Routing and Wavelength Assignment

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Routing and Wavelength

Assi nment



Introduction• Wavelength-division multiplexing (WDM) technology has

been improving steadily in recent years, with existing systems

capable of providing huge amounts of bandwidth on a singlefiber link.

-

telecommunication networks on a point-to-point basis, with

the optical signals being converted back to electronics at each

node.

• The electronic switching and processing costs at the nodes

can potentially be very high, leading to severe performance

bottlenecks and limiting the delivery of optical link bandwidth

to the end users.



Introduction........• Emerging wavelength-routed WDM systems, which

utilize photonic crossconnects (PXCs), are capable of switching data entirely in the optical domain.

• Configuring these optical devices across a network

enables one to establish all-optical connections, orlight paths between source nodes and destinationnodes (Fig. below).

• Data carried on these light paths avoid electronicconversion and processing at intermediate nodes,thereby alleviating the electronic bottleneck.



Wavelength-routed WDM network with light

paths established between nodes.



Challenges involved in designing wavelength-

routed networks

• One of the challenges involved in designing wavelength-routed networks is to develop efficient algorithms and

protocols for establishing lightpaths in the optical network.

• The algorithms must be able to select routes and assign

utilize network resources and which maximizes the numberof lightpaths that can be established.

• Signalling protocols for setting up lightpaths musteffectively manage the distribution of control messages andnetwork state information in order to establish aconnection in a timely manner.



Routing and Wavelength Assignment• The problem of finding a route for a lightpath and assigning a

wavelength to the lightpath is often referred to as the routing andwavelength assignment problem (RWA).

• The objective of the problem is to route lightpaths and assignwavelengths in a manner which minimizes the amount of networkresources that are consumed, while at the same time ensuring that

no two lightpaths share the same wavelength on the same fiberlink.

• Furthermore, in the absence of wavelength conversion devices, theRWA problem operates under the constraint that a lightpath must

occupy the same wavelength on each link in its route.

• This restriction is known as the wavelength-continuity constraint.



Types of RWA Problems• The RWA problem can be considered under two different traffic assumptions.

• The static RWA problem applies to the case in which the set of connections is

known in advance, and a lightpath must be established for each connection.

• ,

remain for some amount of time before departing; thus, lightpaths must not onlybe established dynamically, but must also be taken down.

• In both the static and dynamic RWA problems, the routing component and the

wavelength assignment component of the problem are often decoupled into two

separate sub problems in order to make the problem more tractable.

• It is assumed that no wavelength conversion is available.



RWA for Static Lightpath Establishment

• A number of studies have investigated the RWAproblem for setting up a static set of lightpaths.

• These studies formulate the problem using integerlinear program (ILP) formulations, or rely on heuristica roaches in an attem t to minimize the number of

wavelengths required to establish a given set of lightpaths.

• The ILP formulations are NP-complete and thereforemay only be solved for very small systems.

• For larger systems, heuristic methods must be used.



Heuristics• In engineering, a heuristic is an experience-

based method that can be used as an aid tosolve process design problems, varying from

.

• By using heuristics, time can be reduced

when solving problems.



• The routing problem is formulated as an ILP in which the objective is tominimize the number of wavelengths required to establish a fixed set of

lightpaths.

• The search space of the problem is reduced by restricting the set of linksthrough which a lightpath for a given source-destination pair may traverse.

• The resulting ILP is then solved by relaxing the integer constraint, solvingthe resulting non-integer linear program, and then utilizing a randomizedrounding approach on the result to obtain an integer solution.

• Other heuristics consider only alternate shortest-hop paths between a

source-destination pair, and choose one of the paths according to apredefined policy .



Shortest Path Method

• A shortest-hop path is randomly chosen for

each source-destination pair.

• Each source-destination pair is then

considered individually, and the route for the

pair of nodes is switched to an alternateshortest-hop path.

• The wavelength-assignment sub-problem of

the RWA problem can itself be formulated

using graph colouring Problem.



RWA for Dynamic Lightpath Establishment• When lightpaths are established and taken down dynamically,

routing and wavelength assignment decisions must be made

as connection requests arrive to the network.

• It is possible that, for a given connection request, there may

e nsu c ent networ resources to set up a g tpat , n

which case the connection request will be blocked.

• The connection may also be blocked if there is no common

wavelength available on all of the links along the chosen

route.



