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32 Optical Networks Magazine March/April 2003

Provisioning and restorationin the next-generationoptical core

Lu Shen

Byrav RamamurthyDepartment of Computer Science and EngineeringUniversity of Nebraska – LincolnLincoln, NE USA

1 Introduction

As today’s ever-increasing IP-centric data becomesthe predominant traffic in the metro/backbonenetworks, legacy transport networks designed

for voice-predominant or leased line data traffic are fac-ing pressures from the demands of increased bandwidthand dynamic services. WDM appears to be a promisingtechnology to meet the bandwidth requirement in thenext-generation optical networks. Research is underwayto develop intelligent control planes for the metro/back-bone optical WDM networks. There is an emerging con-sensus that incorporating GMPLS technology into theoptical control plane can lead to an intelligent opticalcore, which provides customers with an automated andreal-time service provisioning, as well as increased opera-tional flexibility, enhanced network survivability andinteroperability.

ABSTRACTResearch is underway currently to develop intelligent

control planes for the next-generation optical transport

networks, which can provide customers with automatic,

flexible, and real-time provisioning as well as enhanced

network survivability and interoperability. An intelligent

optical core appears to be viable by incorporating

Generalized Multiprotocol Label Switching (GMPLS)

technology into the optical control plane and deploying

reconfigurable optical network elements, such as recon-

figurable optical crossconnects, tunable transceivers,

and reconfigurable optical add-drop multiplexers. Much

of the work in this area has focused on proposing net-

work architectures, solving the dynamic RWA problem,

developing distributed protection/restoration schemes,

standardizing network interfaces (e.g. UNI and NNI),

and extending existing Internet routing /signaling proto-

cols for WDM optical networks. In this paper, we present

an overview of the role of GMPLS in the next-generation

optical core, concentrating on both the issues and the

challenges in automatic lightpath provisioning and net-

work restoration. First, we discuss the evolutionary trend

and architectures of the next-generation optical network.

Then, we present an overview of dynamic provisioning

problems, followed by a discussion of various con-

straints and unique requirements for lightpath establish-

ment in WDM optical networks. We close by discussing

the challenges in optical network restoration.

Optical Networks Magazine March/April 2003 33

In WDM optical networks, data can be transmittedover hundreds of fibers, each of which is capable of carry-ing up to tens of different wavelengths (frequencies) withbit rates from 2.5 Gbps to 10 Gbps simultaneously [1].A lightpath in a WDM optical network is an end-to-endtunnel, usually along a single wavelength (maybe severalwavelengths), spanning several links, which are con-nected along the way from the source to the destination.Optical crossconnects (OXCs) are able to switch a wave-length from an input to an output. In this paper, we useOXC to specify all the categories of optical crossconnects,irrespective of the internal architecture. Reconfigurableequipments, such as programmable OXCs, tunable opti-cal transceivers, tunable filters, reconfigurable opticaladd-drop multiplexers (OADMs) and so on, allow opticaltransport networks to be automatically manageable, incontrast to the current statically configured transport net-works. With those equipments available for use, an intelli-gent control plane is necessary to coordinate the operationof the network.

Multiprotocol Label Switching (MPLS) technologyevolved from tag switching with the original aim to im-prove packet-forwarding efficiency of the switching routers[2]. Its capability for implementing traffic engineering wasfound later. In an MPLS-based network, the explicitlyrouted point-to-point paths can be accomplished and arereferred to as explicitly routed label switched paths (LSPs).The detailed specifications of MPLS are beyond the scopeof this paper and can be found in recently published books[2], papers, and IETF drafts. Generalized MultiprotocolLabel Switching (GMPLS) is formulated by extendingMPLS to encompass time-division (e.g. SONET ADMs),wavelength-division (e.g., optical wavelength or lambda),and spatial switching (e.g., incoming port or fiber). ForWDM optical networks, the label operation becomes theoperation for optical wavelengths by implicitly using wave-lengths as labels at OXCs. Substantial efforts have been ex-pended on extending MPLS protocols to support the inte-gration of GMPLS into optical control plane [3-8]. Thesolutions discussed in those papers are leveraging the exist-ing MPLS protocol suite. Explicit lightpath computation isimplemented at source node, with the aid of extended IPlink state routing (e.g. OSPF or IS-IS) and constraint-based RWA algorithms. The lightpath establishment andteardown can be achieved by signaling protocols, such asextensions to RSVP-TE or CR-LDP [9,10].

