Cost-effective metropolitan area fiber network at the TASK Academic
Computer Network in Gdansk
Submission to TERENA Networking Conference 2004
Mscislaw Nakonieczny <mnak@task.gda.pl>
Slawomir Polomski <sp@task.gda.pl>
TASK Academic Computer Center
Gdansk University of Technology
ul. Narutowicza 11/12, 80-952
Gdansk, Poland
Keywords
fiber metropolitan area network, CWDM, passive optics, coupler, circulator, bidirectional transmission
Abstract
The constant increase of network traffic combined with the demand for versatile
services forces rethinking of network architecture. Instead of expensive
exchanging or laying down new cables, methods of putting more signals on a
single fiber are worth considering. The CWDM technology deployed in the TASK
Academic Computer Network in Gdansk, using simple 1x2 optical couplers,
OADM and color GBIC tranceivers, has proved to be a cost-effective method of
transporting more data and separating services in a metropolitan backbone
network.
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1. Introduction
The substantial increase in the number of Internet users and development of new bandwidthdemanding network services have stimulated a reconstruction of the TASK backbone network
and a change of the applied technology. In the 10 years of the TASK network's operation
various network technologies have been used, including leased circuits, FDDI and Ethernet,
followed by ATM and finally replaced with Gigabit Ethernet. Due to the need for greater
bandwidth and continual network development, it is necessary to apply the backbone network
both in 10GE / n x 1GE and ATM technologies.
Fiber links turned out to be the only stable element of the puzzle. Due to its unprecedented
capacity, optical fiber is an ideal transmission medium for high volume data. These days,
plugging into PIONIER, the national optical backbone network of n*10Gb/s capacity, and
concurrent participation in establishing a local information community necessitates a different
approach to the creation of a metropolitan area network.
Apart from delivering bandwidth, the ability to create virtual networks dedicated to a
restricted group of users is becoming a key factor. An example of such networks are dedicated
and secure networks for supercomputer centers, data filing and distributed calculations. The
needs of universities and other users require strictly separated services of Transparent LAN
type such as: network for university staff and laboratory assistants, for student houses, for
dean offices and financial and accounting services.
Not all network technologies are capable of providing versatile services. The first advanced
network technology, FDDI, did not allow virtual networks. Only the introduction of the ATM
technology made it possible to offer many classes and kinds of services in a single network.
Bandwidth requirements of some user groups are significantly beyond the possibilities of the
ATM backbone currently used in the TASK network. The GE/10GE technology makes it
possible to build networks of great capacity, but does not guarantee a full separation of data of
different users, which is an obstacle in an attempt to simultaneously offer high degrees of
security and reliability. This technology does not allow networks transparent for different
protocols.
Possibilities of deploying independent networks based on separated fiber links are limited
especially by an old fiber cable base with few links. Furthermore, there is a great demand for
transparent transmission channels from the side of local self-government and state
administration. It results in the necessity to increase the number of fibers available.
The majority of cables installed between 1993-97 contains from 6 to 16 fibers. The process of
cable exchanging, though necessary, is time and money consuming. It also requires
reorganization of the whole additional infrastructure: rooms, cable ducts, cabinets and optic
fiber distribution boxes. The modernization is planned between 2004 and 2007. Within this
transitory period it is the simplest and cheapest to use passive optics in the access layer and
xWDM technology in the backbone, what enables to multiply network capacity in physical
layer by four times.
There are two possible directions in which the TASK network can be developed in order to be
capable of offering many kinds of services:
Building a new backbone in layer 3 using the IP/MPLS technology to offer various services,
or
Building a new backbone in layer 1/2 using the CWDM technology.
Considering rapid developments in optical network building components, a decision has been
made in favour of a test CWDM installation. Experience from this project will be used in
deciding the future development of the network.
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2. Optical building components in the TASK’s access layer
In the course of modernization, within the access layer, we change bi-directional transmission
to mono-directional one using wideband couplers 50/50. In case increased error rate is
indicated on access link, couplers are replaced with circulators. Such movement is entirely in
compliance with standard to be accepted this year by IEEE: 802.3ah Ethernet in the First
Mile, where bi-directional transmission using a single optical fiber is the basic way to have
access to an end user.