RWA…….Contd.• Thus, the objective in the dynamic situation is to choose a

route and a wavelength which maximizes the probability of

setting up a given connection, while at the same timeattempting to minimize the blocking for future connections.

,problem can also be decomposed into a routingsubproblem and a corresponding wavelength assignmentsubproblem.

• Approaches to solving the routing subproblem can becategorized as being either fixed or adaptive, and asutilizing either global or local network state information.



Fixed routing• In fixed routing, a single fixed route is predetermined for each source-destination

pair. When a connection request arrives, the network will attempt to establish alightpath along the fixed route.

• If no common wavelength is available on every link in the route, then theconnection will be blocked.

• A xe rout ng approac s s mp e to mp ement; owever, t s very m te n

terms of routing options and may lead to a high level of blocking.

• In order to minimize the blocking in fixed routing networks, the predeterminedroutes need to be selected in a manner which balances the load evenly across thenetwork links.

• Fixed routing schemes do not require the maintenance of global network stateinformation.



Adaptive Routing Based on Global

Information

• Adaptive routing approaches increase the

likelihood of establishing a connection bytaking into account network state information.

• For the case in which global information is

available, routing decisions may be made with

full information as to which wavelengths areavailable on each link in the network.



Centralized Versus Distributed Routing• Adaptive routing based on global information may be implemented

in either a centralized or distributed manner.

• In a centralized algorithm, a single entity, such as a networkmanager, maintains complete network state information, and isresponsible for finding routes and setting up lightpaths forconnection requests.

• Since a centralized entity manages the entire network, there doesnot need to be a high degree of coordination among nodes;however, a centralized entity becomes a possible single point of failure.

• Furthermore, a centralized approach does not scale well, as thecentralized entity would need to maintain a large database tomanage all nodes, links, and connections in the network.



Alternate-Path Routing• One approach to adaptive routing with global

information is alternate-path routing.

• Alternate-path routing relies on a set of predetermined fixed routes between a sourcenode and a destination node.

• When a connection request arrives, a singleroute is chosen from the set of predeterminedroutes, and a lightpath is established on this

route.• The criteria for route selection is typically based

on either path length or path congestion.



Alternate Path Routing

• An example of a routing algorithm based on path length is the K-shortest paths algorithm , in which the first K shortest paths aremaintained for each source-destination pair, and the paths areselected in order of length, from shortest to longest.

• A connection is first attempted on the shortest path. If resourcesare not available on this path, the next shortest path is attempted.

• A path selection policy based on path congestion examines theavailable resources on each of the alternate paths, and chooses thepath on which the highest amount of resources are available.

•

Choosing the shortest-path route consumes less network resources,but may lead to high loads on some of the links in the network,while choosing the path with the least congestion leads to longerpaths, but distributes the load more evenly over the network



Unconstrained Routing• Another adaptive routing approach utilizing global information is

unconstrained routing which considers all possible paths between a

source node and a destination node.

• In order to choose an optimal route, a cost is assigned to each linkin the network based on current network state information, such aswave engt ava a ty on n s.

• A least-cost routing algorithm is then executed to and the leastcostroute . Whenever a connection is established or taken down, thenetwork state information is updated.

• Two examples of unconstrained routing approaches are link-state routing and distance-vector routing.



link-state routing• In a distributed link-state routing approach, each node in the

network must maintain complete network state information . Each

node may then find a route for a connection request.

• Whenever the state of the network changes, all of the nodes mustbe informed. Therefore, the establishment or removal of a lightpathn t e networ may resu t n t e roa cast o up ate messages to

all nodes in the network.

• The need to broadcast update messages may result in signficantcontrol overhead. Furthermore, it is possible for a node to have

outdated information, and for the node to make an incorrectrouting decision based on this information.



distance-vector approach• A distance-vector approach to routing with

global information is also possible .

• This approach doesn‘t require that each node

maintains complete link-state information at

each node.• Each node maintains a routing table which

indicates for each destination and on each

wavelength, the next hop to the destination

and the distance to the destination.



Problems with global information

Algorithms• While near-term emerging systems will be fairly

static, with lightpaths being established for long

periods of time, it is expected that, as networktrafic continues to scale up and become moreburst in nature, a hi her de ree of multi lexin

and flexibility will be required at the optical layer.

• Thus, lightpath establishment will become more

dynamic in nature, with connection requestsarriving at higher rates, and lightpaths beingestablished for shorter time durations.