Due to the inherent difference between packet-switched Internet and wavelength-routed optical trans-port networks, however, GMPLS can only subsume asubset of functionality of MPLS. Furthermore, the physi-cal constraints, some peculiarities of routing and wave-length assignment algorithm (RWA), and the special re-quirements of protection/restoration in optical networksimpose additional challenges in designing GMPLS-basedoptical networks. Many issues and challenges arise fromthe unique characteristics of optical transport networks.

Some of them appear to be solvable. For example, linkbundling is proposed to enhance the scalability of routingand signaling in optical networks [30]. And, link man-agement protocol (LMP) is proposed to maintain connec-tivity status of links and channels between two adjacentnodes [35]. Some issues are still not clearly understood; forinstance how to detect and localize fault in all-optical net-works is still unclear. In this paper, we mainly concentrateon the issues and challenges in integrating GMPLS intoan intelligent optical core. We only consider the non-packet (i.e., circuit switching) forms of optical switchingin mesh networks.

The remainder of this paper is organized as follows:Section 2 describes the evolutionary trend and architec-tural alternatives of the next-generation optical transportnetworks. Section 3 presents an overview of automaticprovisioning in optical networks, taking into account theRWA problem, physical layer impairment constraints,wavelength conversion capability, link bundling and linkmanagement protocol, inter-domain routing and signal-ing, bi-directional lightpath establishment, and crankbackrouting. Section 4 presents issues and challenges in net-work survivability. Section 5 concludes this paper.

2 ArchitectureDramatic increase in data traffic, driven primarily by

the explosive growth of the Internet, is challenging today’stransport networks, which are essentially statically config-ured and voice-optimized. In addition to the bandwidthproblem, the slow provisioning time provided by cur-rent transport networks cannot meet the demand fromthe frequently changing IP services. Today’s transport net-works consist of SONET ring and are widely deployed inthe metro networks, which are connected by backbonenetworks. Data traffic in such networks is carried onleased circuit through TDM channels. Given the inher-ently bursty nature of IP data traffic, the fixed-bandwidthpipes of TDM transport may not be an efficient solu-tion. As indicated in [12], the traditional approach ofbuilding SONET-based ring networks fails to handle cur-rent traffic growth because of its long deployment time,difficulty of equipment scaling, and high operationalcosts. In contrast, making use of GMPLS in WDM op-tical networks has the potential to provide ATM-like QoScapability and SONET-competitive restoration time, re-sulting in an efficient optical core with unlimited band-width. Figure 1 shows the expected evolution of transportnetworks from a layered perspective. Figure 1(a) is thecurrent status of layered transport networks. Althoughvarious approaches can be chosen to meet the require-ments for different types of traffic, multi-layering be-comes an overhead for the transport networks. For exam-ple, a low efficiency is introduced by layering best-effortIP data over ATM or SONET (or both), because theATM layer brings in significant overhead to meet QoS re-quirements and SONET is voice-optimized. In the next-


generation networks, as shown in Figure 1(b), a simpler,lower cost, and more responsive network with two layers(MPLS-based IP layer and smart GMPLS-based opticallayer) is likely to replace the four layered transport net-works. In the expected future network architecture asshown in Figure 1(b), the ATM-like QoS can be imple-mented at IP/MPLS layer and SONET-competitiverestoration capability can be achieved at the GMPLS-based optical layer.

