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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 1 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.) 2 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 3 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. 4 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. 5 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. 6 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. 7