Adaptive Routing Based on Local

Information• The alternative is to implement routing schemes which rely only on

local information.

• A number of adaptive routing schemes exist which rely on localinformation rather than global information.

•

The advantage of using local information is that the nodes do nothave to maintain a large amount of state information; however,routing decisions tend to be less optimal than in the case of globalinformation.

• Two examples of local-information-based adaptive routing schemesare alternate routing with local information, and deflection routing.



Alternate routing• While alternate-path routing schemes typically rely on

global information, variations exist which utilize only local

information.

• In this scheme, the choice of a route is determined by the.

• Two variations of the scheme are considered: the case inwhich wavelength availability information is known alongthe entire path,

• and the case in which only local information is available.



Alternate Routing• In the first approach, the decision making entity is

aware of the wavelength availability information for allof the links in each of the alternate paths.

,greatest number of wavelengths which are availablealong all of the links in its path.

• For example, in FIGURE(a), if we consider two alternateroutes from source node A to destination node D, withavailable wavelengths as shown on each link.



Alternate Routing



ALTERNATE ROUTING….• Then two wavelengths (1 and 3) are available

along the entire length of route 1, while only onewavelength (2) is available along the entire length

of route 2; thus, route 1 will be chosen.

• The limitation of basing the route selection

decision on full path information is that the

information may be difficult to maintain or

difficult to obtain in a timely manner.



Deflection Routing• Another approach to adaptive routing with limited information is

deflection routing, or alternate-link routing . This routing scheme

chooses from alternate links on a hop-by-hop basis rather thanchoosing from alternate routes on an end-to-end basis.

• The routing is implemented by having each node maintain a routingta e w c n cates, or eac est nat on, one or more a ternate

outgoing links to reach that destination.

• Other than a static routing table, each node will only maintain

information regarding the status of wavelength usage on its ownoutgoing links. When choosing an outgoing link for routing, thedecision can be determined on either a shortest-path or least-congested basis.



Shortest Path Routing



Shortest Path routing….• The default shortest path in this example is along the path A,B,C,D.

When the request reaches node C, it cannot continue over link CD,

since no common wavelength is available on linksAB, BC, and CD.

• The request is therefore deflected to node F, where it can continueto the destination along link FD. The wavelength selected for the

g tpat w e .

• Note that, in the absence of any deflections, the default routing forany connection will be shortest-path routing.

• Also, once the routing for a lightpath is deflected at a node, thedefault routing from the point of deflection onward will again beshortest-path routing if no further deflections take place.



shortest path criteria• Under the shortest path criteria, the routing scheme

will first attempt to choose the outgoing link whichresults in the shortest path to the destination.

,

then the routing scheme will attempt to choose analternate outgoing link which leads to the next shortestpath to the destination.

• The routing scheme proceeds in this manner until thedestination is reached or the connection is blocked.



Least Congested Path Routing



Least Congested Path Routing• In a least-congested deflection routing approach, the routing scheme

chooses, from among the alternate outgoing links, the link which has thelargest number of feasible wavelengths.

• The set of feasible wavelengths consists of the set of wavelengths whichare available on all of the previous hops as well as the next outgoing link.

•Least-congested deflection routing is illustrated in Fig(b) for a connectionfrom node A to node D. On the first hop, link AB is selected, since it hasthree available wavelengths, while link AE has only two availablewavelengths.

• When the connection request arrives to node B, it will be routed to nodeE, since there are three feasible wavelengths (1; 2; and 4) available on linkBE, and there is only one feasible wavelength (1) available on link BC.



Problems in deflection routing• Problem which may arise is that a connection request may be

deflected a large number of times, leading to an unreasonably long

route for the lightpath.

• Possible solutions to this problem include limiting the maximumlength or number of hops in a lightpath, or limiting the number of

e ect ons t at a route can ta e.

• When a connection request message reaches its limit on themaximum number of hops or deflections, the connection attemptwill be blocked.

• Further restrictions may also be placed on the selection of possibleoutgoing ports in order to prevent routes from heading backtowards the source node.



Wavelength Assignment• In general, if there are multiple feasible wavelengthsbetween a source node and a destination node, then awavelength assignment algorithm is required to select

a wavelength for a given lightpath.

• The wavelength selection may be performed eithera ter a route has been determined, or in parallel withfinding a route.

• Since the same wavelength must be used on all links in

a lightpath, it is important that wavelengths are chosenin a way which attempts to reduce blocking forsubsequent connections.