Optical couplers are used for branching or combining of optical signals. They are used in
optical fiber networks to serve as a passive distribution and collection points for optical data
transmission (telephone, cable TV etc.). Fusion couplers are characterized by the following
properties:
 Low insertion loss
 High return loss
 Wavelength-selective or broadband properties
 High thermal and mechanical stability
 For any coupling ratio (1 % ... 50 %)
 Manufactured to customer specifications
Fusion couplers are manufactured by the so-called FBT method (fused biconical taper), in
which coupling zones are created by fusion and simultaneous pulling and narrowing
(tapering) of optical fibers (fig. 1).
Fig. 1 Schematic diagram of a fused biconical taper coupler
By manipulating the tapering process and by special pre-treatment of the optical fibres to be
fused, couplers with differing transmission and coupling properties can be made.
Four different types of fusion couplers can be identified:
 Standard couplers (SSC = Standard Singlemode Couplers) for a wavelength with minimal
 Single window couplers (WFC = Wavelength Flattened Couplers) for a wavelength range,
e.g.: 1310 ±40 nm.
 Dual window couplers (WIC = Wavelength Independent Couplers) for two wavelength
ranges, e.g.: 1310 ±40 and 1550 ±40 nm.
 Wavelength multiplexers (WDM = Wavelength Division Multiplexers) for separating two
wavelengths, e.g.: 1310 and 1550 nm.
3
Fig. 2 Coupling ratio as a function of pull length for a fused biconical taper coupler made of
identical fibres [1]
As can be seen in fig. 2, the coupling ratio depends both on the tapering length and on the
working wavelength. If the tapering process is stopped at a specific point, a specific coupling
ratio is achieved for one wavelength. Point A marks a standard coupler with a coupling ratio
of 50 % at 1550 nm.
If this coupler is operated with a wavelength of 1310 nm, the coupling ratio is approx. 20 %.
Point B marks a standard coupler with a symmetrical coupling ratio at 1310 nm.
Point C marks a single window coupler for 1550 nm. At this turning point, the coupler is
highly insensitive to wavelength changes. However, this characteristic is not required to occur
at 100 % coupling, so one of the two fibers is pre-treated by etching or pre-tapering. This
enables the turning point to be reduced to 50 % (fig. 2, point C). Point D marks a dual
window coupler. At this intersection, the coupling ratio is the same for two wavelengths, but
asymmetrical (approx. 10 %).
Here, too, an intersection with a symmetrical coupling ratio is achieved by fusing two fibers
that are pre-treated differently (fig. 2, point D).
Point E marks a specific coupler, the so-called wavelength multiplexer. At this point, 100 %
of the signal at 1550 nm and 0 % of the signal at 1310 nm is coupled. This means that a
WDM, like a filter, can separate two wavelengths so that each output carries only one
wavelength.
Fiber optic circulator
A fiber optic circulator is a nonreciprocating passive device which transports an optical signal
from one port to the next port, only in one direction (i.e. 1 to 2, or 2 to 3). They may be used
to separate forward and backward propagating signals with typically 50dB of isolation and a
crosstalk figure of better than 60dB.
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Fig. 3 Idea of circulator
It provides a means by which the telecommunications rate may be immediately doubled on
existing optical fiber carrier infrastructure. Separation of the signals is based only upon
propagation direction; no additional losses are imposed on transmitted signals, as in the case
of conventional directional couplers.
Short analysis of bi-directional line using coupler 50/50.
In the article “Cross-talk in bi-directional, single wavelength, single fiber Gigabit Ethernet
links” Vipul Bhatt [3] showed that thoeretically there may be bi-directional transmission with
error rate ( BER) on the level of 10-12 within a single optical fiber at distances up to 10 km.
This type of link is basically limited by optical return loss not lower than 12 dB.
Fiber
Transmitter
Mechanical
splices and
connectors
Pi
Pr
ORL = - 10 log
Pr
Pi
Fig. 4 Definition of optical return loss (ORL)
Optical return loss, or system reflection, is the ratio between the total reflected power (the
sum of all reflections) and the incident power. ORL is a positive dB value—the equation has a
negative sign to change the logarithm of the Pr/Pi ratio into a positive number.