Hence, the transport infrastructure is moving towardsa model of high-speed routers interconnected by intelligentoptical core networks. The next step is to interconnectthese two layers, resulting in automatic end-to-end connec-tivity via standardized routing and signaling methods.Three interconnection models, i.e. peer model, overlaymodel, and augmented model, were first proposed in [11].Under the peer model [11], the IP/MPLS layer nodes act aspeers of the nodes in optical transport networks, such that asingle routing protocol instance runs over both IP/MPLSand optical domains. The interior gateway protocol, suchas OSPF or IS-IS, with appropriate extensions, can be usedto distribute topology information. Under the overlaymodel [11], the IP/MPLS routing, topology distribution,and signaling protocols are independent of those at the op-tical layer. Each layer defines its own approaches for routingand signaling. Under the augmented model [11], althoughIP and optical domain use separate routing instances, infor-mation from one routing instance is passed through to theother, depending on the administration policies and otherfactors. This is analogous to the routing approach in today’sInternet, where different interior gateway routing protocolsmay be used in different autonomous systems (ASs), whileinterdomain routing is used to exchange reachability infor-mation between ASs, such that each AS can have summaryinformation about others.

The peer model is not practical for implementationdue to its poor scalability and high complexity. The over-lay model imposes administrative control boundaries be-tween core and edge, strictly limiting the network infor-mation within each domain. As a result, explicit routing

across several domains is difficult to implement. Onepoint of view is that the augmented model appears tomeet the requirement to achieve a simple, effective, andscalable transport network. However, as discussed in [13],diversity of protocol stacks, architecture choices, and net-work applications in the transport networks will continueto exist in the near future. So, although the augmentedmodel would seem superior architecturally, the overlaymodel will prevail in practical networks for many yearsdue to the existence of these diversities.

3 Issues and Challenges in AutomaticProvisioning

Provisioning of new services in today’s transport net-works involves activities, such as adding new access devicesand ADMs, mapping new paths, verifying link connectiv-ity, signing up Service Level Agreement (SLA) and so on.These processes are extremely manual and generally takeseveral weeks (or months) to accomplish. By employingdistributed GMPLS optical control planes, all of theseprocesses (or part of them) can be automatically imple-mented, resulting in faster provisioning, lowered operatingcost, and improved network resource management.

However, some unique characteristics and require-ments are critical for the implementation of automaticprovisioning in the GMPLS-based optical core and willcause vital problems if they are not adequately addressed.We will review issues and challenges in automatic provi-sioning in this section: we give an introduction to therouting and wavelength assignment problem in Section3.1; we introduce physical layer impairment constraint inWDM optical networks in Section 3.2; we present the is-sues regarding wavelength conversion in Section 3.3; wediscuss the routing scalability and link bundling in Section3.4; we introduce the link management protocol in Sec-tion 3.5; we present the issues in bi-directional lightpathestablishment and crankback routing in Section 3.6and Section 3.7 respectively; we discuss the challenges ininter-domain routing and signaling in Section 3.8.

Figure 1: The evolution of transport networks — a layered perspective.


3.1 Routing and wavelength assignment(RWA)3.1.1 Background

In order to implement automatic provisioning, therouting and wavelength assignment (RWA) problem mustbe solved for each lightpath connection request either inan on-line or in an off-line manner. RWA is referred to asthe scheme/algorithm to determine a route and corre-sponding wavelength on each fiber along the route for thelightpath. The objective of a RWA problem is to optimizethe network performance, in terms of network blockingprobability or network utilization. The RWA problem forstatic traffic is known as the static lightpath establishment(SLE) problem. For dynamic traffic, the RWA problem isreferred to as the dynamic lightpath establishment (DLE)problem, which is aimed at setting up a lightpath in amanner so as to reduce the network blocking probabilitywhen a connection request arrives.

In earlier work, the SLE problem has been formulatedas an integer linear program, which is NP-Complete. In[1], a multi-commodity formulation combined with ran-domized rounding is introduced to calculate the routes forlightpaths. Wavelength assignments are performed basedon graph-coloring techniques. The work in [14] developsan integer linear programming (ILP) formulation of theRWA problem, from which it derives a generic RWA algo-rithm’s performance bounds — an upper bound on thecarried traffic, or equivalently, a lower bound on the light-path blocking probability. Today’s voice-optimized metroand backbone transport networks are designed mainly forthe highly predictable voice data and leased line traffic.Provisioning in such networks can be implemented bysolving SLE problem when the networks are built up.

