
1
customer 1
working fibre
2
customer 3
customer 2
Wavelength Add Drop Multiplexer
Figure 6 - Architecture with Coloured Node Ring (CNR)
The ring full length is according to the number of OADM within 28 dB budget loss
and without optical amplification. An optical connector (0.3 dB) is installed at the end
of each fibre section. The LEX OADM includes an optical coupler for multiplexing
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and a grating or multilayer demultiplexer. The distance between OADM and CP is
assumed to be less than 300 meters.
Our investment studies evaluate from today cost if DWDM is an attractive alternative
to SDH.
Transmission Netw ork
Transmission Netw ork
OADM
SDH ADM
Netw ork
Manager
Netw ork
Manager
STM-x
SDH-ADM
1, 2, 3, 4
OADM
Acces Netw ork
STM-x
STM1
4
STM-x
Acces Netw ork
1
STM-X
customer 4
protection fibre
customer 1
w orking fibre
customer 4
protection fibre
3
STM-x
STM-x
customer 3
customer 2
customer 1
w orking fibre
2
customer 3
SDH Add Drop Multiplexer
customer 2
Wavelength Add Drop Multiplexer
a) SDH ring
b) WDM ring
Figure 7 – SDH ring and the correspondent WDM solution
Basically the comparison of the SDH ring with the WDM ring was considered, where
each customer is served by one wavelength, in fact the ring is converted in a point to
point topology. The study starts from the assumption that the SDH STM-4 ring is
saturated and at this time, a change from the SDH ring to a WDM ring is a favourable
situation for WDM as far as a technical aspects comparison is concern.
WDM is used to duplicate the SDH rings capacity, considering that fibre is not
available in place.
SRD ring / WDM ring
100.000
90.000
80.000
70.000
60.000
Cost/customer
50.000
40.000
30.000
20.000
10.000
0
3 n SDH
1 customer/node
3n WDM
6 n SDH
6 n WDM
Nº of Nodes
2 Customers/node
8 n SDH
8 n WDM
4 Customers/node
Figure 8– Cost comparison, SDH ring / WDM ring
The graphic in the Figure 8 shows that for both technologies SDH/WDM the cost per
customer decrease with the number of customers. With exception of the SDH case
with three nodes and one customer per node, this was due to the fact that we are overdimensioning the SDH, in any other case we couldn’t find a DWDM solution less
expensive than the electric option, revealing again that the motivation to install
DWDM can not be only the solution cost.
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3.6
DWDM technologies for access networks
Network evolution paths towards DWDM networks for
business customers
Looking for the existing situation we identified three main applications of DWDM in
the access network:

The SDH point to point links capacity multiplication or service multiplexing by
DWDM. Referred to as case 1.

The proprietary coloured loops as an alternative of MAN network, to join one
client’s different locations. Referred to as case 2.

The fibre sharing between several client/application. Referred to as case 3.
Of course these three cases don’t preclude of any other interest that might emerge
especially with appliances towards full optical networks featuring wavelength
continuity with the core network. This will most probably occur when optical
routing/switching will be available and affordable. The lower level time domain
existing switches will then be likely to disappear.
3.6.1
Enhanced star structured copper networks (xDSL)
While the two first cases were referring to very big business client the one here rather
refers to small and medium business. Therefore it is much more likely to generate
volumes sufficient to induce learning curves for the technology and significant cost
reductions. Therefore, even if it is further away both in time and technical feasibility,
than the first cases it should be given a special care.
Due to fierce competition, some operators are making service offers through ADSL,
thus all know that within a short time frame the capacity will be too limited and while
migrating towards other DSL technologies, the reach and technical constraints will
lead to much higher costs. Therefore, there is a need for a graceful migration path
towards optics at lower costs with just in time providing constraints, compatibility
with the existing architectures, services, management and interfaces. So what we
identified were the following 3 steps scenario:
Step one: deploy the DSL technology
The shot has been already fired. Both DSLAMs and client modems have been defined,
dimensioned, service offers and requirements been published and interfaces are
known.
Step two: opticalize the DSL existing systems and areas, and enhance them.
In order for the network planner to avoid blocking situations due to rapid growth of
the demand, it could be wise for the operators to gradually install fibre in the access
directly to some clients, under the conditions of not being obliged to roll out a fully
new technology.
Step Three: densify the use of existing optical links
If both step1 and step2 are successful, it is time to envision as soon as possible to
ameliorate the use of the pioneer optical links through the use of DWDM. Here again
the challenge is to remain transparent to the clients already linked and the existing
systems, architecture and NE already deployed.
The plan is then to accommodate several clients or NEs (including why not radio base
stations ) on the same fibre, through the use of a passive DWDM (Phasar or grating) in
the plant, each branch/drop after the DWDM would then be dedicated to one
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wavelength. This solution might fit as well the evolving path of the early PON
implementations.
Constraints are:

Insertion of passive DWDM, provided the optical budget are respected some
technologies used in the first generation modems should further work.

Replacement of early customers modems unless they were achromatic or
wavelength tuneable from the start.

Installation of the additional network terminations (Modems, ATU-R whatever
they are called).
The operators aren’t likely to be willing to manage wavelength dedicated NEs so that
this aspect of the technology should be hidden to the plant crews. Two ways are then
possible, either self-tuning devices extracting their operating window from the
downstream information or to be wavelength insensitive.
Limitations brought and to overcome:
3.6.2

At the present early stages, a passive DWDM dedicates one wavelength per drop,
so that transmission has to be duplex to work on a single fibre. No coarse diplex
or Diplex compatible devices are foreseen till now.

Due to the same fact further evolution should be preferred in the time domain,
since wavelength allocation will be fixed!

Here again some standardising work could ease the availability of a reduced set
of solutions.
Enhanced Point to Point optical links (SDH)
This case has been already used for point to point SDH systems upgrade within
several access network operators.
Impacting only between the end user and the central office where 2 modems are to be
placed, two alternative provider sources can be found:

One by the traditional Telecom product manufacturers: who propose expensive
but reliable material.

And the other by the LAN and computing manufacturers: who propose efficient
low cost systems.
If the first group will bring reliable products, the costs of the LAN world will be likely
to be very attractive, both of them are to be considered provided they make use of the
single mode G.652 fibre.
No special requirements are to be expressed as long as no interworking between
products of different providers but the following features will be particularly valued:

The respect of the ITU-T grid: because it induces possibly the lowest cost
solutions through the use of common laser sources.

A standard remote modem management facility for easy interfacing with the
operator’s TMN, which enables him to justify the quality of service offered to the
end user, thus avoiding expensive hassles.

The scalability is to be provided especially as long as the price per wavelength is
as high as it is. The operator and client should be able to upgrade the system to
their actual need through plug-in units, with no other hardware operations.
DWDM transparency to the operating and maintenance and installing crews: This
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DWDM technologies for access networks
issue covers mainly the unawareness of technology for the operators. They do not
have to know about the wavelength affected to any end user. The system should
afford for self start-up & calibration, wavelength recognition and associating of
user identity or channel allocations with wavelengths at system level. Other types
of technologies may be forwarded transparently through the access network from
user site to user site, provided that the global path optical budget is respected and
that the DWDM plan is compatible on both links. Examples are FDDI, ESCON,
FICON, but not many others are available in telco’s core network only SDH VCs
and FR are likely to be accepted.
The case of several channels only to have a broader bandwidth hasn’t been shown
since it’s obvious.
3.6.3

Step 1: is a self standing one: Diplex working of existing 2 fibre systems

Step 2: is the upgrade of wavelength unaware systems into coloured ones and the
introduction of DWDM combiners and splitters on both sides of the optical link.

Step 3: eventually upgrade of the number of channel.
Point to multipoint links (ITU-T G.983 ATM – PON)
This architecture and topology is comprehensively addressed in the next section,
where all the upgrading schemes and options along with the related issues are given.
3.6.4
Enhanced cable Ring architectures
This case refers to cable ring infrastructures where one fibre pair all the way long is
dedicated to one customer/node (physical point to point topology). On each
customer/node, an active dedicated terminal multiplexer has to be inserted. This
implies for the operator in front of each of the customer/node locations to extract one
or two fibre pairs from the passing cable, as close as possible to the users’ premises.
Of course this is very expensive and of relative poor revenue. Therefore, if several
clients could take advantage of the same fibre through DWDM, the cost efficiency
would raise. Some obvious constraints are to be met:

Presence of several client locations on common spots, else no fibre sharing will
be cost effective or possible.

Flexible and accurate use of the wavelength allocation plans, passive and tunable
add/drop multiplexers, in order to extract from the fibre loop the wavelength
needed on each spot
It should be a technique affordable both from the cost point of view and technical (low
loss and easy to handle) point of view.
Most of the constraints on the active elements quoted in case 1 have to be respected.
3.6.4.1
Particular issues regarding CR configurations
In order to propose specific ring architectures for DWDM access networks, the
following issues have to be addressed:

The number of wavelengths within the CR and its usage (e.g. one wavelength per
BC, one wavelength shared by a number of BCs, one wavelength per type of
service etc.).

The type of protection and restoration (electrical or optical) type of signal clients
(SDH or others).
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
3.6.4.2
Deliverable 10
The possibility of using more advanced network elements (e.g. Optical CrossConnects -OXCs-, configurable OADMs etc.) on the basis on their (future)
availability.
Proposed ring configuration
In the configuration to follow, the following assumptions have been made:

Only optical distribution networks (ensuring path-protection to the corresponding
BC) are considered, The ADD and DROP function should be optical (use of
OADM in the nodes)

A tributary wavelength, may be either dedicated to a specific BC or shared by a
number of BCs. In case that a number of BCs use the same wavelength, those
BCs may be either connected to different OADMs (forming a logical "sub-ring"
within the CR) or form a separate "sub-ring" connected to a single WADM.

The rate of the SDH signals carried by the individual wavelengths of the CR can
be STM-1, STM-4 or STM-16, depending on the requirements of the particular
BCs, served by the ring. The transmission of different SDH rates by the various
wavelengths is possible.

OADMs providing BCs with the assigned wavelengths are installed either in an
outdoor cabinet or preferably in an indoor location controlled by the local PTT.

