INTRODUCTION
TO
OPTICAL NETWORKS
Presentation Overview
Why Optical Networks..?!
• Generations of Optical Networks
• The Classical Layered Hierarchy
• The Optical Layer
•
• Functions of Optical Layer
• Advantages of Layering
Architectures Of Networks
• Access Networks: Introduction
•
• Why Passive Optical Networks..?!
• Passive Optical Access Network
• Ethernet Passive Optical Network (EPON)
• Downstream and Upstream Operation
• WDM-Passive Optical Network (WDM-PON)
• Ring-Based WDM-PON Architecture
• Downstream and Upstream Operation
Why Optical Networks ...??
Dramatic changes in the telecommunication
industry.
• Need for more capacity in the network.
• Tremendous growth of the Internet and the World
Wide Web in terms of
 number of users & the amount of time
 bandwidth taken by each user – internet traffic growing
rapidly.
•
•
•
•
Businesses rely on high speed networks.
Need for more bandwidth.
Deregulation of the telephone industry.
Need of providing quality of service(QoS) to carry
performance sensitive applications ( real-time voice,
video etc.)
Optical Networks

Definition: An Optical Network is a telecommunication network
• with transmission links that are optical fibers and
• with an architecture that use designed to exploit the unique features if fibers.

High performance lightwave network –involve complex combination both
optical and electronic devices.

Low-cost broadband services – Internet based applications continues to
increase.

The “glue” that holds the purely optical network together consists of :
• optical network nodes (ONN) connecting the fibers within the network
• network access stations (NAS) interfacing user terminals and other nonoptical end systems to the network

Critical role :
• Reducing communications costs
• Promoting competition among carriers & service providers
• Increasing the demand for new services
Generations of Optical Networks

First Generation:
• Optics used for transmission & provide capacity
• Switching & other intelligent network functions
were handled by electronics
• ex. SONET (synchronous optical network)
•
SDH ( synchronous digital hierarchy)

Second Generation:
• have routing ,switching and intelligence in the
optical layer
• use multiplexing techniques – provide the
capacity needed
The Classical Layered Hierarchy
The OSI Model
 Physical layer
• Provides a “pipe” with a certain amount of
bandwidth to the data link layer.
 Data
•
•
•
•
•
•
link layer
Framing
Multiplexing
Reliable transmission –acknowledgment frames
Error detection and correction
Flow control
Demultiplexing data send over the physical layer.
The Classical Layered Hierarchy
 Network
Layer
• Performs the end-to-end routing function of
taking a message at its source
• And delivering it to its destination
• Controls congestion
 Transport
Layer
• Ensuring the end-to-end
• In-sequence
• Ensuring error-free delivery of the transmitted
messages
The Classical Layered Hierarchy

Session Layer
• Sessions restoration
• Token management
• Synchronization
 Presentation
Layer
• Encoding data
 Application
Layer
• Compatibility between
applications
The Optical Layer
Layered View of the Optical Network
The architecture is composed of an
underlying optical infrastructure
 Physical layer
• Contains optical components executing
linear(transparent)operations on optical signal.
• provides basic communication services to a
number of independent logical networks (LNs).
 LNs
•
are residing in the Logical layer.
Contains electronic components executing
nonlinear operations on electrical signal
The Optical Layer
Layered View of the Optical Network
Functions Of The Optical Layer
Multiplexes lightpaths into a single fiber.
 Allows individual lightpaths to be
extracted efficiently from the composite
multiplex signal at the network nodes.
 Incorporates sophisticated service
restoration techniques.
 Incorporates management techniques.
 Provides lightpaths – used by SONET and
IP network elements.

Advantages of Layering
1.
2.
Independently control and manage each logical network
simplifying these functions.
Share the total resources of the physical layer among
several logical network  exploiting them more efficiently.
3.
Customize each logical network to provide specialized
user services  improving the QoS.
4.
Dynamically reconfigure each logical network 
equipment failures and changing traffic patterns.
5.
Use both optical and electronic degrees of freedom
provide flexibility, survivability, manageability and capacity
for growth and change.
Architectures Of Networks

Backbone Networks
• networks in the same building, in different buildings in
a campus environment, or over wide areas.
• exchange of information between different LANs

Metro Area Networks (MAN)
• network that interconnects users with computer
resources in a geographic area or region larger than
that covered by even a large local area network
(LAN) but smaller than the area covered by a wide
area network (WAN).

Access Networks
• Distributed EPON architectures
• Distributed ring-based WDM-PON architectures
• Converged Optical/Wireless Access Networks
Architecture Of Networks
Access Networks : Introduction
• Access Networks
• Tremendous growth in both backbone and Metro Access
Network (MAN) capacity.
• End users are becoming more sophisticated
• Rich multimedia
• Real-time services
• The “Last Mile” remains a bottleneck.
• Current “Last Mile” capacity has increased from 56Kb/s (dialup
modem) to a few Mb/s (cable modem or digital subscriber line
(DSL) connection).
• Still far short of the Gigabit line speed necessary to support rich
multimedia and real-time services.
Access Networks …
Central
Office
End
Users
Last/First Mile
Access Networks …

Copper-based access networks will soon no longer be
able to meet the ever-growing consumer demand for
bandwidth.