However, the next-generation optical core is designedas an IP predominant network, where IP-based traffic isdifficult to predict and model. So, the RWA problem insuch an IP-centric data transport network cannot be for-mulated as a SLE problem. Instead, the DLE problem hasto be solved to achieve automatic lightpath establishmentfor the purpose of a fair network performance. For thisreason, recently more attention has been paid for develop-ing heuristics to solve the DLE problem. Due to lack ofknowledge about the network traffic matrix, the DLEproblem is harder than the SLE problem, which is alreadyNP-Complete. If distributed connection management isemployed in the next-generation optical core, anotherproblem arises: how much information does each node inoptical networks need to know in order to solve the RWAproblem? Maintaining a global knowledge of the networkresources has the potential to achieve better network per-formance, but may result in reduced scalability and in-creased overhead.

To establish a lightpath, an optical network normallyrequires a common wavelength to be assigned on all thelinks on the route. This requirement is known as thewavelength-continuity constraint and such a network is

called wavelength-continuous network. The wavelength-continuity constraint is eliminated, if the data arriving ona wavelength at an input port can be transferred on an-other wavelength at the output port at an OXC node.Such a technique is referred to as wavelength conversion[19] and wavelength converters are the devices to operatewavelength conversions. Optical switches embedded withwavelength converters can provide wavelength conver-sion capability to a network [19]. Such a network is calleda wavelength-convertible network.

3.1.2 Heuristics for DLEEssentially, heuristics divide RWA problem into two

independent sub-problems: routing and wavelength as-signment. The routing schemes are classified into threeclasses: fixed routing, fixed-alternate routing, and adaptiverouting [15]. In the fixed routing, the route for each light-path is fixed and computed before connection requests ar-rive. The fixed-alternate routing keeps a set of pre-selectedalternative routes for each lightpath. In case a lightpath es-tablishment fails on one route, another route may be cho-sen from the set of alternative routes. The adaptive rout-ing dynamically selects a route for each lightpath when aconnection request arrives. Given a route for a lightpathin a wavelength-convertible network, the wavelength as-signment becomes straightforward by allocating any oneof available wavelengths on the link to the lightpath. So,wavelength assignment algorithms are mainly referred toas schemes proceeding under the wavelength continuityconstraint.

Numerous wavelength assignment algorithms andtheir variants exist in the literature, such as First-Fit (FF),Random, Least-Used (LU), Most-Used (MU), Least-Loaded (LL), Max-SUM (MS), and Relative Capacity Loss(RCL). The work in [15] showed that RCL has the best per-formance among them and FF is the simplest one to imple-ment with network performance close to RCL. In FF, allthe wavelengths are indexed with consecutive integernumbers and the wavelength with the lowest index amongall the available wavelengths is selected for the lightpathestablishment. The MAX_SUM algorithm was first pro-posed in [17] for WDM ring and tori networks. In [16],MAX_SUM is slightly changed to become RCL. BothMAX-SUM and RCL are designed for assigning wave-lengths under fixed or fixed-alternate routing schemes. Asan improvement to RCL, the distributed RCL (DRCL) isproposed for adaptive routing in [15] and found to havethe best performance among those previously proposed.The better performance of DRCL is originated from itsadaptive routing, which considers network load whenselecting a route. For a better understanding of theseheuristics, please refer to [15–17].

The simulation experiments in [18] showed that theheuristic simultaneously considering both routing andwavelength assignment is better (in terms of network uti-lization) than the one separately solving them when com-


puting lightpaths. This requires a global knowledge ofwavelength usage in the whole network. By flooding linkstate advertisement (LSA) in the network, each node cankeep the same picture of the complete network resources.This method can yield a fair network performance and ob-taining global knowledge of the network resources in anoptical network appears to be feasible by extended Internetinterior gateway protocols (e.g. OSPF or IS-IS). As a result,some new distributed adaptive routing and wavelengthassignment heuristics, based on global knowledge of thenetwork, have been proposed in the recent literature[18,21,22].