5
Tx1
single fiber
Coupler
Tx2
Coupler
Rx1
Rx2
TP3
TP2
Fig. 5 Block diagram of a single-fiber link
ODN – Optical Distribution Network
Intenal reflection
(outside spec scope)
Reflection due ORL
Transceiver
Near
Transceiver
Far
fiber
(High Power Pmax)
(Low Power Pmin )
Reflection due to ODN
(far end terminated)
PRx-signal=Pmin xChannel Loss
2
PRx-reflect =Pmax x Channel Loss
ORL + Pmax x ODN
x
range L (km)
Channel Loss:
Feber attenuation (0.5dB/kmx10km)
Connector (0.5dBx4)
Link Penalty:
MPN and others – 3dB allotment
Fig. 6 Link reflections – transmit issues
Too high power of transmitters may also be problematic. It causes power increase of received
reflected signal. This is confirmed by our practical experiments, in which error rate decrease
has been achieved by adjusting the link with variable optical attenuators.
In case of lines exceeding 10 km, circulators having much better parameters, can be used.
To sum up theoretical considerations Vipul Bhatt’s [3] conclusion could be quoted:
“It appears feasible to develop a low cost single-fiber EFM transceiver that can support a link
length of 10 kilometers. This can be done with a single wavelength of operation. Single
wavelength links overcome some of the disadvantages of using WDM links. Performance
degradation due to crosstalk resulting from reflections at the transceiver cable interface can be
overcome by making a simple threshold adjustment at the receiver, and by paying a small
power penalty”
6
Various methods of TASK’s access link realization
POP
CWDM
One fiber
1310
bidirectional
End User
coupler
coupler
Fig. 7 Option 1 using standard couplers
POP
CWDM
One fiber
WDM
coupler
User 1
1310/1550
1550
bidirectional
1310
Wideband Coupler
Wideband Coupler
WDM
coupler
User 2
Fig. 8 Option 2 using standard and WDM couplers and two bi-directional transmissions
within a single fiber.
ADM
ADM
ADM
CWDM MUX
Fig. 9 Option 3 using WDM multiplexers and OADM elements
Cost analysis of access link realization
Access line 1
260 Euro per user
Access line 2
1760 Euro per user
Access line 3
4700 Euro per user
The major expense in case of option 2 are GBIC 1550 nm tranceivers, in case of option 3
color GBIC and CWDM multiplexer. In case access technology becomes widespread, the
costs of option 2 per user, should approach the costs of option 1.
Widespreading of such solutions will depend on standardization of EPON technology
(ethernet passive optical networks) IEEE 802.3 ah EFM Task Force
http://grouper.ieee.org/groups/802/3/efm/
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3. Optical building components in the TASK’s backbone
Networking equipment usually uses multi-mode or single-mode fiber optic interfaces
transmitting data at 850nm, 1310nm or 1550nm. Each input light stream has to be converted
by a transponder to a particular wavelength corresponding to a CWDM or DWDM channel.
Eventually, we can use color GBIC's operating directly on one of the ITU wavelength
channels.
The input data streams operating at a unique color are combined by a multiplexer into a single
fiber trunk. A demultiplexer is executing the opposite function, taking a fiber trunk and
separating it into individual fibers, each transporting a different wavelength signal.
An Optical Add/Drop Multiplexer (OADM) is able to take a single wavelength from a trunk,
pull the signal out, and allow a new signal at the same wavelength to be re-inserted into the
trunk. All the other wavelengths pass through the OADM with only a small loss of power
(usually less than 1dB).
Components used in CWDM relaxed-tolerance systems can utilize totally passive optics.
They do not need power or electronics and are independent of the transmission protocol. The
key performance metrics for passive components include insertion loss, channel isolation and
bandwidth, as well as cost and size.
CWDM
Wavelength Division Multiplexing (WDM) is a method to transmit several optical signals by
the same fiber at different wavelengths (colors of light). Each WDM channel is completely
independent of others, both with regard to bit rates and protocols, so that running a mixture of
network technologies on the same fiber is possible.
Two variants of WDM are specified, one called Dense WDM (DWDM) and the other Coarse WDM (CWDM). Frequency grids for DWDM and CWDM systems are specified by
ITU G.694.1 and G.694.2 recommendations accordingly.
A typical DWDM channel spacing is 200GHz, which corresponds to 0.4nm. There are
systems available of 32, 64 and more wavelengths. DWDM optical systems require an
expensive thermoelectric cooler to stabilize wavelength emission and absorb the power
dissipated by the laser.