First Fit• One example of a simple, but effective,

wavelength-assignment heuristic is First-fit.

• In first-fit, the wavelengths are indexed, and alightpath will attempt to select the wavelength

a wavelength with a higher index.

• By selecting wavelengths in this manner, existing

connections will be packed into a smaller numberof total wavelengths, leaving a larger number of wavelengths available for longer lightpaths.



Random Selection• Another approach for choosing between

different wavelengths is to simply select oneof the wavelengths at random.



Most Used and Least Used• Other simple wavelength assignment heuristics include the most-

used-wavelength heuristic and the least used- wavelength heuristic.

• In most-used wavelength assignment, the wavelength which is themost used in the rest of the network is selected. This approachattempts to provide maximum wavelength reuse in the network.

• The least-used approach attempts to spread the load evenly acrossall wavelengths by selecting the wavelength which is the least-usedthroughout the network.

• Both most-used and least-used approaches require globalknowledge.



Wavelength Assignment…..• A number of more advanced wavelength assignment

heuristics which rely on complete network stateinformation have been proposed.

future lightpath connections is known in advance.

• For a given connection, the heuristics attempt to

choose a wavelength which minimizes the number of lightpaths in the set of future lightpaths that will beblocked by this connection.



Signalling and Resource Reservation• In order to set up a lightpath, a signalling protocol is required to

exchange control information among nodes and to reserveresources along the path.

• In many cases, the signalling protocol is closely integrated with therouting and wavelength assignment protocols.

• Signalling and reservation protocols may be categorized based onwhether the resources are reserved on each link in parallel,reserved on a hop-by-hop basis along the forward path, or reservedon a hop-by-hop basis along the reverse path.

• Protocols will also differ depending on whether global informationis available or not.



Parallel Reservation• The scheme, which is based on link-state routing, assumes that each nodemaintains global information on the network topology and on the currentstate of the network, including information regarding which wavelengthsare being used on each link.

• Based on this global information, the node can calculate an optimal routeto a destination on a given wavelength.

• The source node then attempts to reserve the desired wavelength on each

link in the route by sending a separate control message to each node inthe route.

• Each node that receives a reservation request message will attempt toreserve the specified wavelength, and will send either a positive or

negative acknowledgement back to the source.

• If the source node receives positive acknowledgements from all of thenodes, it can establish the lightpath and begin communicating with thedestination.

P ll l R ti Ad t d



Parallel Reservation Advantages and

Disadvantages• The advantage of a parallel reservation

scheme is that it shortens the lightpathestablishment time by having nodes process

.

• The disadvantage is that it requires global

knowledge, since both the path and thewavelength must be known ahead of time.



Hop-by-Hop Reservation• An alternative to parallel reservation is hop-by-hop reservation in

which a control message is sent along the selected route one hop ata time.

• At each intermediate node, the control message is processedbefore being forwarded to the next node.

•

When the control message reaches the destination, it is processedand sent back towards the source node.

• The actual reservation of link resources may be performed eitherwhile the control message is travelling in the forward direction

towards the destination, or while the control message is travellingin the reverse direction back towards the source.

F d R i



Forward Reservation

• In forward reservation schemes, wavelength resources arereserved along the forward path to the destination on ahop-by-hop basis.

• The method of reserving wavelengths depends on whetheror not global information is available to the source node.

•

If the source node is maintaining complete stateinformation, then it will be aware of which wavelengths areavailable on each link.

•Assuming that the state information is current, the sourcenode may then send a connection setup message along theforward path, reserving the same available wavelength oneach link in the path.



Forward Reservation• For the case in which a node only knows the status of

its immediate links, the wavelength selection becomes

more complicated, as the source node doesn't knowwhich wavelength will be available along the entire

ath.

• The source node may utilize a conservative reservation

approach, choosing a single wavelength and sendingout a control message to the next node attempting toreserve this wavelength along the entire path.

A i R i



Aggressive Reservation

• Multiple wavelengths may be reserved on each link inthe path, with the expectation that at least onewavelength will be available on all links in the path.

• In a greedy approach, all feasible wavelengths will bereserved at ever link in the ath.

• The source node will first reserve all availablewavelengths on the desired outgoing link.

• A connection request message containing thewavelength reservation information is then sent to thenext node along the path.

Forward Reservation of Wavelengths when



Forward Reservation of Wavelengths when

Establishing a Lightpath• At each intermediate node, the subset of wavelengths consisting of

the intersection of the wavelengths reserved on the previous link andthe wavelengths available on the next link will be reserved.

