WAVELENGTH ROUTING NETWORKS

Second-generation optical networks

In first-generation networks, the electronics at a node must handle not only all the data intended for that node but also all the data that is being passed through that node In 2nd generation n/w data could be routed through in the optical domain. Second-generation optical networks have routing, switching and intelligence in the optical layer.

WAVELENGTH ROUTING NETWORKS

WDM wavelength Routing Networkkey network elements optical line terminals multiplexes multiple wavelengths into a single fiber and demultiplexes a set of wavelengths on a single fiber into separate fibers. OLTs are used at the ends of a point-to-point WDM link.optical add/drop multiplexers (OADMs) takes in signals at multiple wavelengths and selectively drops some of these wavelengths and let others pass through. It also selectively adds wavelengths to the composite outbound signal. An OADM has two line ports where the composite WDMsignals are present, and a number of local ports where individual wavelengths are dropped and added .

WDM wavelength Routing Network ( contn.)optical crossconnects(OXCs)An OXC essentially performs a similar function as OADM but at much larger sizes. OXCs have a large number of ports (ranging from a few tens to thousands) and are able to switch wavelengths from one input port to another.Both OADMs and OXCs may incorporate wavelength conversion capabilities.

Features of WR network:Wavelength reuse.:multiple lightpaths in the network can use the same wavelength, as long as they do not overlap on any link. This spatial reuse capability allows the network to support a large number of lightpaths using a limited number of wavelengths.Wavelength conversion.:Lightpaths may undergo wavelength conversion along their route. Wavelength conversion can improve the utilization of wavelengths inside the network. Wavelength conversion is also needed at the boundaries of the network to adapt signals from outside the network into a suitable wavelength for use inside the network.

Transparency.:Transparency refers to the fact that the lightpaths can carry data at a variety of bit rates, protocols, and made protocol insensitive. This enables the optical layer to support a variety of higher layers concurrently. Circuit switching.:The lightpaths provided by the optical layer can be set up and taken down upon demand. today these lightpaths, once set up, remain in the network for months to years. With the advent of new services and capabilities offered by todays network equipment, this process is more dynamic, both in terms of arrivals of lightpath requests and durations of lightpaths.

Survivability.:The network can be configured such that, in the event of failures, lightpaths can be rerouted over alternative paths automatically. This provides a high degree of resilience in the network.

*NODE DESIGNSDegree of wavelength conversionMultiple fiber networksDegree of transparencyRealizations

Degree of wavelength conversion

Degree of wavelength conversion

Multiple fiber networks

Optical layer cost tradeoffDesign parameters that need to be considered in determining the cost of the network:Router ports: use the minimum possible number of IP router ports to support the given traffic.minimizing the number of ports is same as minimizing the number of lightpaths that must be set up to support the traffic.WavelengthsAt the same time, we would also like to use the minimum possible number of wavelengths since using more wavelengths incurs additional equipment cost in the optical layer.

Optical layer cost tradeoffHops:This parameter refers to the maximum number of hops taken up by a lightpath.For the PWDM ring, each lightpath takes up exactly one hop. more difficult to design the transmission system as the number of hops increases ,which again increases the cost of optical layer equipment.

Routing and wavelength assignmentRouting and wavelength assignment (RWA) problem is defined as follows. Given a network topology and a set of end-to-end lightpath requests determine a route and wavelength(s) for the requests, using the minimum possible number of wavelengths.

Routing and wavelength assignmentThe RWA problem can be divided into a lightpath routing (LR) problem and a wavelength assignment (WA) problem. The LR problem is to find routes for a collection of lightpaths, The objective of the LR problem is to minimize the maximum, over all fiber links, of the number of lightpaths using a fiber link. An alternative objective of the LR problem is to minimize some network cost such as bandwidth, ports, switching, or regenerator cost. The WA problem is, given a collection of lightpaths and their routes, assign wavelengths to the lightpaths. The objective is to minimize, over all fiber links, the maximum wavelength used on a fiber link.