Each BC is served by an Optical Network Unit (ONU) installed in the BC
premises. This ONU must be equipped with an SDH ADM (of the appropriate
level n= 1, 4 or 16 depending on the STM rate carried by ?i) and connected to the
OADM that provides the BC with the pre-allocated wavelength.
OADM (proving optical or
electrical protection)
Transmission Network
Network
Manager
1, 2, 
n
Acces Network
n
1
STM-1 to 16
BC ONU m
BC ONU 1
protection fibre
3
working fibre
2
BC ONU 2
BC ONU 3
WADM
sharing ?2
Figure 9 – Proposed final step in the evolution (ring structure)
3.7
Conclusions
The demand to justify WDM in the access network is not there yet, but an initial huge
deployment of fibre in the access is required.
Equipment available is not primarily developed for the access network, so is
expensive and is more adapted for point to point topologies, the cost of technology is
high even considering the short distances in the access. Components for flexible all
optical networks are not yet available.
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The increase of data traffic brings a paradigm shift in the network structure concept
and DWDM will be a key technology as far as transmission is concerned. This fact is
a signal for the operators that should plan their infrastructures to enable a DWDM
deployment in a smooth manner towards to the all-optical networks.
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4
DWDM Upgrade of existing PON
4.1
Introduction
In 1995 the Full Services Access Network (FSAN) initiative was founded to support
the co-operation between operators and equipment vendors aiming to recommend and
establish a standard fibre access system widely accepted among the involved operators
and cost efficient by minimising the custom requirements on the system.. This work
leaded to the FSAN recommendation, which was incorporated in 1998 into the
International Telecommunications Union (ITU) standard G.983 “BROADBAND
OPTICAL ACCESS SYSTEMS BASED ON PASSIVE OPTICAL NETWORKS (PON)”.
This standard is the basis to make the attractive ATM PON solution for optical access
networks a mass-market technology. First operators (NTT) started with
implementation in their access network and many other operators plan to follow in a
short time frame. It is assumed that the FSAN PON concept will determine the optical
access network for the next years and therefore the fibre infrastructure. Due to this fact
network operators intend to build new future proof networks avoiding significant cost
expensive changes on the installed fibre infrastructure for many years.
It is expected that the predicted data traffic for the next years, driven mainly by the
internet access, will cause capacity bottlenecks in the access networks so that upgrade
concepts for the installed FSAN PONs are necessary. Because of the high flexibility
and the capability to provide very high bandwidth DWDM is a very attractive
technology for upgrading existing PON systems. Compared with the possibility to
upgrade the PON in the time domain (TDMA/TDM systems, e.g. 2.5 Gbit/s PON),
DWDM could be the more flexible and cost effective solution.
4.2
Passive optical Networks (PON)
4.2.1
General PON architecture
In PON architectures an Optical Line Termination (OLT) is connected with multiple
Optical Network Units (ONUs) by a passive optical distribution network (ODN). This
point to multipoint connectivity is established by a passive optical power splitter (1 x
N star coupler) serving as passive remote node (RN). In general the OLT is located in
the CO and provides the interface between the access network and the service node.
ONUs can resides on different locations of the Access Network (FTTH, FTTC,
FTTCab). Depending on the ONU’s location, the customers are connected either
directly or by a copper or radio drop.
OLT
Passive Branching
Device at the
Remote Node
1,5 m
DFB Laser
Burst - modereceiver
m
m
ONUs
m
m
1 x 16
Power
splitter
 m
 m
Receiver
Fabry-Perot
Laser
Receiver
Fabry-Perot
Laser
Figure 10 - Example of a schematic PON layout compliant with ITU G.983.
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DWDM technologies for access networks
Extracted from the ITU recommendation G.983.1, in Table 2 some key parameters
describing the Physical Layer requirements are listed.
Parameters
Architectures
Fibre type
Optical path loss
Requirements
FTTH, FTTB/C, FTTCab
single mode 1260nm – 1580nm (ITU G.652)
10 – 25 dB (G.982 class B)
15 – 30 dB (G.982 class C)
Differential path loss
 15 dB
Fibre distance between S/R  20 km
(OLT) and R/S (ONU/ONT)
points
Split ratio
Restricted by path loss and ONU addressing limits
1:16 or 1:32 with passive splitters (G.983.1)
Overall reflection in ODN at  -32 dB
R/S reference points
Downstream
Upstream
Bi-directional transmission
single fibre PON:
1550 nm (1480-1580 nm) 1310 nm (1260-1360 nm)
dual fibre PON:
1310 nm (1260-1360 nm) 1310 nm (1260-1360 nm)
Nominal bit rate:
Option 1 (FTTCab/C/B/H)
155.52 Mbit/s
155.52 Mbit/s
Option 2 (FTTCab/C/B)
622.08 Mbit/s
155.52 Mbit/s
Point-to-Multipoint
Time Division Multiplex Time Division Multiple
transmission
(TDM)
Access (TDMA)
Table 2 - Key parameters of Physical Layer of FSAN ATM PON extracted from
ITU Recommendation G.983.1
4.2.2
Limitations of Power splitting PON
There are some serious limitations due to the passive power splitting approach:
4.3

The optical power intended for one ONU is split between all ONUs, leading to a
N-fold power budget penalty.

Because the PON bandwidth is shared between all subscribers it is possible to
allocate the bandwidth in a flexible and dynamic way. This important advantage
has the counter part, that the OLT and all ONUs must work on aggregate bit rate.

Since the downstream information is broadcast to all ONUs concerns about
privacy will be stimulated.

Network diagnostics and fault localisation of the outside plant are problematic.
Drivers for the WDM upgrade
Despite its great potential the ATM PON has also some serious limitations, explained
in before, which will limit the system performance and make the upgrade of such a
system difficult. With WDM it is possible to construct wavelength based point to point
links on a shared PON fibre infrastructure and therefore to overcome these limitations.
The high flexibility, capacity and transparency, also makes WDM an attractive
upgrade solution offering additional degrees of freedom due to the insertion of various
wavelength to support both entertainment services and more demanding business
applications as LAN-LAN connections.
In summary the main drivers for FSAN PON upgrading are:
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Increased bandwidth demands by business customers - The main drivers for
business customers are similar to the generic ones driving other sectors of the market
bandwidth increase. This will in most cases be a bi-directional requirement so that it is
possible to construct a virtual LAN connecting various sites of the same company,
requiring high levels of security and availability. WDM utilise the PON infrastructure
but implements a point-to-point dedicated link.
Increased bandwidths demands at single ONUs - Additional to the described
requires by specific business customers (e.g. SME) in the early phase of arising
capacity shortage the more and more increasing variety of data based services and the
steady growing number of residential customers cause the need to upgrade certain
ONUs. DWDM could solve the problem delivering bandwidth for this point.
Increased gross downstream bit rate demands on the PON – The expected demand
for internet browsing and digital TV may bring a bottleneck in the downstream PON
capacity, WDM is an option to overcome the problem with minor changes in the
network.
4.3.1
Further requirements on future networks:
The hard competition in access market will lead to a pressure on the cost of future
networks. This means equipment and installation cost as well as the operation and
maintenance cost of such a network. The cost pressure will bring demands like:
Increased network flexibility - The competition will force dynamic response to
customer requests. So the Network operator has to react and to provide the requested
services faster and faster. This can be solved only by a flexible and “more intelligent”
system and aspects like

dynamical consideration of various QoS aspects,

rapid provisioning of unexpected traffic demands,

and the possibility to quickly provide services and introduce new services
will become of great importance. Also for the customers these aspects will have a
strong impact on their decisions to select service and network provider.
Reuse of the existing fibre infrastructure - This demand covers two parts, network
upgrade and optimal exploitation of the existing network infrastructure.
Network upgrade must take into account at least the reuse of the fibre infrastructure
without significant changes, because the installation of the fibre plant is a main cost
factor. Therefore it requires a long life of the fibre plant. So an upgrade will mainly
affect the installation of new equipment at the OLT, RN and ONU locations.
The second part aims optimal exploitation of the existing fibre infrastructure by means
of delivering all services on same fibre infrastructure and combining different
networks. Technological advances have opened the possibilities of new services and
service types, while fibre-optic advances have permitted improved networks, such as
fibre-reinforced cable networks, to evolve. These newer networks signal the
possibility of competition between service providers to provide ever-improving
services at ever-decreasing costs.
Technical remedies can be found for this case and utilise WDM technology. In such a
system, passive power splitters are used to convey multiplexed information between
the head-end or central office (CO) and the subscribers. At the same time different
service providers use the different regions of the optical spectrum, sharing the same
physical network but operating with distinct services and even distinct terminal
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DWDM technologies for access networks
equipment. In a similar way, WDM can further be used to separate distinct services
or/and service types (leased lines).
This will reduce network, operation and maintenance costs. Therefore the following
aspects are of great importance:

Combination of broadcast and point to point links.

Handling of different QoS aspects.

Multiplex / Segregation of services, service types and service providers.
Service Segregation/Multiplex - WDM systems can be used for multiplexing
different services on the same architecture. For example it has been proposed by NTT
[19] to multiplex narrow band ISDN (N-ISDN) services with broad band ISDN (BISDN) services and video distribution services. A two way service uses two different
wavelengths for downstream and upstream. A video distribution service needs only
one wavelength. In total six wavelengths are multiplexed in the 1.5µm wavelength
range.
Customer segregation -Customer segregation could be another application for WDM.
Here the technology presents interesting advantages, allowing independent overlay
networks sharing the same fibre infrastructure and differentiating customer per gross
bite rate used as well as by the quality of service required.
Upstream capacity upgrade - The maturity and cost reduction in the WDM laser
allow the use of WDM solution to be implemented in the upstream direction, in point
to point fibre infrastructures, Passive optical networks or CATV networks.
Segregation of different QoS - With the announced evolution in optical switching it
could be interesting to use it to guaranty quality of service in the IP environment. This
may open another field of application for the technology with a dynamic WDM
allocation network.
Dynamic selection of service provider on optical layer - From a user perspective the
freedom of selecting service provider may be regarded as a service. Every service
provider is connected to an OLT covering a segment of the AN. Whether the service
given by the SP is a broadcast or point-to-point service will decide the means for
transmitting the signal downstream from the OLT. Also multicast services can be
provided if similar signals from one SP is modulated on a number of tuneable lasers.
The ONU are able to select the correct wavelength.
4.3.2
Basic Upgrade concepts
WDM PON networks can be divided in two broad categories:

Broadcast and Select Networks

Wavelength routing networks.
The first category - Broadcast and Select Networks is based on the passive power
splitter at the remote node. The wavelength insensitivity of the power splitter allows
the broadcasting of signals at different wavelengths from the input port to the output
ports. Figure 11 shows this behaviour for 1 x 4 splitter. The WDM signal at input port
appears at all output ports with a reduced optical power by factor 4.
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1
1, 2, ... , N
2
1, 2, ... , N
...
...
1, 2, ... , N
Splitter
M=1,
N=4
...
...
...
1, 2, ... , N
4
...
Figure 11 - Wavelength broadcast behaviour of a power splitter
Due to the passive optical splitting there is N-fold power budget penalty like in the
PS-PON. Additional losses have to be considered, since filters must be implemented
to select the allocated wavelengths in the ONUs and in the OLT wavelengths must be
multiplexed in the transmitter and de-multiplexed in the receiver.
The upstream and downstream separation can be established by a dual fibre PON or
by Coarse WDM diplex in the one fibre PON. Because of the broadcast approach the
network has the same limits like the PS-PON, except that the transmission bit rate and
protocol per wavelength is not shared between all ONUs. Therefore the bite rate and
transmission protocol per wavelength is devoted to the allocated ONU. A great
advantage of the concept is that the temperature dependence of the power splitter can
be neglected, because of no significant impact on wavelength transmission behaviour.
In Figure 12 a generic layout of the single fibre Broadcast and Select-PON is given.
OLT
Passive Remote Node
Source
Array
Mux



ONUs
1 x16
Power
splitter
Router



Source


Receiver Demux
Array
Receiver
Receiver


Source
Figure 12 - Generic layout of a single fibre WDM Broadcast and Select PON.
The second category - Wavelength routing PON – is based on a passive device at the
remote node, known as waveguide grating router, arrayed waveguide grating
multiplexer or optical phased array. For simplicity, it will be designated in the
following consideration as Wavelength Router (WR). The WR in general is a M
(input) x N (output) device, but for our application it is an 1 x N device such as the
power splitter. One of the remarkable properties of this device is the capability to
direct downstream light from the input (1) to unique output ports (N) as function of
optical wavelength. In this direction it works as a wavelength demultiplexer. Figure 13
1, 2, ... , 4
Wavelength
Router
1
1
2
2
...
...
M=1,
N=4
N=4
4
Figure 13 - Wavelength router as demultiplexer
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WR designs permit WDM bi-directional links in two ways. In first approach two parts
of the 1,5µm transmission window are used for the downstream and upstream
transmission. In the second approach, WR works as wavelength router in the
downstream direction in the 1.5 m window and as a simple power combiner in the
upstream direction for the 1.3µm window.
Figure 14 shows schematic layout of the general WDM PON architecture based on
Wavelength Routing concept.
OLT
Source
Array
Mux
ONUs
Passive Remote Node




Wavelength
Router
16x


Receiver Demux
Array




Receiver
Source
Receiver
Source
Figure 14 - Schematic layout of the general WDM PON architecture based on the
Wavelength routing concept
In this architecture, wavelength based point to point connections are established using
the properties of the WR, improving the network integrity and the privacy
significantly. The power budget penalties of the optical path are lower when compared
with a power splitter approach, even if the number of connected output port is high.
Further on the optical transmission in downstream and upstream direction is bit rate
and transmission format transparent. The architecture offers huge bandwidth and high
flexibility.
Considering the architecture of FSAN PON and assuming that at least the installed
fibre infrastructure is the fixed part of the network, there are 3 points to upgrade the
network - OLT, Remote node and ONU. The WDM implementation on the PON
topology generally requires multi wavelength devices. The OLT has to transmit and
receive different wavelengths, the ONU has to transmit and receive one or more
wavelengths. At the remote node a passive multipex / demultiplex and/or a
broadcasting component has to be realised and an appropriate approach for allocating
and selecting the different wavelengths to the individual ONUs have to be defined.
Taking into account the upgrade drivers, four categories of upgrade concepts can be
identified:

WDM upgrade only of the downstream transmission and maintain the upstream
transmission of the PS PON – for single links, single ONUs or the entire PON.

