LIGHT TREE

BY SMRITHY T.M

Contents

Introduction Block DiagramComparison between CWDM and DWDM Light PathLight TreeArchitecture of wavelength-routed optical networkMulticast Switch An MWRS based on a splitter bankMWRS based on a “drop and continue” switch.

contentsThe Optical layerUnicast ,Broadcast and Multicast Traffic.Advantages and DisadvantagesFuture enhancementConclusion

Introduction Today, there is a general consensus that in near future wide

area networks (WAN) will be based on Wavelength Division Multiplexed (WDM) optical networks. 

Depending on the underlying physical topology networks can be classified into three generations:

1st Generation: They employ copper-based or microwave technology. eg. Ethernet.

2nd Generation: These networks use optical fibers for data transmission but switching is performed in electronic domain. eg. FDDI.

3rd Generation: These networks both data transmission and switching is performed in optical domain. eg. WDM. 

Block diagram

Traditional Digital Fiber Optic Transport

Single Pair of Fibers

Single Pair of Fibers

Digital Transceiver

Digital Transceiver

Digital Transceiver

Digital Transceiver

Digital Transceiver

Digital Transceiver

Digital Transceiver

Digital Transceiver

Single Pair of Fibers

Digital fiber optic transport using WDM

Digital Transceiver

Digital Transceiver

Digital Transceiver

Digital Transceiver

Digital Transceiver

Digital Transceiver

Digital Transceiver

Digital Transceiver

Single Pair of Fibers

WDM MUX WDM MUX

Fiber Optic Transport using WDM

Light PathA light path is an all-optical channel which may be used to

carry circuit switched traffic and it may span multiple fiber links.

A light path can create logical (or virtual) neighbors. A light path carries direct traffic between nodes it interconnects.

Major Objective of light path communication is to reduce the number of hops.

Under light path communication the network employs an equal number of transmitters and receivers because light path operates on point to point basis.

Light Tree

A light tree is a point to point multipoint all optical channel which may span multiple fiber links.

It enables single-hop communication between a source node and a set of destination nodes.

A light tree based virtual topology can reduce the hop distance, thereby increasing the network throughput.

Architecture of Wavelength-Routed Optical Network

NSFNET backbone topology

Architecture of Wavelength-Routed Optical Network

A WDM control network may require efficient delivery of broadcast traffic. which may be modelled as a layered graph in which each layer represents a wavelength and each physical fibre has a corresponding link on each wavelength layer.

The switching state of each wavelength-routing switch (WRS) is managed by a controller.

A light tree based broadcast layer may provide an efficient transport mechanism for such multicast applications.

Multicast Switch :

Linear divider combiner (LDC)

Multicast Switch :

linear divider combiner with two input fibers (the Pi’s), two output fibers (the Po’s) two dividers and four control signals (the αjs).

The LDC acts as a generalized optical switch with added functions of multicasting and multiplexing.

The values of α1, α2, α3, α4 control the proportion of the input power that can be sent to the output links.

Let Pi1 and Pi2 be the power on the input links, and let Po1 and P02 be the output powers. Then,

Po1= (1-α1) (1-α3) Pi1+ (1-α2) α3Pi2 and Po2= α1 (1-α4) Pi1+α1α4Pi2

An MWRS based on a splitter bank

An optical splitter splits the input signal into multiple identical output signals. Since an optical splitter is a passive device the power from at least one output signal of an n-way optical splitter is less than or equal to 1/n times the input power.

MWRS based on a “drop and continue” switch

• In a drop and continue switch a light path can be terminated at a node and simultaneously an identical copy of the light path can be allowed to continue to another node in the network.

• Technique: It is the special case of light tree , By employing a drop and

continue switch we can construct a chain of nodes which are connected by a drop and continue light path.

All nodes on the chain will receive transmissions on a drop and continue light path where light is dropped .

The Optical Layer

Definition:The optical layer provides light paths to the higher layers. In

addition to the pass through capability provided by the optical layer other features include are:

TransparencyWavelength reuseReliabilityVirtual topologyCircuit switching.

Unicast , Broadcast and Multicast Traffic

Unicast Traffic:

IP/TV on Demand use unicast traffic. Each user can request the program at a different time. with the number of simultaneous users limited by the available bandwidth

Unicast traffic is sent from a single source to a single destination IP address.

Unicast traffic

Example of Single Unicast Traffic

Broadcast Traffic:

Broadcast traffic uses a special IP address to send a single stream of data to all of the machines on the local network. A broadcast address typically ends in 255. (for example, 192.0.2.255) or has 255 in all four fields (255.255.255.255).

Example of Broadcast Traffic

Multicast Traffic:

Unlike unicast addresses, when a data stream is sent to one of these addresses potential recipients of the data can decide whether or not to receive the data.

If the user wants the data the user's machine receives the data stream if not the user's machine can ignore it.

Example of multicast traffic

Combining Unicast and Multicast Traffic:

Example of Combined Multicast and Unicast Traffic

Combining Unicast and Multicast Traffic

If the routers in a network are not capable of handling multicast IP/TV can use unicast transmissions to send the multimedia content across the non multicast -enabled router.

A server on the other side of the router can then use multicast transmission to deliver the content to its local users.

Advantages and Disadvantages:

Advantages:

Minimize the network-wide average packet hop distance

Increased Bandwidth.Broadcasting and Multicasting.

Disadvantages:

Difficulties arising from limited number of transceivers per node.Difficulties arising from limited number of wavelengths.

Future enhancement

To Minimize wavelength cost.Minimize the number of transceiversWavelength assignment algorithm should be

explored in future reasearch.

ConclusionA novel WDM WAN architecture based on light trees that are

capable of supporting broadcasting and multicasting over a wide-area network by employing a minimum number of optoelectronic devices.

Such WDM WAN can provide a very high bandwidth optical layer which efficiently routes unicast, broadcast and multicast packet-switch traffic.

Preliminary results show that if we employ a set of light trees, then significant savings can be achieved in terms of the number of optoelectronic devices that are required in the network.

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