• For example, if Sn is the set of wavelengths available on the nth link,then the set of wavelengths reserved on the first link in the path willbe S1, the set of wavelengths reserved on the second link will be S1

.

• The set of wavelengths reserved on the third link would be S1 ∩ S2 ∩S3, etc.

•

When the connection request reaches the destination, onewavelength of the remaining set of wavelengths will be chosen, and allof the other wavelengths will be released.



FORWARD RESERVATION…



Disadvantage of Over Reservation

• One disadvantage of over-reserving resources

is that, during the time that the resources are

reserved, the reserved resources cannot be

utilized by other users, even if these resources

will never be used by the connection.



Disadvantages………..

• In order to reduce the amount of time that anunused wavelength is reserved on a link, the

wavelength may be released as soon as it isapparent that the wavelength is not viable for a

iven connection.

• For example, if wavelengths λ1; λ2, λ3; and λ4 areavailable on the first link, then all four of thewavelengths will be reserved on this link.

•



Second Approach



Second Approach

• One disadvantage of over-reserving resources is that, during the time thatthe resources are reserved, the reserved resources cannot be utilized byother users, even if these resources will never be used by the connection.

• In order to reduce the amount of time that an unused wavelength isreserved on a link, the wavelength may be released as soon as it isa arent that the wavelen th is not viable for a iven connection.

• For example, if wavelengths λ1; λ2, λ3; and λ4 are available on the firstlink, then all four of the wavelengths will be reserved on this link.

• However, if it is subsequently discovered that only λ1, λ2, and λ4 are

available on the second link, then not only will λ1, λ2, and λ4 be reservedon the second link, but λ3 will immediately be released on the first link.

Third Approach



Third Approach• Another approach to limiting the number of wavelengths that are

reserved is to divide the wavelengths into groups. When reservingwavelengths on a link, a node will reserve only those wavelengths whichbelong to a specific group.

• The choice of the group is made at the source node and is based on thenumber of available wavelengths in each group.

• The source node will find the group with the largest number of available

wavelengths, and the node will reserve all of the available wavelengths inthat group before sending the request on to the next node.

• The size of the group is a critical parameter.

• If the group is too large, then too many resources will be reserved, but if the group is too small, then the likelihood of establishing a lightpath willbe smaller.



Third Approach



Disadvantages of Forward Reservation

• However, there is no guarantee that the selectedwavelength will be available along every link in the

path.

,

select a different wavelength and reattempt theconnection.

• The limitation of this approach is that it may result inhigh setup times, since it may take several attemptsbefore a node can establish a lightpaths.

B k d R i



Backward Reservation• To prevent the over-reservation of resources altogether,

reservations may be made after the control message has reachedthe destination and is headed back to the source. Such reservationschemes are referred to as backward reservation schemes.

• By reserving wavelengths in the reverse direction, the reservedwavelengths are idle for less time than if the wavelengths are

v w .

• Another advantage is that the connection request message cangather wavelength usage information along the path in the forwarddirection.

• This information can then be used by the destination node toselect an appropriate wavelength to reserve.

B k d R i



Backward Reservation….

• Figure illustrates the backward reservation

scheme. In general, backward reservationschemes outperform forward reservation

wavelength conversion.

B k d R ti



Backward Reservation

E l ti



Explanation…….

• As the control message propagates from A to

D, it records the set of available wavelengths.

•Node D then chooses a wavelength andreserves the wavelength on each link by

sending a reservation message back towards

the source.

D b k f B k d R ti



Drawback of Backward Reservation

• One possible drawback of a backward reservation

scheme is when multiple connection are beingset up simultaneously.

• It is possible that a wavelength that was availableon a link in the forward direction will be taken by

another connection request and will no longer be

available when the reservation message traversesthe link in the reverse direction.

Holding Policies



Holding Policies

• To improve the connection setup probabilityat the cost of higher setup times, it is possibleto hold or buffer connection requests atintermediate nodes if wavelength resourcesare not immediately available .

• If an appropriate wavelength becomes

available, the connection request will continuetowards the destination.

Holding Policies



Holding Policies……

• If, after waiting for some time, the appropriateresources do not become available, then the

connection is blocked.

probability without significantly increasing setup time.

• However, it is also shown that a holding policy reduces

the network throughput compared to a policy in whichcalls are blocked immediately if resources are notavailable.