Single direction WDM upgrade (downstream) as overlay without affecting the
operation of the PS PON – for single ptp links or as a broadcast channel for the
entire PON.

Bi-directional WDM upgrade as overlay without affecting the operation of the PS
PON– for single links, single ONUs or the entire PON.

Reuse only of the fibre infrastructure and substitute the PS PON by a new WDM
PON.
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4.3.3
Upgrade constraints, restrictions and limitations
4.3.3.1
System constraints
The starting situation is an existing FSAN PON. Therefore the ATM PON ITU-T
G.983.1 specification must take into account, especially the power budget, the
wavelength ranges, possible architectures, transmission methods and the bit rates.
Spite of WDM being a good solution to upgrade the downstream direction in some
ONUs, there are various system-level difficulties involved with the introduction of
WDM dedicated channels in a pre-existent PON system, the most important of which
regards the control of the TDMA upstream channel, requiring attention.
In the case of a dedicated channel in only the downstream direction the ONU still
needs the TDMA signalling information in order to properly transmit to the OLT. In
this case there are 2 alternatives: either the control information is also present on the
dedicated channel, or the ONU must continue to receive the original broadcast channel
solely for the purposes of control. Of course, in the case of bi-directional transmission
there is no need for the control information, the link being effectively point-to-point.
The first of these solutions poses difficulties as an upgrade scenario since normally
OLT equipment does not output the control information on an electrical interface to be
used by a second transmitter. That is, there is no intrinsic limitation to the addition of
downstream-only dedicated channels but without access to control data, normally not
available outside the OLT, the TDMA channel cannot be implemented
Another potential problem is the fact that the data destined for the dedicated link must
be output from the ATM switch on a separate interface from the OLT. This should
normally be a simple question of switch configuration, since there are usually several
OLTs connected to the same switch. If however there are not enough free ports on the
switch then there is a bottleneck which can only be overcome by increasing the
number of ports.
4.3.3.2
Technological constraints
The use of DWDM in the upgrade of a PON system can only be achieved if a number
of technological conditions are met, since the original system must continue to
function after the upgrade. The authors identified possible constraints in the following
parameters, that should be visited before a DWDM upgrade option:
4.3.3.3

Increased losses due to the new components installation.

Crosstalk between channels, present because wavelengths are not perfectly
separated in WDM components.

Back-reflections from components, the figures given for the return loss of optical
components are generally very small, and it is unlikely that difficulties will arise
in this area.

DWDM devices are temperature sensitive passive, requiring special attention.
Economic Constraints
To upgrade an existing FSAN PON with WDM overlay techniques naturally contain
the installation of new components or equipment in the system. In this connection
some economical aspects have to be taken into account that cause any kind of
expenses or expenditures to be avoided or at least to be minimised:
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4.3.3.4
DWDM technologies for access networks

Advance payment (in the planning and introduction phase) to be minimised with
a decision for a system concept that allows a low-priced introduction for the first
customers or services and that can nevertheless be e.g. modularly extended with
little working and installation expenditures and interruption free service
provision.

The cost for components and equipment is the most important part to note. So the
aim is to use as few as possible separate components. The decision about the kind
of new or reuse of existing components at the three relevant locations OLT,
Remote node and ONU/ONT has to be made in comparison of the component
cost, the installation expenditures, the technical properties and quality
advantages.

The scope of changes and new equipment at the ONU has to be minimised above
all because of the comparatively large number of these locations per PON system.

Personnel costs for the installation of the equipment and for putting the system in
operation and for Operation and Maintenance have to be planned. To limit these
personnel payments the system once put in operation has to run automatically
without constant or permanent need for maintenance or servicing.

Service life of the technique/equipment at least for the time until investment
depreciation and the future capability in view of the compatibility, functionality
and further upgrade possibilities of the technology.

Financing of investment for the WDM upgrade to be balanced with adequate
secure income of new offered applications and services.
Operator Constraints
The necessary extensions of the system features like increase of capacity (with chance
to offer more services) and better exploitation of fibre infrastructure are desirable
improvements wanted by the operator. But the upgrade of a running system cause
interruptions for installation of the new components. In the starting time system
problems can often cause service works and reduction of functioning. So customers
connected to the FSAN PON who usually enjoy the smooth work are annoyed.
Therefore an everlasting change of the network like installing discrete components
step by step according to the present effort is not the idea of the operator. More
expensive but avoiding the described disadvantages is to include a single integrated
component in one advance step.
Other constraints concerning the operator are related to economical aspects like
service security, service life, simple OAM and to build a flexible and future secure
platform.
With view of the whole operator network it is desired to come closer to an uniform
technology over all hierarchical steps to an all optical network.
4.4
WDM Upgrade solutions
4.4.1
Solutions for business customers
The solutions with the aim to increase the bit-rate of certain existing ONUs for
business customers fall into two main categories: downstream only, and bi-directional
dedicated links. There is also an additional driver to provide a dedicated channel
between two ONUs present on the same PON.
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Since it is likely that the first users of WDM channels on a PON will be the business
customers it is necessary to envisage solutions which provide very few (even just one)
dedicated channel – these will be the entry points of WDM in the PON, leading
eventually to widespread use among all sectors. Here we will present only one
example.
4.4.1.1
Downstream dedicated links in a single-fibre PON
The addition of a small number of downstream channels to a single-fibre PON is
complicated by the fact that the upstream signals at 1.3µm use the same fibre as the
downstream signals, both broadcast and dedicated, around 1.55 µm. Hence the
behaviour of wavelength selective components must also be specified at 1.3µm, or
extra components must be used to bypass critical parts of the network. The first
solution is not an option at present since no commercial WDM components specified
for the 1.55 µm window have a defined behaviour at 1.3 µm. The solution, shown in
Figure 15, introduces extra losses, but can nonetheless be viable in cases where the
initial power margin is sufficient. In fact the total extra loss on the broadcast channel
is 6.45 dB, that on the upstream channel is 3 dB, and the worst case total internal loss
on the WDM channel is (7.8 + M 0.95) dB where M is the total number of WDM
channels.
FP Tx 1.3
Rx 1.5
Rx 1.3
Tx 1 DFB
1:N
FP Tx 1.3
Rx 1.5
...
...
0
Tx 2 DFB
...
FP Tx 1.3
Tx M DFB
1
Tx 0 DFB
2
M
Rx 1.5
...
Figure 15 - Addition of WDM channels (
-fibre PSPON
It is also possible to use the AWG to multiplex the broadcast channel, thus saving an
add/drop component. However this entails that the broadcast channel suffers
additional losses due to two AWGs in series, and it is unlikely that the power margin
of a typical PSPON will allow such an architecture Figure 16.
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DWDM technologies for access networks
FP Tx 1.3
Rx 1.5
0
...
FP Tx 1.3
Rx 1.5
Rx 1.3
1:N
0
0
Tx 0 DFB
FP Tx 1.3
Rx 1.5
Tx 1 DFB
...
DE
MUX
M
...
...
MUX
FP Tx 1.3
Tx M DFB
Rx 1.5
1
Figure 16 - Architecture for the addition of WDM channels to single-fibre PSPON
using AWGs.
4.4.1.2
Bi-directional dedicated links
4.4.1.2.1 Bi-directional upgrade based on discrete components
As has been pointed out in the chapter system constraints, unless the necessary MAC
protocol is also present on the dedicated downstream channel it is not possible to
control the upstream TDMA channel of the ONU. One solution is the addition of a
dedicated upstream link each time a downstream link is installed. For the two-fibre
PON case this is simple – it is enough to implement the same architecture as for the
downstream direction. For the single fibre PON a bi-directional link can be achieved
using the architecture of Figure 17.
FP Tx 1.3
Rx 1.5
Rx 1.3
1:N
0
DFB Tx 1.5
Tx 1 DFB
Rx 1.5
Rx  2 DFB
1
2
Tx 0 DFB
Figure 17 - Upstream and downstream channels using cascaded discrete
components in a single fibre PON
In Figure 17 the grey OADMs could be red/blue band WDMs to allow the use of
multiple broadcast channels in the downstream direction. The architecture allows the
addition of as many pairs of channels as the system power margin can support, with
the same number of components as for the downstream-only case.
4.4.1.2.2 Bi-directional upgrade based on AWG wavelength routers
The same approach can be used here as for the discrete component case. The
1.3µm/1.55µm WDM components on the ONU side of the splitter are replaced by
OADMs and each ONU uses 2 channels of each AWG. The additional losses here are
the same as in the discrete component case, approximately 1.2dB Figure 18.
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FP Tx 1.3
Rx 1.5
Rx 1.3
1:N
0
DFB Tx 1.5
Tx 1 DFB
Rx 1.5
Rx  2 DFB
Tx 3 DFB
DFB Tx 1.5
Rx 1.5
Rx  4 DFB
Tx 0 DFB
Figure 18 - Upstream and downstream channels using AWGs in a single fibre
PON
Another approach involves use of the cyclic properties of the AWG. the upstream
channels can use a wavelength 1 free spectral range away from the downstream
wavelength. The upstream and downstream signals use exactly the same path, and are
separated only at the terminals. The OADM on the ONU side of the AWG in the
remote node is therefore relocated to the OLT Figure 19 so the losses are the same, but
twice the number of channels can be offered.
FP Tx 1.3
Rx 1.5
Rx 1.3
1:N
0
Tx 1 DFB
DFB Tx 1.5
Rx 1.5
Rx  2 DFB
Tx 3 DFB
DFB Tx 1.5
Rx 1.5
Rx  4 DFB
Tx 0 DFB
Figure 19 - Upstream and downstream channels using the cyclic properties of
AWGs in a single fibre PON
4.4.1.2
Conclusions
From the calculation of the impact on single- and two-fibre PSPONs of the addition of
downstream dedicated WDM channels it appears that it is possible to add channels
gradually with losses comparable to, if not better than, those currently available using
AWG routers. In the case of the single fibre solution the power margins of the original
broadcast and upstream channels need to be fairly high. This is true for channel
numbers up to about 16, after which it will probably become attractive to remove the
splitter altogether and provide each ONU with a dedicated channel. The advantage of
this approach is that initially, when perhaps there is some doubt about the rate of
growth of WDM applications, one can provide connections as requests occur without
having to predict the total number of connections to be provided in the future.
Making all WDM overlay channels bi-directional, achieved by adding one or two
components to each optical path, avoids the difficulties encountered in TDMA control
when only downstream WDM channels are implemented. The extra loss amounts to
about 1.2dB per channel, but the number of ONUs with dedicated channels is halved,
for a given initial power margin, with respect to the unidirectional case. Another
possible solution is the use of the cyclic properties of AWGs which allow the same
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DWDM technologies for access networks
optical path to be shared by two different wavelengths. The losses are the same as for
the previous cases, but the number of ONUs that may be connected is not affected.
The current costs of the components required for the various upgrade scenarios are
such that the solution based on discrete approach is more economical, though it is
probable that the costs of routers will fall more quickly than those of OADMs, for
example.
4.4.2
Solutions for increased demands at single ONUs
In previous section “Solutions for business customers” it was discussed how to
upgrade downstream or bi-directional capacity of single dedicated links to business
customers. Another driver described in Section “Drivers for the WDM upgrade” is the
increased bit rate demand at single ONUs. From technical point of view there is no
difference concerning the possible solutions. So this section refers to the results of
investigations made there.
4.4.3
Solutions for increased gross Downstream bit rate
In the upgrade of downstream capacity at the PON system it is thinkable to assign
additional dedicated wavelengths to every ONU (wavelength per ONU) or to provide
a new service using one wavelength to all ONUs (wavelength per service). It is
possible to make use of the broadcast and select architecture, routed wavelengths
architecture or a combination of both.
4.4.4
Wavelength per Service:
It is possible to use this technique both for single or two fibre PONs. Because in twofibre PONs the transmission in upstream and downstream direction is managed on two
separate fibres in the downstream fibre only the 1.3µm is occupied. So the free
1.55µm window can be used for transmission of upgrade wavelength(s). If one wants
to provide a broadcast service to all ONUs an additional wavelength or a WDM
channel can be inserted in the downstream fibre using a 1.3/1.5 WDM. This
wavelength will be transmitted to all ONUs connected to the passive power splitter. A
coarse WDM at the ONUs detours it to a new receiver.
Upgrading a single fibre PON is of course more complicated. The original
downstream TDM channel and the upgrade channel both must use the 1.55µm
window. So additional to the wide band 1.3µm/1.55µm WDMs add/drop WDMs at
1.55µm have to be added at OLT and ONUs. This will make the upgrade much more
expensive then the two-fibre solution.
The ONU has to select multiple wavelength, this factor increase the complexity and
cost of the ONU.
4.4.4.1
Wavelength per ONU:
By wavelength per ONU upgrade there have to be wavelength-based point to point
links between the OLT and each ONU. So each ONU has one dedicated wavelength
channel on the same fibre infrastructure without affecting the installed fibre cable.
These allow different applications by assigning the wavelength to specific issues like
increasing of up- and downstream bit rates, multiplexing (Adding or Segregation) of
services (with special format), segregation of different operators or segregation of
different QoS (e.g. IP traffic with QoS classes and different services level agreement).
This upgrade could be done with two possible solutions:
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
Broadcast and Select concept