Although these heuristics can achieve a fair networkperformance, they have scalability problems due to therequirement of global knowledge of network state. In aWDM optical network employing hundreds of fibers withtens of wavelengths on each fiber, it is a great burden foreach node to maintain large amounts of information ofwavelength availability (and probably a routing table foreach wavelength). Furthermore, the requirement of largeoverhead capacity and relatively long routing convergencetime will cause the network performance to be lower thanexpected. Link bundling is a proposed solution to dealwith this scalability problem in the next-generation opticalcore. The aggregation due to link bundling may cause in-formation loss at the lower level link status. Link manage-ment protocol (LMP) has been proposed to solve thisproblem. The relevant discussion of link bundling andLMP is provided in the later sections (see Section 3.4 andSection 3.5)

3.2 Physical layer impairment constraintThe RWA problem as discussed above remains at a

theoretical level without considering optical layer impair-ments. However, physical layer impairment is a criticalconstraint for designing an automatic switched opticalnetwork in reality. To support high-speed end-to-enddata communication in a large-scale WDM optical net-work, a lightpath may traverse a long distance usingwavelength switching. The quality of the signal degradesas it travels through several fiber spans and optical com-ponents. The optical amplifiers, e.g. erbium-doped fiberamplifiers (EDFAs), may compensate for some loss, butintroduce noise at the same time. Furthermore, the OXCintroduces crosstalk, which may interfere with a partic-ular channel. The accumulated impairments withoutregeneration will result in a high bit error rate (BER),which in turn increases the network blocking probability.If the BER of the signal in a lightpath at destination nodeis higher than a pre-defined threshold, the lightpath maybe blocked. In an opaque optical network, the regeneratorson each node conduct optic-electronic-optic (O-E-O) con-version and 3R regeneration (regeneration, reshaping,and retiming) to clean up the signals carried on the fiber.However, the expensive regenerators increase the cost ofthe network and the O-E-O conversion becomes a bottle-

neck, which limits the network data rate. In a transparentoptical network, lightpaths bypass the expensive elec-tronic signal processing at intermediate nodes. However,transparent optical networks are difficult to be practicallydeployed on a large scale due to the impairments.

Authors in [23] studied the impact of transmissionimpairments on the routing in the optical layer. The lin-ear and nonlinear impairments were investigated. Thelimitation of transparent fiber length is calculated by con-sidering the polarization mode dispersion (PMD), andthe maximum number of spans between optical ampli-fiers is calculated from the problem formulation of ampli-fied spontaneous emission (ASE). PMD and ASE are lin-ear impairments. It is more complex for routing to dealwith the nonlinear impairment, such as crosstalk. One ofthe solutions to conquer the impairments is to divide net-works into sub-domains with regenerators deployed ateach edge node, such that the scale of each domain issmall enough to limit impairments in an acceptable level.Regeneration only happens at each edge node when thelightpath exits or enters a sub-domain. In such a network,the signal quality is ensured so that dynamic routing doesnot need to consider impairment constraints. The impair-ment analysis in [23] offers some insight into designingsuch networks from a routing perspective.

Another solution relies on the concept of the recentlyproposed translucent network [24]. In a translucent opti-cal network, a signal is made to traverse in optical layer aslong as possible before its quality falls below a thresholdvalue. Because the signal is regenerated only if necessary,we need only sparse regeneration deployment in the net-work. Authors in [24] described the architecture of atranslucent optical network and proposed several algo-rithms to allocate the regeneration resources in the net-work. In a typical WDM optical network, most interme-diate nodes are attached to access nodes. Each access nodeinherently has a regenerator, which can be reused to con-duct regeneration on a bypass lightpath when it is idle.The authors of [25] propose two dynamic routing heuris-tics (MRHBC and MBRHC) fitted into a GMPLS-basedframework, taking into consideration regenerator’s loca-tion and availability, signal bit error rate (BER), andlightpath distance. In order to implement these twoheuristics, a logical network topology is generated basedon the knowledge of available regenerators’ location andwavelength availability. Lightpaths are calculated on thelogical topology with the objective of minimizing bothregenerator hops and BER. A lower network cost isachieved by dynamically using the idle regenerators in-herited from the access nodes, which are attached to theOXC nodes. In order to implement dynamic routing intranslucent networks, additional information, such aslocation of available regenerators, needs to be added intothe GMPLS routing protocols. However, a problem mayarise if there are no additional regenerators in the network.An OXC cannot drop a signal to the access nodes attached