PON-based fiber-to-the-curb/home (FTTC/FTTH)
systems are considered as possible successors to
current copper-based access solutions.

Two most viable architectures:
• Single channel Time-Division Multiplexed PON (TDM-PON)
• Multi-channel Wavelength-Division Multiplexed PON (WDMPON)
Why Passive Optical Networks?
A natural step in access evolution
Point-to-Point links
PON
 Minimum fiber
CO
usage/ N+1
transceivers
 Path transparency
Concentration Switch in
the neighborhood
 Passive network
elements
 Much longer distance CO
(~20km) than DSL
(~5.5 km).
 Higher bandwidth
PON
due to deeper fiber
Passive Star
Coupler
penetration.
SC
CO
 Downstream video
broadcasting.
~20 km
18 ~1 km
Passive Optical Access Network
Multiplexing Techniques
Time Division Multiplexing (TDM) :
A type of multiplexing that combines data
streams by assigning each stream a different
time slot in a set. TDM repeatedly transmits
a fixed sequence of time slots over a single
transmission channel.
Wavelength Division Multiplexing (WDM):
A technique of sending signals of several
different wavelengths of Light into the Fiber
simultaneously. In fiber optic communications,
wavelength-division Multiplexing (WDM) is a
technology which multiplexes multiple optical
carrier signals on a single Optical Fiber by
using
different
wavelengths
(colors)
of Laser light to carry different signals.WDM is
similar to frequency-division multiplexing
(FDM).
Optical Network Terminal and
Optical Network Unit
ONT (Optical Network
Terminal):
ONT is a media converter that is
installed either outside or inside your
premises, during fiber installations.
ONU (Optical Network
Unit):
An
The
ONT converts fiber-optic light
signals to copper/electric signals.
Three
wavelengths of light are used
between the ONT and the Optical Line
Terminal :
•1310 nm voice/data transmit
•1490 nm voice/data receive
•1550 nm video receive
Each ONT is capable of delivering:
 Multiple POTS (plain old telephone
service) lines
 Internet data
Video
•An Optical Network Unit (ONU)
converts optical signals transmitted
via fiber to electrical signals.
•These electrical signals are then sent
to individual subscribers. ONUs are
commonly used in fiber-to-the-home
(FTTH) or fiber-to-the-curb (FTTC)
applications.
Transmission between ONT and ONU
Example …
•Using different wavelengths for each service makes it possible
to transmit high-speed Internet and video services at the same
time.
• The 1310nm and 1490nm bands are used for Internet
transmissions on the uplink and downlink, respectively,
•The 1550nm band is used for multi-channel video broadcasts.
•Wavelength multiplexing is performed at the central office and
a wavelength demultiplexing mechanism is provided at the
customer's house.
OLT – Optical Line Terminal

OLTs are located in provider’s central switching office.

This equipment serves as the point of origination for FTTP (Fiber-to-thePremises) transmissions coming into and out of the national provider’s
network.

An OLT, is where the PON cards reside. The OLT's also contain the CPU and
the GWR and VGW uplink cards. Each OLT can have a few or many dozens of
PON cards.

PON = Passive Optical Network
GWR = Gateway Router
VGW = Voice Gateway

Each PON card transmits 1490nm laser data signal to the ONT, and receives
the ONT transmission of the 1310nm laser data signal.

The one-way 1550nm laser video signal to the ONT is injected into the fiber
at the CO.
Optical Splitter and Combiner
Fiber optic splitter is used to split
the fiber optic light into several
parts at a certain ratio.
For example, a 1X2 50:50 fiber
optic splitter will split a fiber
optic light beam into two parts,
each get 50 percent of the
original beam.
An optical combiner is a
passive device that
combines the optical
power carried by two input
fibers into a single output
fiber.
Ethernet PON (EPON) Architecture
Upstream operation
Downstream operation
Passive Optical
Splitter/Coupler
10-20 km
Downstream:
 Operates as Broadcast & Select Network
 Each ONU extracts those packets that contain
the ONU’s unique MAC address
Upstream:
• ONUs employ arbitration mechanism to avoid
collisions.
•
OLT arbitrates transmissions via a Dynamic
Bandwidth Allocation (DBA) module.
•
A Multi-point Control Protocol (MPCP) was
developed.OLT and ONUs exchange control
messages, namely, REPORT and GATE messages.
•
REPORT message contains the ONU’s
bandwidth requirements. GATE message has
the start time and the duration of the granted
time slot.
•
The average dedicated bandwidth per user is
limited to a few percent of the channel capacity,
i.e., a few tens of Mb/s.
EPON - Frame Transmission

EPON employs a point-to-point emulation
mechanism, which makes the EPON medium behave
as a collection of point-to-point links.

Emulation mechanisms rely on tagging Ethernet
frames with a unique value called the Logical Link
ID (LLID).

To allow point-to-point emulation, the OLT must
have N MAC ports (interfaces), one for each logical
link .