Wavelength routing concept
Deliverable 10
The broadcast and select architecture to provide separate wavelength channels to all
ONUs, upgrade wavelengths are multiplexed by AWG and broadcast to all ONUs.
Discrete filters or add/drop WDMs must be inserted at the receivers to let only the
destined wavelength pass to the ONUs.
In the Wavelength routing concept, the PON architecture uses power splitter as
combiner for upstream transmission and wavelength router for downstream capacity
upgrade.
4.4.5
Solution for increased bi-directional capacity demand
A way to increase the bi-directional capacity of a two-fibre PON is to add an WDM
overlay of permanent wavelength paths in the region of 1.55µm. This system concept
gives a capacity increase without changing the fibre network structure. The existing
TDM/TDMA PON system will not be infected.
1,3 µm
1,3 µm
3 db
1,3 µm
1/N
3 db
.....
1,5 µm
1
3 db
1p
1
3 db
1p
.....
i
A
W
G
1/
N
3 db
ip
1,3 µm
1,5 µm
1,5 µm
A
W.....
G
1/
N
1,5 µm
3 db
i
3 db
ip
Figure 20 bi-directional upgrade with WDM PON overlay concept [20]
To separate the original TDM/TDMA PON from the new overlay at the central office,
at the ONUs and to detour the Power splitter at the Remote Node coarse 1.3/1.55 µm
WDMs have to be inserted. The key element used for multiplexing, demultiplexing
and routing functions of different wavelength signals is the arrayed waveguide grating
(AWG). At the OLT as well as at the Remote Node wavelength routers AWGs) have
to be inserted. Because of the frequency selective routing function of the AWG cost
effective receivers can be used. This also considerably improves the protection against
interference and eavesdropping. The individual ONU source wavelength must match
with its related wavelength of the wavelength router port, so technical problems like
temperature dependency of both components (ONU-laser, wavelength router) and
controlling and stabilisation of the ONU Laser sources have to be solved.
4.4.6
Future networks
In future various upgrade possibilities are thinkable that only use the fibre
infrastructure of the former PON system. So the power splitter at the remote node can
be replaced by a router (e.g. AWG or active device) and to all ONUs point-to-point
wavelength connections can be realised. More then one wavelength per ONU for
much higher demands are thinkable too.
The most freedom and flexibility will be provided by dynamic wavelength access. The
access to wavelengths for transmitting and receiving between the individual ONUs
and the OLT are allocated in dynamic and flexible way. In this case a procedure and a
related Medium Access Control (MAC) protocol must be implemented to provide a
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DWDM technologies for access networks
resource management and handle the wavelength allocation between OLT and ONU.
Further on, tuneable components such as tuneable lasers and filters are required. These
will lead to more complex and expensive network.
These solutions are further steps of not upgrading then more exchange of PON for
much more powerful systems and depend also on development of future services and
demands as well as availability of commercial and mature components and trade off
between cost and performance.
4.4.7
Component issues
All the components necessary for the architectures presented before are currently
available. Table 3 shows the prices, as of June 1999, of DWDM components
necessary for the addition of dedicated downstream channels in the downstream
direction only.
Table 3 - Prices (June 1999) of WDM components
WDM device &
Quantity
Channel spacing
1–9
10 - 24
100
Wide band WDM 1.3- ~200 EUR
1.55µm
OADM 200GHz
936 EUR
831 EUR
719 EUR
OADM 400GHz
811 EUR
718 EUR
619 EUR
8-channel AWG 200GHz
8044 EUR
7306 EUR
6382 EUR
8-channel AWG 400GHz
7047 EUR
6398 EUR
5588 EUR
Based on this information, it is possible to extrapolate the cost per port of the
wavelength routers and hence to estimate the total component cost, for a given number
of added channels, for the discrete component and AWG based solutions for
downstream-only dedicated channels. However, the OADM devices have been
available for some time and their cost seems to have bottomed out, whereas the router
technologies (AWGs, for example) are relatively new and it is foreseeable that their
cost will continue to decrease for some time. Clearly the results of Figure 21, although
calculated for the downstream-only case, are valid for the bi-directional case since the
additional components necessary are the same.
Figure 21 - Cost per channels added for 1 fibre (left) and 2 fibre PONs with
downstream-only dedicated channels
The composite component necessary for the reception of the broadcast channel even
by ONUs with dedicated links could perhaps be integrated into a single device, and in
this case the approach would be more attractive. Otherwise this particular architecture
performs badly from the point of view of the power budget and it would probably be
more sensible to go directly to the bi-directional solution.
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The figures quoted by manufacturers suggest that crosstalk will not be a great problem
in these upgrade scenarios, at least in the cases anticipated for the solutions for
business customers in which there are relatively few dedicated channels present on the
PON. However, in more evolved networks in which there is a large number of
dedicated channels serving both business and residential customers it is likely that
crosstalk will be more of an issue.
4.4.8
Management and protection aspects
The management and protection in the PON procedures and technique are fully
covered by the FSAN G.983 specification. In fact from the system point of view the
protection is implemented by a parallel PON systems over the same infrastructure.
The upgrade scenarios using WDM to provide dedicated point to point links in the
downstream direction suffer from the problem of the distribution of OAM information
in the PON. While in new WDM PONs it is possible, but very difficult, that the
problem be overcome at the hardware level in the OLT, in non-green field situations
the two solutions are either to always install bi-directional links, or else to maintain
the original downstream broadcast channel even in the ONUs with a dedicated link.
The second option further reduces power margins in the system, particularly in onefibre PONs, and necessitates the use of several additional components per ONU. The
first option, on the other hand, is the most attractive because it is likely that the first
users of dedicated channels be business customers interested in high bandwidth twoway traffic.
The independence of the FSAN-PON and the WDM systems simplifies the OAM
function, only need to certify that both systems are compliant to the specification as
far as survivability is concerned. This case could be more complex in a WDM
scenario where the wavelength channel is routed from the core network without any
conversion in the edge near the OLT. In this case special attention should be taken to
the optical element that makes the interface with feeder section in the access network
in order to certify its protection capabilities.
The scenario of dynamic wavelength allocation in the access network is the final step
in the upgrading scenario, more than an upgrade option it is a target architecture. In
this case we are talking about an advanced system that needs a complex wavelength
MAC protocol to assign the different wavelengths. The complexity of such a MAC
protocol requires the existence of OAM information flows between the network
elements that should integrate all management and survivability mechanisms
embedded on it.
4.5
Evaluation of the upgrade solutions and migration path
Corresponding to each upgrade driver there is a set of upgrade solutions. One has to
evaluate each of the upgrade solutions with respect to the set of the following criteria:

Cost,

Technological Feasibility,

Upgradability,

Flexibility

and identified constraints.
Each evaluation should provide the “best” solutions. It is also quite likely that a
“trade-off” between different criteria might be required to be able to determine the
best solutions. The process is repeated for each driver. The next step remains in
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DWDM technologies for access networks
determining the time variation of upgrade drivers. A good deal of uncertainty is
involved in this task and therefore involves some guessing and “speculation”. When
done, this corresponds to arranging different drivers in a chronological order, and
might lead to several different scenarios. Arranging the best solutions along the same
scenario should provide one/several migration paths. An evaluation might again need
to be performed to determine how good the “best solution” for a given upgrading step
is and how smooth the migration provides it, when one takes into account the
migration from an earlier upgrade step.
This quantitative evaluation requires a detailed analysis with large set of parameters.
So for instance it must take into account a fixed FSAN reference model, assumptions
on expected services and bit rates and large database with technical parameters and
cost of the required components. Only in this way the different upgrade solution are
comparable in cost and performance. Of course the evaluations is “trade off” between
cost and performance. Beside the technical and economical aspects, the strategy and
philosophy of network operator will have strong impact on the migration path.
In the following table (Table 4) a qualitative evaluation is given. It takes into account
the identified drivers and upgrade concepts as well as the WDM PON Networks
discussion before. The large “X” indicates a promising solution, the squares marked
slight grey indicates a possible solution. One may identify different migration paths.
One is based on the broadcast and select approach the other on the wavelength routing
approach. For the first demands (it is assumed this will be business customer
demands) an upgrade can be started with discrete components. But with growing
demands integrated components will be required, which promise an easier handling
and installation, compactness, higher assurance, easier fault diagnostic and at least
lower costs (of curse only these components will be commercially available in a
sufficient quantity).
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WDM technologies for access networks
D e d ic a t e d
some O NUs
a ll O N U s
e n t ire P O N
L in k s
Inc re se d
b a n d w id th
D r iv e r s
de m a nds by
b u s in e s s
c u s to m e r s
Inc re a se d
Inc re a se d
b id ir e c tio n
I m p le m e n ta tio n
d o w n s tr e a m b it
a l b it r a te
o f a d d itio n a l
d o w n s tr e a m b it
b id ir e c tio n a l b it
r a te d e m a n d s a t
de m a nds
B roa dc a st
r a te d e m a n d s o n
Inc re a s e d gros s
r a te d e m a n d s o n
Inc re a se d gros s
s in g le O N U s
a t s in g le
c ha nnne l
th e P O N
th e P O N
D if f e r e n tia tio n o f
s e r v ic e s a n d
p r o v id e r s
Inc re a s e d
n e tw o r k f le x ib ility
H a n d lin g o f
d if f e r e n t Q o S
a s p e c ts
ONUs
Upgrade concepts
E x p e te d c h a n g e s o f d ir v e r s w ith th e tim e
B r o a d c a s t & S e le c t
W D M upg ra de o nly o f the
X
w a v e le n g th p e r O N U
do wns tre a m tra ns m is s io n a nd
m a inta in the ups tre a m
d is c r e te c o m p o n e n ts
tra ns m is s io n o f the e x is ting
X
X
W a v e le g th r o u tin g
PS PO N
in te g r a te d
X
c o m p o n e n ts
B r o a d c a s t & S e le c t
S ing le dire c tio n W D M upg ra de
X
w a v e le n g th to a ll
X
ONUs
(do wns tre a m ) a s o v e rla y witho ut
d is c r e te c o m p o n e n ts
a ffe c ting the o pe ra tio n o f the
( a s p tp lin k )
W a v e le g th r o u tin g
PS PO N
in te g r a te d
( w ith s p e c tr a l
(a s p t p lin k )
c o m p o n e n ts
d is c r e te c o m p o n e n ts
W a v e le n g th r o u tin g
B i-dire c tio na l W D M upg ra de a s
O v e r la y
o v e rla y witho ut a ffe c ting the
in te g r a te d
c o m p o n e n ts
o pe ra tio n o f the P S P O N
B r o a d c a s t & S e le c t
O v e r la y
w a v e le n g th p e r
ONU
s lic in g )
X
X
X
X
X
X
X
X
X
d o w n s tr e a m
C o m p o s ite P O N
X
d e d ic a te d p tp lin k s
lin k s c o m p o s ite w ith
R e us e o nly o f the P S P O N fibre
infra s truc ture a nd s ubs titute the
W D M P O N ba se d on
w a v e le n g th r o u tin g
P S P O N by a n ne w W D M P O N
(future ne two rk s )
W D M P O N ba se d on
b r o a d c a s t & s e le c t
d e d ic a te d b id ir e c tio n
X
p tp lin k s w ith f ix e d
w a v e le n g th a llo c a tio n
b id ir e c tio n a l W D M
b r a o d c a s t w ith f ix e d
w a v e le n g th s e le c tio n
w a v e le n g th b a s e d
F u tu r e n e tw o r k s
X
d y n a m ic
X
X
r e c o n f ig u r a b le
Table 4 Solutions to the identified drivers (slightgrey – possible solution, darkgrey/X– promising solution)
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4.6
WDM technologies for access networks
Guidelines for Network planners
In the competitive telecommunications environment, network planners face a common
problem; how to upgrade networks in a profitable way and minimising risk, due to the
uncertainty in service demand predictions and huge needed investments. There are
several other factors in addition which will influence the specification for the new
network. The planner can then make a shortlist of competing architecture solutions,
each of which meet the basic specifications. The economics of these solutions may
then be compared. At this stage variations in the introduction scenarios may be
discussed to further improve the profitability of a particular solution. An effort is made
to group all the factors and requirements which a planner has to keep in mind while
introducing the WDM technology in upgrading the existing PON systems.
In the first stage a planner has to make some serious evaluation whether he wishes to
upgrade the existing network using WDM. The main factors influencing this decision
are shown in the flow chart (Figure 22) and are divided in four broad categories:
Existing equipment - The extent to which the existing equipment has to be integrated
in the upgrade solution with a new technology like PON.
Traffic - A comprehensive forecast has to be made to predict the expected traffic and
traffic types in the access network. A corresponding revenue earning forecast may also
be needed.
The new system - What properties are expected from the new system. Future
scalability and upgradability are one of the major issues. How manageable are the
network management systems? Is backward compatibility important, etc.
The other influences - As the name suggests, these involve the decisions made by the
others outside the planner’s reach. These include the decisions made through
internal/external politics and business strategy actions taken by the competitors.
Once that is done then he has to consider the most compelling drivers for the required
upgrade. In previous chapters a detailed description of different drivers has been
performed. In the Flow chart Figure 22 such factors are grouped into nine broader
categories to be able to give an overview. Some of these categories therefore might be
overlapping:
Before we presented the possible solutions for various stages in upgrading of PON
networks with WDM, and several possibilities for corresponding migration paths are
described. Every driver may be fulfilled through several different technological
concepts as shown in the Figure 22. For every concept there may also be several
technological implementations described as solutions in the Figure 22. E.g. a WDM
upgrade of the downstream transmission may be realised both through broadcast &
select and wavelength routing. Further each of these two routing mechanisms can be
achieved through either discrete or integrated components etc.
The choice of a particular solution has its pros and cons as described in earlier
chapters. At this stage an evaluation has to be performed by the planner. The
evaluation criteria can include both quantitative parameters such as Payback period
and Net Present Value. For details of these methods see f.ex. [9]) or/and qualitative
parameters such as upgradability and flexibility of a network solution. The inputs to
this evaluation may include the costs (equipments and new installations f.ex.of extra
fibers) and expected revenues from these new investments, which in turn may require
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WDM technologies for access networks
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a more accurate traffic forecast. A negative outcome of this calculation/evaluation
would imply that another concept or another implementation should be chosen.
Driver
Concept
Alt. Solutions
Increased
bandwidth
demands by
business
customers
•Existing topology/network
•Existing cables/fibres etc.
Traffic
•Present vendor relationship
•Competitors decision
•Traffic forecasts
•Future revenues
•Future traffic mix/types
Choice of WDM
upgrade concept
Increased
downastream
bit rate
demands at
single ONUs
WDM upgrade only of the
downstream transmission and
maintain the upstream trenamission
of the existing PSPON
Broadcast
& select
Other influences
Existing Equipment
The new system
•Compatibility with existing
systems
•Equipment availability
•Scalability
•Future upgradability
•Protection ?
•Maintenance costs
Increased
bidirectional
bit rate
demands at
single
ONUs
Implementation of
additional
broadcast
channel
Single direction WDM upgrade
(downstream) as overlay without
affecting the operation of the
PSPON
Wavelength
routing
Wavelength
routing
overlay
Increased
gross
downstream
bit rate
demands on
the PON
Increased
gross
bidirectional
bit rate
demands on
the PON
Differentiation of
services and
providers
Bidirectional WDM upgrade as
overlay without affecting the
operation of the PSPON
Broadcast
& select
overlay
Composite
PON
Increased
network
flexibility
Handling of
different
QoS aspects
Reuse only of the PSPON fiber
infrastructure and substitute the
PSPON by a new WDM PON
WDM
PON based
on
wavelength
routing
Future
networks
Evaluation Criteria
Rank solutions f.ex. in
terms of shortest PP and
highest NPV
Payback period
Evaluate each solution
Traffic and
revenue estimates
Net Present Value
Upgradability etc.
Equipment costs
Figure 22 - Simplified methodology to provide guidelines for technology
introduction
4.7
Conclusions
The limitations observed in the FSAN Power splitting Passive Optical Networks,
relating to increasing bit rate demands caused by foreseen drivers, have been shown.
The high flexibility and the capability to provide very high bandwidth and possibility
of using the existing infrastructure makes WDM a very attractive technology for
upgrading existing PON systems.
Various technical and architectural concepts for WDM upgrading of existing PONs
have been introduced, the architectures have been described and analysed, advantages
and disadvantages have been discussed. The following technical concepts that meet
the drivers are considered to be the possible WDM upgrades of FSAN PON systems:

Downstream WDM upgrade as overlay without affecting the operation of the PS
PON– for single ptp links or as a broadcast channel for the entire PON.

Upgrading only the downstream capacity and maintain the upstream transmission
of the existing PS PON.

Bi-directional WDM overlay upgrading without affecting the operation of the PS
PON.
The drivers and solutions have been joined together derived from the technical and
architectural concepts. As the beginning step it is foreseen to upgrade a few business
customer ONUs with dedicated links. So the solutions for this driver have been
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WDM technologies for access networks
discussed most detailed and the physically practicability of WDM for this start
scenario was demonstrated
The solutions have been evaluated resulting in possible migration paths depending on
development of drivers and customer demands. Aspects of management and
protection have been investigated.
Various circumstances are influencing the practical realisation of WDM upgrade of
PONs in the access. An important aspect is the commercial and cost effective
availability of components like wavelength routers (AWGs) and transmitters (for
instance LASERs, Super LEDs). Another constraint is the suitability of wavelength
routers for use in the outside plant (at the remote node). Consequently an essential
property of wavelength routers is temperature independence.
It is mentioned that today no perfected, commercial products and components are
available. To introduce the solutions in the network the costs of mentioned
components must decrease. It is expected that through the practical operation of WDM
in the Metro network component prices will fall and so in future the possibility of
introduction in the access part will become more economically.
As described in “Guidelines for network planners” a lot of concrete complex
conditions for the special situations have to be took into account and a detailed
analysis and a trade off between costs and performance have to be made.
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5
Convergence of DWDM Networks
5.1
Reference Optical Metropolitan Access Architecture
The network convergence through the DWDM technology towards all optical
networking shall be examined through a reference General network considering the
three domains, core, metro and access as presented in the first chapter. This model is
partly based on the highlights of the documented studies [15] – [18]. In this model
emphasis is given on the network functional description and technology requirements
rather, than on detailed description of architecture. It is important that the reference
architecture is simplified as much as possible while keeping at the same time a
functional structure which is capable of describing both the future DWDM access
networks and migration to those from the current networks. The legacy access and
metropolitan area networks can be identified as parts of this reference network.
Figure 23 shows a high level network architectural view consisting of three segments:
core network, metropolitan network and access network. For the rest of this document
we use this topology for illustrative purposes, however the principles are fundamental
to a range of possible topologies. The metro network has a ring topology
interconnecting Access Nodes (AN) and Hub Nodes (HN). An access node typically
hosts switching facilities (LEX, ATM, IP) and is designed to be highly configurable.
Hub nodes serve as the interface between the metropolitan access network and the
backbone network. The access network interfaces directly the customer premises and
is responsible for delivering and collecting traffic. Some traffic aggregation may take
place in the distribution network as well. The topology of the distribution network can
be tree, bus, ring or a combination of them. Legacy active rings, e.g. SDH rings, can
also be accommodated.
IP Backbone
Network
ATM Backbone
Network
HN
HN
HN
AN
AN
ADM
HN/AN
METROPOLITAN
NETWORK
AN
ADM
ACCESS
NETWORK
ADM
SDH ring
CORE
NETWORK
Passive tree
Passive bus
AN = Access Node
HN = Hub Node
= Passive Splitter
or Add/Drop
= Customer Premises
/ Aggregation Point
Figure 23 - A reference metropolitan access network architecture.
The reference metropolitan access network covers an area of approximately 250 to
1250 km2, serving about 500 to 2000 high-end customers. Note that a head-end that
serves thousands of residential customers could be a “customer” of this network. The
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feeder ring circumference (metro network) is expected to be in the 40-150 km range,
with customers located no more than a few km from an access node. In densely
populated areas, it may be necessary to deploy a number of smaller rings. However the
optimal size of the reference network and the optimal size and the placement of the
feeder ring within that region will depend on factors such as location of existing fibre,
geographic constraints, existing serving areas, customer location etc.
5.2
Functional description of DWDM access node
A schematic view of a DWDM Access Node (AN) is shown in Figure 24. The AN
supports various functionalities with respect to wavelength by-pass and wavelength
add-drop between the metropolitan (feeder) and access (distribution) network
segments. The possible functional scenarios are:

Fibre by-pass - In this case a feeder fibre carrying transit traffic simply bypasses
the AN without interfacing any equipment in the node. No DWDM functions
occur in the AN.