to it, when all regenerators on this OXC are occupied bybypass lightpaths. Further study is needed to investigatethis problem.

3.3 Wavelength conversion capabilityAn OXC embedded with wavelength converters pro-

vides wavelength conversion capability, which may result inan improved network blocking performance [19]. Authorsin [20] give an extensive review on the importance of wave-length converters in dynamically reconfigurable WDMoptical networks. The opaque networks inherently havewavelength conversion capability at each intermediatenode due to the O-E-O operation. In translucent net-works, nodes with available regenerators have wavelengthconversion capacity. However, in all-optical transparent net-works, wavelength conversion capability depends on thefunction of optical switches. By sparsely deploying wave-length conversion nodes, the network performance of trans-parent networks can be dramatically increased [18]. In thiscase for RWA computation, each node should know thelocation of the nodes with wavelength conversion capabilityeither through manual configuration or by flooding routingmessages. The latter method needs an extension to the rout-ing protocol.

3.4 Scalability and link bundlingIn a DWDM optical network, two adjacent OXCs

may be connected by hundreds of fibers, each of whichmay have tens of wavelengths transmitted together. Thetraditional LSA update method for these network re-sources results in a large volume of control message over-head. It also causes scalability problems to interior gate-way protocols, such as OSPF and IS-IS. Link bundling[30] is proposed by IETF working group to improve net-work scalability by abstracting information on traffic engi-neering (TE) links. The routing messages are dramaticallyreduced by disseminating abstracted link information,rather than exchanging the status for all the links. The re-sources between two adjacent OXCs are identified by tu-ples in the form of �Bundled Link ID, Component LinkID, Label�. As defined in [31], a bundled link (or TElink) is a logical construct that represents a way togroup/map the information about certain physical re-sources and their properties. Link bundling abstracts theresources between two label-switched routers (LSRs) intodisjoint sets of component links. Each bundled link maycontain a set of component links. A combination of�Bundled Link ID, Component Link ID, Label� unam-biguously identifies the appropriate resources. Compo-nent links will not be disseminated in routing messagesinto the network. Determining a component link is a mat-ter for the local LSR. The rules for bundling must be con-sistent across all the LSRs in the same domain.

Now, we take a closer look at link bundling in WDMoptical networks using an example. In Figure 2, each OXC

connects to its neighboring OXCs with five fibers, repre-sented by {F1, F2, F3, F4, F5}. Each fiber can simultane-ously transmit 40 wavelengths, represented by {�i � 1 � i �

40}. The link bundling can be implemented according tothe following rules: every group of 5 fibers, which arelinked to the same neighboring OXCs, is mapped to abundled link (TE link); each fiber is mapped to a compo-nent link; every wavelength is mapped to a label. In thisway, network resources, i.e., wavelengths, can be identi-fied unambiguously. For instance, the �30 at fiber 4 fromOXC B to OXC A can be represented as �BA, 4, 30�.After link bundling is finished, the status of a bundledlink is defined as:• alive – if any one of the component links is alive;• dead – if all the component links are not alive.In this example, if a conduit cut causes all the fibers to godown between B and A, only one update message, insteadof five messages, is flooded into the network to informother nodes that link BA is down.