Switching Elements



g

• Obviously, switching elements are essential component of any network.

• The concept of switching originated from electronics field. Thus, accordingto the signal carriers, there are optical switching and electronic switching.

• In the switching granularity point of view, there are two basic classes:circuit switching and cell switching.

• In optical field, circuit switching is corresponding to wavelength routing,

and cell switching is optical packet switching and optical burst switching.

• As far as the transparency of signals is considered, there are opaqueswitching and transparent switching.

• In the section, switching devices are classified into two basic classes: logicswitching and relational switching.

Optical Add-Drop Multiplexers



(OADM)• Optical Add/Drop Multiplexers (OADMs) are

elements that provide capability to add and drop

traffic in the network .

(bidirectional) fiber pairs and enable a number of wavelength channels to be dropped and added,thereby reducing the number of unnecessaryoptoelectronic conversions, without affecting thetraffic that is transmitted transparently throughthe node

OADMs



OADMs

• An OADM can be used in both linear and ring network architectures and inpractice operates in either fixed or reconfigurable mode.

• In fixed OADMs, the add/drop and through channels are predeterminedand can only be manually rearranged after installation.

,transparently through the node can be dynamically reconfigured asrequired by the network.

• These are more complex structures but more flexible as they provide on-demand provisioning without manual intervention; therefore, they can beset up on the fly.

• The reduction of unnecessary optoelectronic conversions through the useof OADMs introduces significant cost savings in the network.

OADM Figure



OADM Figure

Types of Reconfigurable OADMs



• Reconfigurable OADMs can be divided into two categories:one is partly reconfigurable architecture and the other isfully reconfigurable architecture.

• In partly-reconfigurable architectures, there is capability toselect the channels to be added/dropped, but there is alsoa redetermined connectivit matrix between add droand through ports, restricting the wavelength assignment

function.

• Fully-reconfigurable OADMs provide the ability to selectthe channels to be added/dropped, but they also offerconnectivity between add/drop and through ports, whichenables flexible wavelength assignment with the use of tunable transmitters and receivers

Types of Fully Reconfigurable OADMs



Types of Fully Reconfigurable OADMs

• The two most common examples of fully-

reconfigurable OADMs, i.e., wavelength

selective (WS) and broadcast selective (BS)

architectures.

WS OADM



WS OADM



• The WS architecture utilizes wavelengthdeMultiplexing/multiplexing and a switch fabric

interconnecting express and add/drop ports.

• The BS is based on passive splitters/couplers andtunable filters. The overall loss introduced by thethrough path of the BS solution is noticeablylower than the loss of the WS approach,

significantly improving the optical signal-to-noiseratio (OSNR) of the node

BS Architecture



BS Architecture

Optical Cross-Connect (OXC)



• An optical cross connect (OXC) switches optical

signals from input ports to output ports.

•

These type of elements are usually consideredto be wavelength insensitive, i.e., incapable of

demultiplexing different wavelength signals on

a given input fiber.

OCX Types



OCX Types

Types of OXC



Types of OXC

• Opaque OXCs are either based on electricalswitching technology or on optical switch fabricssurrounded by optical-electrical-optical (OEO)conversions, imposing the requirement of expensive optoelectronic interfaces.

• In transparent OXCs, the incoming signals arerouted through an optical switch fabric withoutthe requirement of optoelectronic conversions,

thereby offering transparency to a variety of bitrates and protocols

In OXC architectures, optical crosspoint elements have



been demonstrated using two types of technologies:• (1) the generic directive switch, in which light

is physically directed to one of two differentoutputs.

• (2) the gate switch, in which optical amplifier

gates are used to select and filter input signals

to specific output ports.

Other Switches Switches



• Other types of switches include the

mechanical fiber-optic switch and the thermo-

optic switch.

• These devices offer slow switching (about

milliseconds) and may be employed in circuit-

switched networks

Thermo Optic Waveguide Switches



p g

• Thermo-optic waveguide switches, on the

other hand, are fabricated on a glass substrateand are operated by the use of the thermo-

.

Gate Switches



• In the N x N gate switch, each input signal firstpasses through a 1 x N splitter.

• The signals then pass through an array of N 2 gate ,

combiners and sent to the N outputs.

• The gate elements can be implemented using

optical amplifiers which can either be turned onor off to pass only selected signals to the outputs.

Gate Switches…..