Wavelength by-pass - This refers to the case where some wavelengths of a feeder
fibre bypass the AN while one or more other wavelengths of the same feeder
fibre are dropped in the AN. This implies the use of wavelength multiplexers /
demultiplexers and OADM.
ACCESS
NETWORK
DMX
DMX
MUX
METROPOLITAN
NETWORK
`Fixed W avelength
Connection
Wavelength
Converter
DMX
Distribution
Fibers
MUX
Configurable
Opt. Add/Drop
ATM
switch
or
IP router
etc.
Fiber Bypass
DMX
MUX
MUX
Wavelength
Bypass
Feeder Fiber
Ring
Figure 24 - Multiple feeder fibres enter an access node. Both fibre and
wavelength bypass, and configurable add/drop are supported. Some wavelengths
are terminated on an electronic switch, while others are routed transparently to
the distribution network.
For the added-dropped wavelengths the following possibilities exist:

Fixed wavelength interconnection - A fixed wavelength from a feeder fibre is
dropped in the AN and directly routed to a distribution fibre, and vice-versa.

Configurable wavelength interconnection - This scenario uses configurable
OADM to select the wavelength which is dropped from a feeder fibre in the
AN.The selected wavelength is directly routed to a distribution fibre.

Wavelength conversion - This solution overpasses the problem of using the same
wavelength in both network segments by making use of wavelength converters. A
selected wavelength is still dropped from a feeder fibre in the AN using an
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OADM, however it is also converted to an appropriate wavelength which is
vacant in the distribution fibre.
5.2.1
Use of OXC instead of OADM
All the above functions could also be realised using an Optical Cross-Connect (OXC)
in the place of OADMs. This choice would depend on techno-economic reasons based
on the number of wavelengths used.
5.2.1.1
Optical-electrical-optical conversion
Finally, in case the AN hosts a switch (ATM, IP router, etc) o-e-o conversion could
not be avoided for wavelengths carrying relevant services. In this case DWDM
convergence has the meaning that the e-o interfaces of the switch should be able to
transmit at selected wavelengths in order to match with the available wavelength slots
on either side of the AN.
5.2.2
Network level interfaces
The ring topology has gain substantial importance in networking architectures,
regardless of the type of signal (electrical/optical), service or even network
segmentation. This is basically due to its inherent characteristics related to the
topology robustness towards network failures which is based on simple protection
schemes and to the simplicity of node conception (only one direction for information
flow – only one input side and one output side)
In the following sections we will address the problem of feeder ring interconnection
with either ring or PON access architectures, , regarding traffic flow and protection
issues under the agreed scenario for optical DWDM networking migration towards the
access network segment.
5.2.2.1 Ring-Ring interconnection
Ring-ring interface can assume several forms, depending on the desired functionality
and perspective of evolution.
The interface nodes within AN are “principal” nodes in interfacing with the above ring
(feeder). All traffic incoming/outgoing the distribution ring passes these nodes. Two
strategies are possible with different implications in traffic routing and network
protection following a failure (Figure 25):
In single hubed rings all traffic incoming/outgoing the ring passes the same node;
there is therefore only one common node to both rings (feeder and distribution). Two
options regarding traffic routing can be taken: Pure hubbed traffic distribution or
mixed hubbed traffic distribution.
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WDM
Ring
Feeder



AN



 
OADM
 
 
OADM
 


Distribution
In/Out traffic
Inner Ring traffic
Mixed traffic Dist
Hubbed traffic Dist