Another example is used to show that the selectiondecision on a component link occurs at local OXCs.When OXC D wants to set up a connection to OXC A,the following steps happen (only steps in the forward di-rection are detailed):• OXC D calculates a path and wavelength, e.g. D-B-A

and �30, according to its RWA algorithm.• OXC D reserves wavelength �DB, 130� according

to wavelength availability on each fiber in a local data-base and sends a signaling message to B (identifier insignaling message uses �bundled link ID, label�, e.g.�BA, 30�).

• OXC B reserves �BA, 4, 30� and sends a signalingmessage to A.

• OXC A configures related equipment and sends a con-firmation message back to D.

When several changes on wavelength usage at a bun-dled link occur simultaneously, one LSA update message,carrying the wavelength usage information of the bun-dled link, will be sent out. Without link bundling, morethan one LSA update message will be flooded into thenetwork. So, link bundling potentially reduces the over-head caused by the additional messages. Correspondingchange in routing and signaling protocols is needed forusing link bundling in a GMPLS-based optical network.For a detailed explanation of link bundling, please referto [30].

Figure 2: An example of link bundling.


3.5 Link management protocol (LMP)The aggregation of network resources by link bun-

dling results in loss of some of the link information, suchas the status of component links, since link bundling ab-stracts the network resource to only two levels, i.e., bun-dled link and label. Link management protocol (LMP) isdesigned to support maintaining a detailed level of net-work resources [35]. The connectivity of fibers (compo-nent links) between a pair of adjacent OXCs and theirport configuration at each side may be maintained by ex-changing LMP messages between two adjacent OXCs.The difference between LMP and other routing/signalingprotocols is that LMP is a protocol for two adjacentOXC, but routing and signaling protocols apply to all thenodes in the network. A key function of LMP is to testthe health of the data-bearing channels (wavelengths).Because the control channel and data channels in an all-optical transport network are very likely to be separate,the status of the control channel is not the same as that ofdata channels. So, sending hello messages periodically onthe control channel can only obtain the status of the con-trol channel, but not the status of the data channels. Oneof the key concerns of LMP is for testing the health ofeach data channel that is not in use. The core proceduresof LMP include [35]:• Control Channel Management

It uses a Config message and a fast keep-alive mechanismover the control channel between adjacent nodes.

• Link Property CorrelationThe link property correlation of LMP is designed toaggregate multiple data channels (ports or componentlinks) into bundled links and to synchronize the prop-erties of the bundled links.

• Link ConnectivityThe link connectivity verification is used to verify thephysical connectivity of the data-bearing links betweenthe nodes and to exchange the interface IDs. In-bandTest messages are sent over data-bearing channels andTestStatus messages are transmitted over the controlchannel.

• Fault managementFault management of LMP is used to exchange status ofdata-bearing channels and to detect and locate faults. Itrelies on physical layer mechanisms to detect faults.

All of the LMP messages for the above functionality,except for Test message of link connectivity, are sent overcontrol channels. In order to carry the Test message overdata-bearing channels, each OXC node should be able tosend and receive messages over any data link. This is anadditional requirement for all-optical switches. After thecontrol channel has been established between two nodes,the data link connectivity can be verified by exchangingTest messages over each data channel specified in the bun-dled link. The verification is done initially when bringingup a link and can be periodically conducted on the idledata channels between two OXC nodes.

When considering LMP in the design of wavelengthstate transition diagram, an additional wavelength statusneeds to be added into the wavelength status pool. Thecurrent status pool of wavelength state transition func-tion contains: idle, reserved, active, and down. A new sta-tus, under verification, needs to be added into the statuspool. How the OXC acts when a reservation request ar-rives to reserve the channel under verification becomes aproblem. This is not addressed in the existing IETF drafts.Two solutions may exist:• The OXC node holds the reservation message until the

verification ends.This increases the reservation time and thus reducesthe network utilization.

• The OXC ends the verification by sending an EndVerifymessage and then reserves the channel immediately.This assumes that the link connectivity configuration isnot changed since last verification. So, the channelconnectivity may be out-of-date.

No matter what decision is made to deal with thissituation, corresponding changes should be made on thewavelength state transition diagram.