• The amplifier gains can compensate for couplinglosses and losses incurred at the splitters and

combiners.

Figure.

• Disadvantage of the gate switch is that the

splitting and combining losses limit the size of theswitch.

2 x 2 amplifier gate switch



A three stage Clos Architechture



Micro-Electro Mechanical Systems



(MEMS)• Currently, micro-electro mechanical systems (MEMS) is widely

believed to be the most promising method for large-scale opticalcross-connects.

• Optical MEMS-based switches are distinguished in being based onmirrors, membranes, and planar moving waveguides. The formertwo are ree-space sw tc es; t e atter are wavegu e sw tc es.

• Furthermore, MEMS-based switches are classified into the twomajor approaches, i.e., 2-Dimensional and 3-Dimensionalapproaches.

• Among these classifications, the 3D optical MEMS based on mirrorsis popular because it is suitable for compact, large scale switchingfabrics.

MEMS….



• The ability of this architecture to achieve input- and output-portcounts of over one thousand is the primary driver of the large scaleOXCs, in which spatial parallelism is utilized.

• In particular, the t pe of switch provides hi h application flexibilitin network design because of low and uniform insertion loss with

low wavelength dependency under various operating conditions.

• Furthermore, this switch exhibits minimal degradation of the

optical signal-to-noise ratio, which is mainly caused by crosstalk,polarization dependent loss (PDL), and chromatic and polarizationmode dispersions.

MEMS…..



• The optical signals passing through the optical fibers at the inputport are switched independently by the mounted MEMS mirrorswith two axis tilt control and then focused onto the optical fibers at

the output ports.

• In the switch, any connection between input and output fibers cane accomp s e y contro ng t e t t ang e o eac m rror.

• As a result, the switch can handle several channels of optical signalsdirectly without costly optical-electrical or electrical-opticalconversion.

• The 3D MEMS-based O-O-O switch has been built in sizes rangingfrom 256 x 256 to 1000 x 1000 bi-directional port machines.

Mems….



• In addition, encouraging research seems to show that8000 x 8000 ports will be practical within theforeseeable future.

• An o-o-o switch is also scalable in terms of throughput.

• A truly all-optical switch is bit-rate and protocolindependent.

• The combination of thousands of ports and bit-rateindependence results in a theoretically future-proof switch with unlimited scalability.

Schematic diagram of a 3D MEMS



optical switch.



• They can lead to compact and stable opticalcrossconnect switches for simple, fast, and flexible

wavelength applications in today's optical networks.

• Optical MEMS are miniature devices with optical,electrical, and mechanical functionalities at the same

time, fabricated using batch process techniquesderived from microelectronic fabrication.

•

Optical MEMS provide intrinsic characteristics for verylow crosstalk, wavelength insensitivity, polarizationinsensitivity, and scalability.

Wavelength Router



• A wavelength-routing device can route signals

arriving at different input fibers (ports) of the

device to different output fibers (ports) basedon the wavelengths of the signals.

• Wave engt routing is accomp is e y

demultiplexing the different wavelengths fromeach input port, optionally switching each

wavelength separately, and then multiplexing

signals at each output port.

Reconfigurable routers



• The device can be either nonreconfigurable, inwhich case there is no switching stage between

the demultiplexers and the multiplexers, and theroutes for different signals arriving at any input

r r x v r r rr

routers rather than switches).

• Second type is reconfigurable, in which case the

routing function of the switch can be controlledelectronically

Non Cofigurable Wavelength RouterA fi bl l h b



• A nonreconfigurable wavelength router can beconstructed with a stage of demultiplexers whichseparate each of the wavelengths on an incoming fiber,

followed by a stage of multiplexers which recombinewavelengths from various inputs to a single output.

•

The outputs of the demultiplexers are hardwired to theinputs of the multiplexers.

•

Let this router have P incoming fibers, and P outgoingfibers. On each incoming fiber, there are M wavelengthchannels.



• The router is nonreconfigurable because thepath of a given wavelength channel, after it

enters the router on a particular input fiber, isfixed.

• The wavelengths on each incoming fiber areseparated using a grating demultiplexer.

• And finally, information from multiple WDM

channels are multiplexed before launchingthem back onto an output fiber.

4x4 non configurable router



Reconfigurable Wavelength-Routing Switch

• A reconfigurable wavelength routing switch



A reconfigurable wavelength-routing switch(WRS), also referred to as a wavelength selectivecrossconnect (WSXC), uses photonic switches

inside the routing element.