OADM
a) Single Hubbed Ring
Feeder
AN




HUB 1






HUB 2


 
OADM
 
 
OADM
 
Distribution


OADM
WDM
Ring
Normal state
Protected state
Out traffic
In traffic


b) Double Hubbed Ring
Figure 25 - Protection in a) single hubbed rings; b) double hubbed rings
Several options exist for the traffic routing in double hubbed rings
The straight forward approach consists on defining one of the hubs as a primary hub
and the other one as backup, being traffic routed as in a single hubbed ring only by
one hub at a time.
Another option consists on independently routing of incoming/outcoming traffic using
one particular hub for each traffic direction. Again here, in a pure hubbed solution the
inner ring traffic should be routed via a hub node and this should be done using the
least occupied hub, which can be allocated dynamically or pre-established according
to the existing or estimated knowledge on traffic pattern distribution.
Also other mixed traffic distribution strategies can be used, being the hub nodes
almost exclusively used for ring interconnection.
5.2.2.1.1 Protection strategies
Protection strategies depend on the already established strategies for each ring
individually. It is assumed here that protection is carried out in the optical domain as
protection can also be implemented by the client, in the electrical domain such as in
the case of SDH clients. In the latter case the optical transport should be considered
unprotected, as mixed multilayer protection strategies involving optical and client
electrical protection are not currently being implemented and add an extra level of
complexity to distribution-feeder interfaces.
For the feeder, which is a multi fibre ring, optical sections can be protected with a n:m
fibre shared strategy, or then a n+n dedicated fibre strategy, on the exact option
depending on the actual traffic distribution and priority level in the feeder.
Protection against hub failures is only possible for double hubbed rings in which case
regardless of existing routing policy, in case of failure of one of the hubs the
interconnection becomes singled hubbed.
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5.2.3
Possible functional/physical implementation of ring interconnections
5.2.3.1
OXC/Configurable OADM functionality
The use of OXC and configurable OADMs in the interconnection nodes is the final
step towards the full optical networks. Both elements are identical in terms of
functionality, as they allow several or one input/output trunk ports (fibre DWDM
carrying signal ports), and one or more single channel input /output ports, being
individual channels routed by means of optical switches (switching matrix) to the
desired output port.
OADM is better suited to distribution-feeder ring interconnection. In this case the
dropped /inserted optical channels (whose number can range from one optical channel
to a few), are used for interconnection of rings, supporting traffic from/to feeder ring.
OXC for the ring interconnection is recommended in the case of high traffic flowing
in the ring interface and the requirement of high flexible assignment of channels and
high demand for direct wavelength routing.
From our study the use of configurable OADMs and OXCs are far from real
deployment because only fixed OADMs are available in the market with limited
functionality and monitoring capabilities, which are necessary to implement a
transparent network.
5.2.3.2
OADM/EADM(EXC)/OADM
This implementation can be useful in a transition phase as a fast straight forward
solution for interconnecting pre-existing rings with OADM nodes. Since this is not a
pure optical solution, the traffic can be further manipulated in the electrical domain
which may present some advantages like routing or aggregating traffic to legacy rings
(eg SDH rings) with a co-located node.
Fj Feeder
Fj Feeder
Fj Feeder
k
k
D
E
M
U
X
Fj Feeder
OADM
Control
M
U
X
Fi Dist.
O/E/O
k
EADM
k
O/E/O
Fi Dist.
OADM
l Add/Drop
Fi Dist.
a)
Fj Feeder
Fj Feeder
k
k
k
k
Fi Dist.
Filter
b)
Legend:
Control
Fi access
Fi Dist.
Coupler
c)
k - WL of optical Ch in WDM signal
l - WL of optical add/drop Ch
Fi - fiber in the distrbution ring
Fj -fiber in the feeder ring
i,j,k,l- order number; from 1 to I,J,K
I - total nº of fibers in the distribution ring
J- total nº of fibers in the feeder ring
K - total nº of optical chs in WDM signal
L - Max nº of add/drop optical chs
Figure 26 - Possible implementations of Access Nodes:
a) reconfigurable optical node; b) Reconfigurable OEO node: c) single channel optical
add/drop; OADM -optical add drop multiplexer; EADM - electrical add drop multiplexer;
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5.2.4
WDM technologies for access networks
Feeder/SDH access ring interconnection
Interconnection with a SDH distribution ring is a particular case because the single
wavelength allocated per fibre in a SDH ring cannot be directly mapped on a DWDM
channel, i.e. one cannot simply connect the optical output of a SDH card on a DWDM
MUX/DEMUX/OADM. This is because the wavelength on the SDH side does not
normally coincide with a wavelength of the WDM grid, and moreover the power,
linewidth and wavelength stability are not consistent with those of an optical channel
within a DWDM comb. Therefore adaptation of the optical SDH card signal to the
optical transport layer is needed.
In the case of low rate SDH distribution rings (STM-4) in which traffic aggregation is
desirable previous to feeder ring interconnection, an intermediate electrical stage EADM – might be needed and the multiplexed signal will then make use of an
interface to feeder as mentioned before.
5.2.5
Feeder/FSAN PON interconnection
As presented before the DWDM introduction on the PON allows service
discrimination, user discrimination and superposed PONS on different wavelengths
but on the same infrastructure. Here we will present some alternatives to root
wavelengths from the access to the metro network.
The interface of the PON to the feeder ring depends on the type of PON and the type
of the services being offered. A distribution PON for CATV for example would
require a very simple interface, while an interactive service providing PON, with
protection requirement for some users or services, would require a more complex
interface. In addition to the required functionality, optical amplification would be most
likely required.
5.2.5.1
Distributive service PON
For distributive service provisioning, e.g. CATV distribution, the functionality
required in the AN is the filtering of the correct wavelength carrying the service,
which must take place before the PON power splitter which is either co-located in the
AN or connected to it by a PON feeder cable, Figure 27 (a). The filter in the AN is
tuned (it can be a fixed filter also) to the wavelength (j) carrying the distributive
service and extracts it from the feeder, while leaving the remaining channels
undisturbed. This channel is then distributed by the PON infrastructure to all ONUs in
the PON.
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Feeder
Deliverable 10
Feeder
Feeder
AN
Fk feeder
1…N
 filter =j
AN
Fk feeder
1… N
AN
Fk feeder
1…N
BBFilter
BBFilter
1… j
1…j
AWG
1:M
1:M
M
1
onu onu
onu
j
M
1
onu
onu
onu
onu onu
onu
onu
onu
a)
j
1
b)
c)
Figure 27 - Distributive DWDM PONs: a) shared; b) c) discriminate
users/services
5.2.5.2
Interactive services PON
This case requires upstream traffic handling. The upstream traffic depends much on
the type of interactive service, but for most services it is considerably less than the
downstream. This is particularly true for BB interactive services such as VOD and BB
Internet access, as in the upstream one mostly has sparse, low bitrate, control
information. In the case of equal distribution of downstream /upstream traffic, e.g.
videoconferencing, or POTS, OADM functionality is required in the feeder ring
interconnection. This is also true for asymmetric traffic, however in this case some
aggregation of upstream traffic could be performed for traffic generated in the several
virtual PONs superposed on the same physical infrastructure. The figure bellow
illustrates three types of DWDM interactive PONs and their interconnection to feeder
ring, for interactive service provisioning. In Figure 27 (a) one wavelength j is used
for both the downstream and upstream traffic. In the AN the traffic is discriminated by
a directional coupler and the interface with the feeder is realised using an OADM.
Case (b) is similar but the interface with the feeder is now done via several
wavelength add/drop channels (one for each coloured PON) and therefore DWDM
DEMUX/MUX is required here. If the number of wavelength channels in the PON is
high, then an OXC could be used and the upstream/downstream DWDM signals could
be trunk inputs of the cross-connect. The same applies to the last case (c) of AWG
PON.
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Feeder
Feeder
Feeder
AN
Fkfeeder
1…N
OADM
j
add/drop =j
AN
Fkfeeder
1…N
OXC/
OADM
1…j
j
Directional
coupler
1…j
1…j
AWG
1:M
1:M
onu
j
M
onu
onu
onu
onu onu
onu
onu
onu
a)
j
1
M
1
1
OXC/
OADM
1…j
BB Directional
coupler
j
onu onu
AN
Fkfeeder
1…N
b)
c)
Figure 28 - Interactive DWDM PONs: a) shared; b) c) discriminate users/services
5.2.6
Wavelength routing policy
The independent wavelength assignment in the different domains of the optical
network (core, feeder and distribution) is an important functionality for the full optical
network concept implementation. This can be based on several mechanisms.
The more evident is the use of wavelength conversion functionality, allowing reusing
of the same wavelength allocation grids and components for all the segments of the
network. This, allied to allocation reconfigurability within rings, also permits direct
wavelength routing (without changing the wavelength) from the core to the access,
Figure 29, and vice-versa. However this is still not for near future implementation,
unless it is based on O/E/O transponders.
Another mechanism is the use of a pre-defined set of rules for wavelength allocation
in the different network segments, associated with network planning and
configuration. One of the possibilities is to use space division multiplexing in the
feeder ring, Figure 30, reserving different fibre rings according to traffic flowing
direction (to the access segment, to the core segment, inner ring traffic), therefore
allowing the use of similar wavelength sets for interfacing with upper and lower rings,
also allowing direct wavelength routing from/to core to/from access. In the same fibre
rings traffic should refer to the same network segment and be routed for maximum
wavelength reuse within the fibre ring. Distribution rings would use for inner ring
routing only a subset of available wavelengths.
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1
CORE
1
C/F interface
Inner ring WL transposition
to free WL channel
FEEDER
Access Node
1
DISTRIBUTION
1
Inner ring WL transposition
to free WL channel
Figure 29 - Direct wavelength routing from core to distribution, with the same
ITU grid, by means of transposition within incoming ring of existing wavelengths
in order to free the required wavelength and therefore transparently route all
way through destination, ie, without converting it.
This situation is shown in Figure 30 where core ring RcoreK directly interfaces the
feeder ring Rfeeder1 in the Core/Feeder interface node. Within feeder ring, Rfeeder 2 to I are
reserved to inner ring traffic while RfeederI interfaces with Rdist of distribution. Within
the distribution ring a part only of the wavelength channels is used.
Rcore 1
Rcore k
CORE
Rcore K
C/F interface
IC
Inner
Rfeeder 1
Rfeeder 2
Rfeeder i
Rfeeder I
FEEDER
Access Node
Inner
ID
DISTRIBUTION
Rdist.
Figure 30 - Wavelength allocation policy with by space division multiplexing,
with different fibre Rings within each network segment interfacing only
particular fibre rings on the neighbour segments. (Rcore, Rfeeder, Rdist)
If less performing technology is to be used in the different network segments, one
possibility is to use different grid spacing but anchored to the same reference, the grid
being narrower in the core and becoming wider towards the access (e.g. 50GHz,
100GHz and 200GHz), Figure 31. For the direction from the core towards the access it
would be possible to route sets of wavelengths grouped for the same network segment
but for different same level rings (the C/F filter extracts a set of wavelengths, with the
red ones directing to ring B and the violet ones to ring A - these are interleaved.) For
routing to a particular ring, wavelength allocation has to respect this level ring grid
spacing. In the inverse direction, at the interconnection of rings, high performance
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components namely in terms of crosstalk rejection should be required in order to
narrow the channel spectrum. This would also allow the upgrade of network domains
and reuse of components on domains of the same network segment. Direct wavelength
routing from core to access network is also possible in this scheme.
1
CORE
To Rings A&B
1
C/F interface
A
FEEDER
B
Access Node
1
DISTRIBUTION
1
Figure 31 - Direct wavelength routing, different ITU Grid
5.2.7
Aggregation of traffic in the distribution network for WDM applications
In this section we analyse the solutions based on DWDM technology for concentrating
traffic in the optical network. The analysis uses some of the arguments and solutions
presented in a detailed published study [15]. Then a comparison of these solutions is
made and it is shown that some solutions have very good performance in
concentrating traffic and reducing the number of switches in an all-optical network.
The access network is by default a section carrying different services and low bit rates
according to customers’ demand. To increase network efficiency, a metro section
appears with a main function of traffic aggregation. As optical switching is far from
the deployment phase, some kind of traffic aggregation is required in the access
network to justify DWDM application.
5.2.7.1
Non-transparent solution for traffic aggregation in the distribution network,
Example : Wavelength per customer
In this solution one wavelength is dedicated to each customer/ONU. The various
electronic services requested by a customer are multiplexed in an SDH multiplexer at
the customer premises, as shown in Figure 32. For example, services from customer 1
are multiplexed on a single wavelength and eventually delivered to a port on a SDH
switch within the access node. The same type of traffic (e.g. ATM traffic from
customer 1 and 2) from different customers is multiplexed together in this switch and
directed to the appropriate switch. Statistical multiplexing of traffic typically occurs in
the switch or router and the traffic is sent out to the feeder network to the
corresponding hub node. As an example, FR traffic is delivered to the hub node that
interfaces with the FR backbone network. The disadvantage of this architecture is that
it requires multiplexing equipment at customer premises and several network switches
in each access node.
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Figure 32 - Wavelength per customer architecture. From [15]
5.2.7.2
Transparent solution for aggregating traffic in the distribution network,
Example: Wavelength per service
Figure 33 - Wavelength per service architecture. From [15]
The second strategy to aggregate traffic involves allocating resources in “simple”
units, such as full wavelengths. For example, an STM-1 ATM connection and an
STM-1 IP connection may be each allocated a full wavelength rather than being more
efficiently multiplexed. Furthermore a wavelength is dedicated per electronic service
type requested by the customer. The solution is shown in Figure 33, where customer 1
would require two wavelengths while customer 2 three wavelengths. The types of
services per customer may be changed by tying the corresponding wavelengths to a
switch in the access node. The great advantage of this architecture is that it eliminates
the need for multiplexing equipment at the customer premises. However, this solution
increases the complexity on the ONU level and makes inefficient use of wavelengths
for relatively small data rates such as STM-1 or below. In such cases it might be
useful to aggregate the traffic from multiple customers before feeding the traffic to the
distribution network.
5.3
Use of DWDM for the reduction and concentration of network
switches
The aim of the all-optical network concept is the possibility of routing a wavelength
transparently from the access network to the core network. The architecture for
“wavelength per service” in such a network will help to reduce further the number of
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WDM technologies for access networks
network service switches concentrating the ANs, which is an objective for all
operators. This can be illustrated as follows:
One option is to place an ATM, IP and FR switch in each AN as shown in Figure 34.
A better option however is to have the wavelengths carrying different types of traffic
not terminated all in the same AN, but have for example IP wavelengths transparently
routed to the next AN where an IP router is located. Similarly, wavelengths carrying
FR traffic are routed to the AN where an FR switch is placed. Thus the ATM switch in
the first node, the IP router in the second node and the FR switch in the third node can
be shared by the customers of all three nodes, Figure 35. This configuration results in
the significant reduction of network switch equipment and correspondingly leads to
lower maintenance and upgrade costs as well as less power and space needs. However,
as mentioned earlier, more wavelengths are needed because the traffic is carried in the
feeder network without first being multiplexed in a switch.
Figure 34 - Several switches are placed in each access node. Traffic is multiplexed
as soon as it enters the access node, goes through O-E-O conversion and is
further multiplexed/converted at the hub node
Figure 35 - Switches are placed only in the hub nodes. Traffic is routed
transparently (specified by double lines) to the feeder ring through the access
nodes. Much fewer switches are needed at the expense of more wavelengths.
However the same number of wavelengths exits the hub node switches in either
architecture.
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5.4
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Network migration concepts towards all optical networks
Although the immediate advantages of introducing DWDM networks is evident, care
must be taken in the course of planning to ensure a smooth transition and reuse of
existing infrastructure and technology, and a sufficient degree of flexibility at the
same time that allow future upgrades.
Currently existing networks in the core, metropolitan and regional network segments
are based on SDH technology, either in ring-ring or in meshed-ring architectures,
supported by optical fibre infrastructure.
As traffic demand growths resulting on increased capacity demand, upgrade via new
SDH systems of the same capacity will soon lead to fibre exhaustion. Going up in
capacity of SDH systems implies substituting all the node equipment to the new
hierarchical SDH level (STM-4 toSTM-16 and to STM-64).
One possibility to immediate upgrade an existing network and also free occupied
fibres or spare existing ones, is to directly map existing SDH rings on DWDM by
colouring the rings, thus creating space for immediate upgrade of capacity on the same
infrastructure.
5.4.1
Migration Phases
The following phases are proposed for a smooth network migration, assuming an
existing hierarchical network which may be of a ring-ring or meshed-ring architecture:

A two level ring-ring SDH topology is assumed.

Introduction of transponders and OADMs in both network levels and
straightforward mapping of existing SDH rings on wavelength rings.

Introduction of OXC on the top level ring.

Capacity upgrade by stacking of rings.

New ring deployment for network expansion.

Installation on new generation DWDM equipment in distinct fibres while there is
fibre available.

Grouping of same generation equipment in same physical infrastructure rings,
reusing older generation equipment in areas with less capacity demand.
Single-
W
P

ADM
T T
W
P
ADM
1
ADM
OADM
OADM
OADM
1
To SDH
WDM
 1-  16
1
Figure 36 - Direct mapping of MS-Spring into DWDM rings
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WDM technologies for access networks
O
O
XC
O
XC
XC
Lower level rings
OADM
OADM
OADM
Lower level ring traffic
Figure 37 - DWDM architecture -introduction of OADM and OXC in both levels
of existing hierarchy.
5.4.1.1
Advantages
The main advantages of this approach are:
5.4.1.2

Use and optimisation of already deployed optical fibre infrastructure by
introducing DWDM.

Use of an architectural concept quite similar to SDH, in architecture,
requirements and functionality, therefore being familiar to the network operator
and allowing better assimilation.

Architectural inherent protection facilities of utmost importance in view of the
high level of traffic aggregation carried in one single fibre.

Allows gradual, follow-up technological upgrade of existing network as well as
future network expansion and capacity upgrade.

Commercial availability of the required optical functionality for the introduction
phase (OADM and transponders).

A reasonable number of used wavelengths, e.g.16, both in technical and
economical terms. This is a moderately low value as DWDM systems are
commercially available with a much higher wavelength density. In spite of this,
not all the wavelengths will be initially used. Spare resources will remain
available to account for future upgrades and optical protection allowance.
Disadvantages
The following disadvantages can be pointed out:

Either coloured optical interfaces for SDH equipment or transponders will be
necessary for the implementation of DWDM-SDH rings. Both solutions are
expensive for the initial phase. The first option seems to be more suitable for new
equipment installation, however unless tuneable lasers are used a great diversity
of spare parts is necessary and the interface must stick to a fixed wavelength grid
which may turn the installation of future generation WDM systems more
difficult. The second option is in all phases expensive but is quite flexible and
assures independence of terminal equipment.

Necessity of DWDM optical amplifiers, wideband to cover the used DWDM
signal and with flat gain within the covered band.

Necessity of optimisation of first approach by straightforward mapping of SDH
rings on DWDM rings in order to minimise the number of used OADMs. This
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WDM technologies for access networks
Deliverable 10
can be achieved by a different effective number of nodes for each of the several
overlapping co-located rings.

As a result of using SDH, MS-SPRing protection, only half of the total capacity is
used, the other half being reserved for protection. This feature will initially be an
advantage but as capacity needs increase and the number of available planned
wavelengths diminishes, it will soon turn out to be a disadvantage. New multilayer protection schemes, including also protection in the optical layer, must be
considered. In particular the diversity of optical routes available due both to the
ring double node interconnection in the lower hierarchical level and to the two
level hierarchy, allows the implementation of other survivability mechanisms,
similar to pre-planned route restoration.

