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Chapter: Optical Communication and Networking - Optical Networks

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Broadcast and select WDM Networks

These networks can be classified as (1) Broadcast-and-select techniques (2) Wavelength-routing networks.

Broadcast and Select WDM Networks


·           All-optical WDM networks have full potential of optical transmission capacity and versatility of communication networks beyond SONET architectures.

·           These networks can be classified as

(1) Broadcast-and-select techniques

(2) Wavelength-routing networks.

·           Broadcast-and select techniques employing passive optical stars, buses and wavelength routers are used for local networks can be classified as

(1) Single-hop networks

(2) Multi-hop networks

·           Single hop refers to network where information transmitted in the form of light reaches its destination without being converted to an electrical form at any intermediate point. In a multi hop network, intermediate electro-optical conversion can occurred.

 

1. Broadcast and Select Signal Hop Network

·           Two alternate physical architectures for a WDM-based local network have n sets of transmitters and receivers are attached to either a star coupler or a passive bus.


·           Each transmitter sends its information at a fixed wavelength.

·           All the transmissions from the various nodes are combined in a pasive star. Coupler  or coupled onto a bus and sent out to all receivers.

·           An interesting point to note is that the WDM setup is protocol transparent.


·           Protocol transparent means that different sets of communicating nodes can use different information exchange rules (protocols) without affecting the other nodes in the network.

·           The architectures of single-hop broadcast-and-select networks are fairly simple, there needs to be careful dynamic coordination between the nodes.

·           A transmitter sends its selective filter to that wavelength.

·           Two sending stations need to coordinate their transmission so tha collisions of information streams at the some wavelength do not occur.

 

2. Broadcast and Select Multihop Network

·           Drawback of single-hop networks is the need for rapidlyt unable lasers or receiver optical fibers.

·           This drawback can be overcome by the designs of multihop networks.

·           Multihop networks do not have direct paths between each node pair.

·           Each node has a small number of fixed tuned optical transmitter and receivers.


·           An example, a four node broadcast and select multihop network where each node transmits on one set of two fixed wavelengths and receives on another set of two fixed wavelengths.

·           Information destined for other nodes will have to be routed through intermediate stations.

·           Considering the operation, a simplified transmission shceme in which message are sent as packets with a data field and an address header containing source and destination identifiers (i.e,. routing information) with control bits.


·           At intermediate node, the optical signal is converted to an electrical format.

·           The address header is decoded to examine the routing information field, which will indicate where the packet should go.

·           Routing information is used to send the electronic packets from optical transmitter to the next node in the logical path toward its final destination.

·           Advantage: There are no destination conflicts or packet collisions in the network.

·           For H hops between nodes, there is a network throughput penalty of at least 1/H.

 

3. The Shuffle Net Multihop Network

 

·           Various topologies for multihop lighwave networks are

(1) The shuffle net graph

(2) The de Bruijin graph

(3) He toroidal Manhattan street network

·           A scheme called the perfect shuffle is widely used to form processor interconnect patterns in multiprocessors.

·           For optical networks, the logical configuration consists of a cylindrical arrangement of k column, each having p nodes. Where P is the number of fixed transceiver pairs per node.

·           The total number of nodes is then



·           a (p,k)=(2,2) shuffle net, where the (k+1)th column represents the completion of a trip around the cylinder back to the first column.

·           Performance parameter for the shuffle net is the average number of hops between any randomly chosen nodes.

·           Since, all nodes have p output wavelength, p nodes can be reached from any node in one hop, p2 additional nodes can be reached in two hope, until all the (pk-1) other nodes are visited.

·           The maximum number of hops is


·           Consider figure above, the connections between nodes 1 and 5 and nodes 1 and 7. In first case, the hop number is one.

·           In second case three hops are needed with routes 1- 6 – 7 or 1 – 5 – 2 -7.?

·           The average of hops Bar H of a shuffle net is


·           In multiphopping, part of the capacity of a particular link directly connecting two nodes is actually utilized for carrying between them.

·           The rest of thelink capacity is used to forward messages from other nodes.

·           The system has Np=kpK+1 links, the total network capacity C is


·           The per-user throughput δ is


·           Different (p,k) combination result in different throughputs, to get a better network performance.

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