• T e R as ncom ng ers an outgo ng

fibers. On each incoming fiber, there are Mwavelength channels.

• Similar to the nonreconfigurable router, thewavelengths on each incoming fiber areseparated using a grating demultiplexer.

RECONFIGU………..

Th t t f th d lti l di t d t



• The outputs of the demultiplexers are directed to

an array of M P x P optical switches between the

demultiplexer and the multiplexer stages.

• s gna s on a g ven wave eng are rec e o

the same switch.

• The switched signals are then directed to

multiplexers which are associated with the

output ports.



• Such switches based on relational devices are

capable of switching very high-capacity

signals.

• The 2 x 2 crosspoint elements that are usedto build the space-division switches may be

slowly tunable and they may be reconfigured

to adapt to changing traffic requirements.

A P x P reconfigurable wavelength-routing switch with

M wavelengths.



• Networks built from such switches are more



Networks built from such switches are moreflexible than passive, nonreconfigurable,wavelength-routed networks, because they

provide additional control in setting upconnections.

• The routing is a function of both thewavelength chosen at the source node, as wellas the configuration of the switches in thenetwork nodes.

• Besides wavelength routing Optical Packet



Besides wavelength routing, Optical PacketSwitching (OPS) and Optical Burst Switching (OBS)have attracted interest in both research andindustry.

• , ,the control of the switching operation isperformed electronically.

• By employing the electronic control, relationship

of data switching is established in optical domain.

Wavelength Conversion

• Consider the network in Figure. It shows a wavelength-routed networkcontaining two WDM crossconnects (S1 and S2) and five access stations (A



g gcontaining two WDM crossconnects (S1 and S2) and five access stations (Athrough E).

• Three lightpaths have been set up (C to A on wavelength λ1, C to B on λ2,and D to E on λ1).

•

To establish a lightpath, we require that the same wavelength be allocatedon all the links in the path. This requirement is known as the wavelength-continuity constraint.

•This constraint distinguishes the wavelength-routed network from acircuitswitched network which blocks calls only when there is no capacityalong any of the links in the path assigned to the call.

An all-optical wavelength-routed network.



Wavelength-continuity constraint in a wavelength-

routed network.



•

Consider the example in Fig. 2. Two lightpathshave been established in the network: (i)

λ1, and (ii) between Node 2 and Node 3 onwavelength λ2.

Fig 2: without converter



• Now suppose a lightpath between Node 1 and Node 3needs to be set up.

• Establishing such a lightpath is impossible even thoughthere is a free wavelength on each of the links along the

.

• This is because the available wavelengths on the two linksare different.

• Thus, a wavelength-continuity network may suffer fromhigher blocking as compared to a circuit-switched network.

Wavelength Conversion

• It is easy to eliminate the wavelength-



It is easy to eliminate the wavelengthcontinuity constraint, if we were able to

convert the data arriving on one wavelength

along a link into another wavelength at an

next link.

• Such a technique is referred to as wavelength

conversion.

Wavelength Conversion……



Wavelength convertor



• A wavelength converter at Node 2 is employed to convert data fromwavelength λ2 to λ1.

• The new lightpath between Node 1 and Node 3 can now beestablished by using the wavelength λ2 on the link from Node 1 toNode 2, and then by using the wavelength λ1 to reach Node 3 from

o e .

• Notice that a single lightpath in such a wavelength-convertiblenetwork can use a different wavelength along each of the links inits path.

• Thus, wavelength conversion may improve the efficiency in thenetwork by resolving the wavelength conflicts of the lightpaths.

Functions of wavelength converter

h f f l h



• The function of a wavelength converter is to convertdata on an input wavelength onto a possibly differentoutput wavelength among the N wavelengths in thesystem.

• In this figure λs, denotes the input signal wavelength;

λc the converted wavelength; &, λp the pumpwavelength.

• Fs the input frequency; fc,, the converted frequency;fp, the pump frequency; and CW, the continuous wave(unmoddated) generated as the pump signal.

Properties of wavelength converters



• transparency to bit rates and signal formats fast setuptime of output wavelength,

• conversion to both shorter and longer wavelengths,• moderate input power levels,

• possibility or same input and output wavelengths (i.e.,

no conversion),• insensitivity to input signal polarization,

• low-chirp output signal and large signal to- noise ratio,

and• simple implementation.

Routing and Wavelength Assignment

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