Routing and wavelength assignmentTo solve the LR problem, route the lightpaths one at a time in some order. Routes can be computed by using shortest path routing algorithms on the network topology. The network topology has weights assigned to each link, so that the shortest path is the least-weight path. The link weights are chosen so that the resulting lightpath routes meet the objective of the LR problem.

Routing and wavelength assignmentFor the WA problem, the assignments must obey the following constraints: 1. Two lightpaths must not be assigned the same wavelength on a given link. 2. If no wavelength conversion is available in the network, then a lightpath must be assigned the same wavelength all along its route.

Routing and wavelength assignmentwavelength assignment Algorithm:first fit. It assumes that the wavelengths are numbered (e.g., 0, 1, ...), and it chooses the smallest numbered wavelength that is available. This tends to pack lightpaths into lower-numbered wavelengths and keeps higher-numbered wavelengths free for future lightpaths.

RWA AlgorithmRandom-1. For a lightpath request between two nodes, choose at random one of the available wavelengths on a fixed shortest path between the two nodes.Random-2. Fix two shortest paths between every pair of nodes. For a lightpath request between two nodes, choose at random one of the available wavelengths on the first shortest path between the two nodes. If no such wavelength is available, choose at random one of the available wavelengths on the second shortest path.

RWA AlgorithmMax-used-1. For a lightpath request between two nodes, among the available wavelengths on a fixed shortest path between the two nodes, choose the one that is used the most number of times in the network at that point of time.Max-used-2. Fix two shortest paths between every pair of nodes. For a lightpath request between two nodes, among the available wavelengths on the first shortest path between the two nodes, choose the one that is used the most number of times in the network at that point of time. If no such wavelength is available, among the available wavelengths on the second shortest path between the two nodes, choose the one that is used the most number of times in the network at that point of time.

*Wavelength Routing Test BedsAfrica ONEAONNTT RingMWTNONTCAlcatels WDM RingMONET

*Africa ONEThe Africa optical network is a project to deploy an undersea WDM/SDH ring around the African continent.The network consists of hub nodes interconnected by two fiber pairs to form a ring.Upto 8 OADMs can be placed along this link

*Africa one

*AON

*AONThe all optical network consortium is an example of a static wavelength routing testbed.The testbed has two level hierarchy.Level 0 corresponds to a LAN and uses a broadcast star architecture.Level 1 corresponds to a MAN and uses a single static wavelength cross connect

*AONThe testbed uses 20 wavelength. The odd numbered wavelengths are used within level 0 and even numbered ones in level 1.Each end node has a tunable transmitter and a tunable receiver.AON provides three service A, B and C.

*AONA service is a transparent service and uses a tunable fiber Fabry-Perot filter.B service is a circuit-switched service that provides pre assigned time slots on a lightpath.C service is a datagram service and runs out of band at 1310nm band as a broadband ethernet.

*NTT Ring

*NTT RingNTTs testbed consists of a unidirectional ring, with one working fiber and one protection fiber.The network has one hub node and several access nodes.The hub node sends out signals at N different wavelengths, all multiplexed into the working fiber.

*NTT RingThere can be up to N access nodes.At access node I, wavelength I is dropped and added.All communication between access node I and hub node takes place through the wavelength I.

*MWTN

*MWTNMWTN- multi wavelength transport Network.Both optical cross connect and add/drop multiplexers were developed.The previous picture shows an optical cross connect.The switched tried where, lithium niobate switches, semiconductor optical amplifier switches,and opto mechanical switches.

*ONTC

*ONTCThe optical network technology consortium uses a reconfigurable wavelength routing architecture.The testbed consists of two interconnected unidirectional rings.Each ring has two nodes with one additional common node.

*Alcatels WDM ringAlcatel has demonstrated a two-fiber ring network with OADMsThis architecture is a mix of optical switching and electronic generators.The regenerators provide wavelength conversion and also retime the signal from link to link

*Alcatels WDM ring

*MONETThe goal of a MONET program is to develop a field-hardened testbeds of wavelength routed networks.It consists of a local ring network with OADMs interconnected to a long distance network via an OXC

*MONET