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Appendix 3
Introduction to Photonic Networks
1. Network classification
Service:
Tele-Communication: voice exchange
Data-Communication: computer data exchange
Community Antenna Television: video/audio delivery
Transmission Protocol:
Legacy TDM (DSn)
SONET/SDH
ATM
TCP/IP
Transmission Technology:
Analog (T&R)
PCM-TDM (Tn)
Line Coding/Modulation (ISDN, xDSL)
Burst Alternative Transfer
Sub-Carrier Multiplexing (CATV)
FDM/WDM
Switched Multi-megabit Digital Service (MAN)
Frame Relay (MAN, WAN)
CDM (LAN)
OTDM
Transmission Media:
TP copper wire
Coaxial cable
RF wireless
Optical fiber
Network Functionality:
Customer premises network
Access network
Switch network
Core network
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Network Topology:
Point-to-point link
Broadcast/distribution network
Local area network
2. Network architecture: from high level protocols to physical layer
End Host
End Host
Application
Layer
Application
Layer
Presentation
Layer
Presentation
Layer
Session
Layer
Session
Layer
Transport
Layer
Transport
Layer
Message
Network
Layer
Packet
Network
Layer
Network
Layer
Network
Layer
Virtual Link:
Hierarchical Protocols
(run on the end hosts only)
Data Link
Layer
Data Link
Layer
Data Link
Layer
Data Link
Layer
Frame
Physical
Layer
Physical
Layer
Physical
Layer
Physical
Layer
Bit
Real Link:
Nodes and Link Media
(offer public service)
More Nodes
Application layer protocols: application functionality management
(These protocols include things like the file transfer protocol, which defines a protocol by
which file transfer applications can inter-operate.)
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Presentation layer protocol: formatting, encoding
(It is concerned with the format of data exchanged between peers, for example, whether
an integer is 16, 32 or 64 bits long and whether the most significant bit is transmitted first
or last.)
Session layer protocol: logical level session initialization, control, synchronization and
termination
(It provides a name space that is used to tie together the potentially different transport
streams that are part of a single application. For example, it might manage an audio
stream and a video stream that are being combined in a teleconferencing application.)
Transport layer protocol: end-to-end data transmission control, network routing
optimization
(It implements a process-to-process channel.)
Network layer: packet assembling/de-assembling, routing and switching
(It handles routing among nodes within a packet-switched network. At this layer, packet
is the unit of data exchanged among nodes.)
Data link layer: frame transmission, error bit check and correction
(It collects a stream of bits into a larger aggregate frame. Network adapters typically
implement this layer. The frames, not raw bits, are actually delivered to the hosts.)
Physical layer: raw bit transmission over a communication link
3. Photonic networks: basic topologies
a) Point-to-point link
Topology: Cascade
Node 1
Tx
Node 1
Tx
Regenerator
Rx-Tx
Amplifier
Regenerator
Rx-Tx
Amplifier
Node 2
Rx
Node 2
Rx
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b) Broadcast and distribution network
Topology: Hub
Hub
Hub
Hub
Hub
Application: local area telecommunication network (for distribution of audio channels
within a city).
Advantage: low cost can be achieved by combining signal transmission over fiber (in the
optical domain) with channel switch over legacy automated cross-connect facility (in the
electronic domain) placed at central locations (hubs).
Problem: reliability.
Topology: Bus
User 1
User N
User 2
Application: CATV distribution network.
Advantage: wide bandwidth for a large number of channels, possible future HDTV
applications.
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Problem: signal loss increase exponentially with the number of taps (subscribers)
described by:
PN  PT C[(1  C )] N 1
where PN ,T are the power available at the Nth tap (subscriber) and the transmitted power,
respectively; C is the fraction of power coupled out at each tap and the fiber loss and the
tap insertion losses are all neglected.
c) Local area network
The main difference between distribution networks and LANs is related to the random
access offered to multiple users of a LAN. The system architecture plays an important
role for LANs, since the establishment of predefined protocol rules is a necessity in such
an environment.
Topology: Bus
Application: Ethernet
Advantage: low cost
Problem: collision detection required (by CSMA/CD protocol)
Topology: Ring
User 1
Node 1
Node 2
User 3
User 2
Node N
Application: FDDI
Advantage: collision can be avoided by ring topology and token set up.
Problem: high cost.
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Topology: Star
User 1
User 2
Star
Coupler
User N
Application: LAN serves for users with high density.
Advantage: more users are allowed due to the lower loss comparing with the bus
topology described by:
PN  PT / N
Problem: generally longer subscriber lines.
4. Photonic networks: incorporating WDM technology
a) Broadcast-and-select WDM networks (single-hop or multiple-hop):
Each Tx sends its information at a different fixed wavelength.
All the transmissions from the various nodes are combined in a passive star coupler or
coupled onto a bus and the result is sent out to all receivers.
Each Rx sees all wavelengths and uses a tunable filter to select the wavelength addressed
to it.
Combined with different network topology, we have the practical WDM networks as
illustrated in the previous chapter.
Advantage: only passive WDM components are required.
Problem: at least as many wavelengths as nodes are required in such networks, unless
several nodes time-share a wavelength.
b) Wavelength-routing WDM networks:
The physical topology of such networks consists of optical wavelength routers
interconnected by pairs of point-to-point fiber links in an arbitrary mesh configuration.
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Each link can carry certain number of wavelengths, which can be directed independently
to different output path at a node. Each node may have logical connections with several
other nodes in the network, where each connection uses a particular wavelength.
Provided the paths taken by any two connections do not overlap, they can use the same
wavelength.
Advantage: less number of wavelengths is required in such networks through wavelength
reuse, wavelength conversion and optical switching.
Problem: active WDM components are required.
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