A CWDM grid is made up of 18 wavelengths defined within the range 1270nm to 1610nm,
which then gives a coarse channel spacing of 20nm between channels. CWDM uses less
precise non-stabilized lasers in combination with broadband filters, as well as lower-cost
passive components. CWDM systems are cost-effective solutions for metropolitan spans of up
to 50km. They can even be installed on previously laid and widely deployed single-mode
optical fibers, although the "water-peak” attenuation has an impact on the reach of the
systems and on the number of available optical channels. Although the basic framework of
CWDM systems has been standardized, system manufacturers are free to pick which of the
wavelengths they use, which results in incompatibilities. Testing CWDM systems is
somewhat difficult, since traditional testing equipment cannot test the wide span of
wavelengths.
8
Filters and multiplexers
To Network
Rx
MUX/DEMUX-4
Tx
Rx Tx
Rx Tx
Rx Tx
Rx Tx
1490
1530
1570
1610
To CWDM GBIC's
Fig. 10 MUX/DEMUX Diagram
All four wavelengths are combined and transmitted to Network side port. Each wavelength is
received on its respective equipment port
OADM
To Network
Rx
EAST
CWDM OADM-1
To Network
WEST
Tx
Tx
Rx
Rx
Tx
Rx
To CWDM SFP
Tx
To CWDM SFP
Equipment Side
Fig. 11 Single lambda OADM
Pilot CWDM network at TASK
A number of WDM connections of differing topologies and network standards have been
tested and applied. A point-to-point connection was made with FOCI optical couplers,
transmitting two-ways two optical signals of 1310nm and 1550nm through a pair of optical
fibers. An ATM OC-12 connection was made in the 1310 nm band, while a Gigabit Ethernet
connection was made in the 1550nm band.
Then a double CWDM optical ring with connection protection was constructed. The ring's
nodes made use of Finisar color GBIC interfaces and FOCI OADM systems of the M-AC
type, operating at the same wavelength. Cisco Systems' Gigabit Ethernet C3508 switches at
each node were connected with the central switch with two links.
9
The tests have been a practical application of the CWDM technology to the MAN network
connecting the cities of Gdansk and Gdynia over a distance of 30 kilometers.
1510nm GBIC
1510nm GBIC
1510nm OADM
EAST
WEST
Univ. of Sopot
1470nm
GBIC
1550nm
WEST
EAST
1550nm OADM
1470nm OADM
Maritime
Academy
Univ. of Gdansk
1470nm
GBIC
GBIC
WEST
EAST
1550nm
GBIC
ACC TASK
1470
WEST
EAST
MUX/DEMUX-4
MUX/DEMUX-4
1510
1550
1590
1470
1510
1550
1590
GBIC
SPARE
SPARE
Fig. 12 Our pilot CWDM network
4. Conclusion
Half-year exploitation of TASK’s network using new access technologies proved that it is
reasonable to increase by four times the number of available transmission channels by using
passive optics and simple CWDM systems. The cost of necessary investments is low enough
compared with the cost of cable exchange or installation of CWDM advanced systems.
Application of such systems is only justified in backbone segments with few fibers. The used
fibers must guarantee low attenuation and signal reflectancy. The key technical problem in
implementation of new technology is to keep optical return loss of optic fiber lines on
properly low level in order to minimize the error rate. PC optic fiber connectors should be
replaced with APC ones.
Due to planned acceptation of EFM standard by IEEE in 2004, and soon implementation of
this technology by producers, it can be expected that solutions suggested by us will have been
replaced by EPON technologies in a few years.
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References
[1] V. J. Tekippe, “Passive Fiber Optic Components Made by the Fused Biconical Taper
Process“, (Invited Paper), Fifth National Symposium on Optical Fibers and Their Application,
Warsaw, SPIE, 1085, (1989). Reprinted in Fiber and Integrated Optics, 9, 97-123, (1990)
[2] IEEE 802.3ah Ethernet in the First Mile Task Force
http://www.ieee802.org/3/efm/index.html
[3] Vipul Bhatt, “Cross-talk in bi-directional, single wavelength, single fiber Gigabit Ethernet
links”, Finisar Corporation,
http://grouper.ieee.org/groups/802/3/efm/public/jul01/presentations/bhatt_1_0701.pdf
[4] Meir Bartur, Tom Murphy,”Single Fiber, Single wavelength, GbE link”, IEEE 802.3 ah
interim, March 2002
[5] FOCI Fiber Optic Communications technical documentation.
[6] DIAMOND technical documentation.
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