When sending a frame downstream (from the OLT
to an ONU), the emulation function in the OLT will
insert the LLID associated with a particular MAC
port on which the frame arrived. Even though the
frame will be delivered to each ONU, only one
ONU will match that frame’s LLID with its own
assigned value, and thus accept the frame and pass it
to its MAC layer for further verification.

MAC layers in all other ONUs will never see that
frame. (discard it)
EPON – Logical Link ID (LLID)
•The LLID replace
two bytes in the
preamble. The OLT
could distinguish
frames of different
ONUs by the LLIDs
and thus the LLID
equals the logical
identification of the
ONU.
EPON frame
WDM-PONs

Separate pair of dedicated upstream/downstream wavelength channels to
each subscriber (≥1 Gb/s of dedicated bandwidth per subscriber).

Provide dedicated optical connectivity to each subscriber with bit rate and
protocol transparencies, guaranteed QoS, and increased security.

WDM-PON systems’ capacity is still too high compared to the access
capacity needed. However, as bandwidth demand increases, the economics
change. In terms of cost per bit rate, WDM-PON is more efficient and
economical.
λ1
Remote
Node
Central
Office
C-Band
ONT
λ2
OLT
ONT
AWG
λ1 λ2 λ3... λN
λ3
ONT
λ’, λ2’, λ3’... λN
L-Band
λN
ONT 28
WDM-PONs
Simple Architecture
WDM-PON
Limitations

Traditional tree-based WDM-PON architectures suffer
from several limitations including:
•
Inability to efficiently utilize network resources. The
unused dedicated channel capacities of lightlyloaded/idle subscribers cannot be shared by any of the
other heavily-loaded users attached to the PON.
•
Inability to provide private networking capability
within a single PON
•
Lack of simple and cost-effective protection and/or
restoration capabilities.
30
Ring-Based WDM-PON Architecture
Olt
Trunk
Ring
Onu
PROCESS-APPEND-FORWARD
TX-1
RX-1
ONU-2
λ1 λLan λLan λ1
λ2 λLan λLan λ2
OADM
OADM
λ1
λN
WDM MUX
TX-N
ONU-1
RX-Lan
λLan
2
3
WDM DEMUX
λN
λ1,λ2,..,λN
1
λ1
RX-N
MUX
2
10-20km
λ1,λ2,..,λN, λLan
λLan
3
Filter
1
90:10
λN
OADM
λLan λLan λN
ONU-N
RXDown RXLAN TXLAN TXUp
λ2
λLan
λLan
λ2
Ring-Based WDM-PON Architecture
•
Efficiently utilizes network resources. Provides dynamic allocation of
unused capacities of lightly loaded/idle wavelengths to heavily loaded
channels
•
Provides truly shared LAN capability among PON end-users.
•
Utilizes a fully distributed control plane among the ONUs that
enables distributed provisioning and fault restoration by the ONUs
•
Eliminates the OLT's centralized task of bandwidth provisioning and
failure recovery
• Reduction of processing complexities and delays at the OLT.
Downstream & Upstream Operation
(Without Sharing)
An upstream flow
An upstream flow
from ONU-1 to OLT from ONU-2 to OLT
λ1
λ2
Scheduler
A downstream
flow to ONU-2
TX-1
λ1
TX-2
λ2
WDM MUX
A downstream
flow to ONU-1
ONU-1
ONU-2
OADM
OADM
MUX
TX-N
Filter
RX-N
WDM DEMUX
RX-1
RX-2
90:10
OADM
RX-Lan
ONU-N
LAN Operation
(ONU-ONU Communication)
ONU-1
ONU-2
OADM
OADM
λLAN
TX-1
WDM MUX
TX-2
MUX
TX-N
Filter
RX-N
WDM DEMUX
RX-1
RX-2
90:10
OADM
RX-Lan
ONU-N
Downstream & Upstream Operation
(With Sharing)
Congestion at downstream buffer, Q1
More downstream
flows to ONU1
(i.e, Rin>Rout)
Scheduler
Q1
Q2
WDM MUX
Arrival of a
downstream
flow destined
to ONU1
λ1
λ2
λLAN
ONU-1
ONU-2
OADM
OADM
MUX
QN
Filter
RX-1
RX-N
WDM DEMUX
Scheduler runs SWS
algorithm searching for a
lightly loaded downstream
buffer to send ONU1’s
newly arriving excess
flows (Assume Q2 is lightly
loaded )
90:10
OADM
RX-Lan
OLT discards all
excess downstream
traffic
ONU-N
ONU2 determines
new flow as ONU1’s
downstream flow and
forwards it to ONU1
over λLAN
Upstream Scheduling Algorithm
(USA)
Arrival of an
upstream flow
Arrival of more
upstream flows
(i.e, Ri,up>λi,up)
ONU-1
Q2
Q3
WDM MUX
Q1
MUX
λ1
λLAN
Filter
RX-2
RX-3
WDM DEMUX
RX-1
RX-Lan
OLT processes all TUS
flows and forwards
them to their
destinations
90:10
ONU-3
ONU-2