Fast evolving DWDM components and systems market, adding up to reduced
standardisation, results in foreseeable incompatibility between the purchased
systems of today and their future upgrades, demanding for some caution on
network planning trade-offs and initial investment. The 16 wavelength DWDM
system may be more suited to the medium term future needs and far cheaper than
higher density systems, which may became completely obsolete within the
lifetime of the first option.

OXC systems are not yet commercially available.

Non existence of efficient optical network supervision and management, allowing
either integration or interworking with existing supervision and management
systems.
5.5
Time frame for the DWDM application
5.5.1
DWDM network element maturity
The deployment of DWDM in the metropolitan and access segments, and especially
their convergence towards a wavelength transparent network is critically dependent on
the maturity of a number of key network elements. Based on the status of technology,
a possible time frame for DWDM solutions could be derived. However, this timeframe
must be considered as indicative since no precise estimation can be made on when a
novel network element will really become available.
Today, only solutions based on fixed OADMs are feasible. This first generation of
OADMs can be used to realise the fixed wavelength interconnection scenario.
All the other DWDM convergence scenarios require that specific DWDM network
elements become available.
Configurable OADM are expected to emerge first, enabling the scenario of
configurable wavelength routing. A possible timeframe for this second generation of
OADM is end of 2001. Tuneable transponders allowing O-E-O wavelength
conversion are also expected in the same timeframe, relaxing the requirements for a
common wavelength allocation scheme between the feeder and distribution network.
First OXCs may be expected in year 2001, however based on transponders (O-E-O
conversion).
Finally, full interoperability using wavelength converters, optical switches or
wavelength routers looks more into future, not foreseen within the next 2 years.
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5.5.2
WDM technologies for access networks
Network Management Systems maturity.
Network management systems have been a limiting factor to the rapid growth of any
lightwave system. Because hardware advances tend to lead software development by 1
to 2 years, new lightwave systems usually become available before complete NMS
packages are written. Today, network management systems for DWDM systems tend
to be element managers, monitoring the performance of the network elements rather
than the wavelength channels themselves.
Nevertheless, convergence of DWDM in metro and access networks would require an
optical layer management system capable of providing functions such as fault
management, including OTDR, configuration management, performance monitoring,
and restoration. These functions are today supported by the SDH layer, therefore it
seems inevitable for OAM to migrate from the SDH layer to the optical layer. For
example, today performance monitoring systems consist of detection of optical power
outside a specific range, which triggers an alarm or an automatic protection switching
module. There is no way at this time for performance monitoring systems to actually
estimate the quality of the client signals. However, to implement transparent all optical
DWDM connections, there is a need to monitor the performance of lightwave signals
without knowledge of what the signal is carrying. In other words, performance
monitoring is required at the analogue optical layer in addition to the digital signal
layer.
Significant deployment of DWDM in the metropolitan access network should not be
expected before suitable optical layer monitor and management systems are
developed, which provide operators with full OAM capabilities.
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6
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Conclusions
In our study we considered the introduction of DWDM technology into the access
networks, to fulfil the increasing bandwidth demand. We focused on two different
application areas, where we expect that DWDM will first emerge:

Connecting large business customers, and

DWDM upgrade of PONs (because we foresee a bandwidth demand that can only
be met by using DWDM)
Furthermore, we investigated the aspect of network convergence, because we believe
that the introduction of DWDM in the access will bring us closer to realise the full
potential of the all optical networks.
When introducing DWDM in the access incumbent operators should consider existing
infrastructures and devise a migration path. Our study indicates that European
operators adopted similar fibre solutions to serve BCs, and also to connect residential
customers. Nearly all of them make use of optical access networks based on a physical
ring topology and SDH transmission. Business customers are connected through point
to point or logical ring connections. Some operators are using PONs to serve
residential users and SMEs. This creates a rather common “present situation” or
starting point from which future networks, such as DWDM access networks will
evolve.
In spite of the commonly claimed “booming” increase of BC capacity demand, most
BC connections still appear to be in the STM-1 range, or below. Since these demands
can be cost effectively served by proven legacy technology a considerable further
increase in demand is needed to boost DWDM deployment in the access network.
For the time being DWDM deployment in the access is solely motivated by bandwidth
upgrade both for the customer drop and for links higher up in the network. Single
provider NE systems as well as homogenous transmission formats are being
employed.
Wavelength per service type application of WDM in the access networks emerges
with the competition between service providers. Solutions with fibre sharing schemes
in either ring topologies or tree branch PON structures are also likely to emerge. In the
first case, passive or tuneable OADMs will be installed all the way along the fibre
path, while in the second topology, a single location may be retained for the passive
DWDM. Presently, however, only fixed wavelength implementations are used with
passive ADM.
The responses to our RFI and the assessment of the state of the art in DWDM systems
show, that DWDM isn’t yet mature nor cost competitive for general introduction in
the access, not even for large business users. Nevertheless, we see two trends that are
major drivers of the deployment of DWDM in the access:

The necessity to rapidly link all LEXs with a broadband service access node, such
as a DSLAM, and provide further bandwidth upgrading capacity.

The emerging demand for VPNs linking all the premises of a business,
(Nowadays provided through a separate fibre or through a dedicated PVC in an
ATM network). DWDM technology allows the creation of dedicated wavelength
channel VPNs giving complete independence from user to user.
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WDM technologies for access networks
At present no ‘telecom quality grade’ DWDM access systems are available.
Nevertheless, DWDM systems developed for LANs may fit in for limited application
fields. Under these conditions, transmission oriented DWDM systems can be suitable
for access links, but for temporary use, only, since there are major issues to be
resolved before a large scale roll-out can take place. These are the following:

Interworking of DWDM equipment from different manufacturers through the
optical interface. Although by now most manufacturers are using wavelength
grids that are compliant to the ITU standard, this isn’t sufficient to ensure full
compatibility.

Full remote supervision of user end NE: configuration, service provisioning, fault
reporting, performance monitoring.
The following conclusions can be drawn from the responses received to the RFI:
Though DWDM offers huge transmission capacities and supports a wide variety of
input signal formats (conditions that are essential for the migration of DWDM to the
access part of the network), it suffers from limited configurability and lacks
networking functionality:

Available transponders emit at pre-selected wavelengths. No dynamic wavelength
allocation is possible, yet.

OADMs are fixed (not dynamically configurable) devices and as such they can
not be used for the dynamic interconnection of DWDM branches and rings.

Optical cross-connects (that would enable the implementation of all-optical fully
configurable networks), are still in the stage of development and their deployment
is believed to be a few years away.

Available DWDM systems at most support point to point connections
multiplexing several channels between a LEX and an end user on a single path or
on a ring (thus providing a 1+1 redundancy with path diversity which is required
for large business clients).
Ultimately the migration of DWDM to the access network is going to be governed by
cost. Cost is a much more complex issue in the access networks than in the core, and
renders necessary the techno-economic comparison of DWDM and non-DWDM
solutions (such as xDSL and SDH/FITL).
The responses to the RFI we have circulated gave very little information regarding
hardware/system costs. One aspect, however, that should be taken into account is that
almost all vendors offer “metro-oriented” WDM solutions that should allow the
development of WDM access network at a lower cost.
Our investment study revealed that today the deployment of DWDM technology in the
access cannot be justified purely on a cost basis.
Another field of application of DWDM in the access that holds a great potential is
PON upgrade. The FSAN Initiative OAN work group considers the upgrade of APON
using DWDM. A wavelength allocation plan is already under discussion.
We also see a high potential in using DWDM technology to upgrade existing PON
systems. Flexibility can be increased and very high bandwidth can be provided by the
re-use of the existing infrastructure and adopting DWDM.
A number of technical and architectural concepts exist regarding how to do this. These
are described in detail, analysed and compared, highlighting the advantages and
disadvantages of each of them.
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We consider the following DWDM upgrades feasible and meaningful for FSAN PON
systems:

downstream DWDM upgrade as overlay without affecting the operation of the
PSPON – for single point-to-point links or as a broadcast channel for the entire
PON.

Upgrading the downstream capacity, only, and maintaining the upstream
transmission of the existing PSPON.

Bi-directional DWDM overlay upgrading without affecting the operation of the
PSPON.
We foresee that the first, most immediate demand will be to upgrade a few business
customers' ONUs with dedicated links. We have demonstrated the feasibility of the
solution to this scenario.
The large-scale deployment of DWDM in the access is hindered by the lack of key
components, such as wavelength routers (AWGs) and transmitters (for instance
LASERs, Super LEDs), that are suitable for use in the access network. Application in
the access network is especially demanding, because a controlled environment can not
be provided, or only at a very high cost. Therefore components need to be thermally
very stable. (Providing a controlled environment for components in the core network
is not that critical.) Furthermore, the cost of the components is also critical, because
unlike in the core networks, they are not shared by that many customers, and also a
much larger number of them is required.
Finally, we provide guidelines for network planners. However, each particular
situation needs to be analysed in detail, and the decision should be made considering
the trade off between cost and performance.
As outlook for the following generation of DWDM access networks will have beyond
this characteristics like reconfigurability and dynamic allocation of wavelengths
depending on capacity demands per area and time period.
A special attention was paid to the interesting aspect of the convergence of DWDM
access and core networks. We have identified the access node that interconnects the
metropolitan and access network domains as the most critical element of DWDM
convergence. This is due to the fact that it interfaces various technologies and
architectures, is cost sensitive, must be flexible to accommodate new services /
demands and has a large impact on OAM.
We looked at the ways wavelength routing can be realised in the access node. A
number of options exist employing a variety of DWDM functionalities, ranging from
fixed wavelength add/dropping to configurable optical cross-connection and optical
conversion. These solutions exhibit vastly different flexibility features depending on
the actual DWDM network elements employed, special attention was taken to the
different degrees of wavelength protection and restoration.
We have found that the immaturity of the technology poses a major barrier towards
convergence of DWDM networks. Simple forms of transparent wavelength routing
can be readily implemented using available fixed OADMs, nevertheless at the expense
of limited flexibility. More flexible solutions allowing the selection of wavelengths to
be added/dropped and effective use of wavelength grids among the different network
segments require more sophisticated network elements such as configurable OADM,
OXC and wavelength converters, which are not available for the time being.
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WDM technologies for access networks
DWDM convergence would also require the evolution of today’s DWDM products
towards systems more focused on access/metro applications. They should justify their
cost effectiveness against conventional solutions based on SDH/ATM and should also
provide full OAM capabilities. Management systems should evolve from the presently
available simple network element monitoring to wavelength channel monitoring, and
realise in the optical layer all the management functionalities provided today by the
SDH layer.
From optical networking point of view, DWDM convergence is envisaged to have a
large impact on network structure and planning because it:

Introduces new networking concepts such as “wavelength per customer” and
“wavelength per service”.

Affects the planning of networks by introducing wavelength routing in the access
nodes, allowing the concentration and reduction of electronic switches.

Introduces new services and applications based on wavelength provisioning to
customers.
The major fields of application of DWDM convergence are:

Wavelength VPNs capable of supporting customers’ native signal formats,

Wavelength leasing,

Wavelength on demand and

Storage networks.
A prerequisite for most of these applications is a large capacity demand exceeding the
STM-4/16 level per customer or aggregation point. Such high capacity is still rarely
demanded nowadays, however the liberalisation of the telecommunication market
might help creating a significant amount of such demands. First users of these DWDM
services are expected to be very large business customers and small operators.
In summary, we consider DWDM convergence as a mid-future issue. Convergence
should be expected when the technology becomes sufficiently mature and capacity
demands justify the use of DWDM. We expect that DWDM will migrate gradually
from the core to the metro and then to the access network. However, when the barriers
will be overcome DWDM convergence will constitute a real paradigm shift in optical
networking, changing the perception we presently have of optical networks.
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7
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References
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