3.6 Bi-directional lightpath establishmentA lightpath tends to be bi-directional in the optical

transport networks. A bi-directional path consists of twoassociated lightpaths in opposite directions routed overthe same set of nodes. The bi-directional lightpath estab-lishment operates under the risk of race conditions, inwhich the end nodes may assign two associated light-paths to different requests simultaneously. There are es-sentially two ways to solve this problem. The first one is togive up the lightpath establishment procedure if the racecondition happens. The second way is to set up some rulesof lightpath establishment for both end nodes to followduring the lightpath establishment procedure. For exam-ple, the higher addressed node always preempts the loweraddressed request when race condition occurs.

3.7 Crankback routingIn a distributed routing environment, the resource

information used to compute a constraint-based pathmay be out-of-date. This implies that a connection setuprequest may be blocked because a link or node along theselected path has insufficient resources. When a setupfailure occurs, a notification is returned to the setup ini-tiator (ingress LSR). In the current CR-LDP, the ingressLSR receiving the notification has to terminate the mes-sage and give up the LSP establishment. If the ingress orintermediate gateway LSR knows the location of theblocked link or node, the LSR can designate an alternatepath and then reissue the setup request, which can beachieved by the mechanism known as crankback routing[36]. Crankback routing requires notifying an upstreamLSR of the location of the blocked link or node. So, a


corresponding extension of signaling protocol is neces-sary to support crankback routing [36].

3.8 Inter-domain routing and signalingAs addressed above, the next-generation GMPLS-

based optical transport network may use routing protocolsto broadcast and maintain network resource status, basedon which the RWA algorithm can dynamically calculate aroute and corresponding wavelengths for each lightpath.Once a lightpath request arrives at the network, a signalingprotocol is initiated to reserve the network resources as-signed to this lightpath by the RWA algorithm. However,these processes are suited for use in a single domain, whichemploys the same routing and signaling protocols. The is-sues in inter-domain routing and signaling appear to bemore complex and need a careful consideration for thepurpose of automatic lightpath establishment in a globalenvironment. The need for inter-domain routing and sig-naling may be triggered by several reasons, such as exis-tence of administrative boundaries, requirement on scala-bility of routing and signaling, security and reliabilityconcern, and topology difference in different areas.

In today’s Internet, BGP or OSPF Areas are usedfor inter-domain routing. The summary of reachabilityinformation is exchanged among edge routers in theAutonomous Systems (ASs). The Internet IP forwardingis designed on the hop-by-hop basis, i.e. only the next-hop for the destination is provided at each node. But,optical circuit switching may require computing explicitconstraint-based routes at source nodes. MPLS’s looserouting allows a source node to specify a route for alightpath in terms of a sequence of optical domain num-bers. In loose routing, an abstract node represents agroup of nodes whose internal topology is opaque to theingress node of the LSP. Using this concept of abstrac-tion, an explicitly routed LSP can be specified as asequence of IP prefixes or a sequence of ASs [37].

Authors in [32] proposed two solutions, based onOSPF, for sub-network routing information exchangewithin the optical network owned and managed by a sin-gle carrier. Both of them can support loose routing atsource node. Each node maintains a complete networkstate information of its own domain and summary infor-mation of other domains. So, a lightpath spanning severaldomains is decided as follows: • The source node decides a route and corresponding

wavelengths in its domain and border nodes in otherdomains.

• The border nodes, selected in the first step, will makeRWA decisions in their own domains.

In most of the current work, signaling is assumed tohappen only in a single domain. Further research is neces-sary to study the signaling schemes across several adminis-trative domains, in which different signaling protocolsmay be employed. Network to network interface (NNI)defines the communication interface between border

OXCs in different domains. Optical NNI interface andsignaling requirements can be found in [33] and [34]. Todate, how to set up a lightpath crossing several domains(inter-domain signaling), has not been addressed in detailin the literature. In summary, how to interoperate with dif-ferent administrative domains, which may employ differ-ent routing and signaling protocols, remains a topic offurther research on automatic provisioning in next-generation optical networks.

4 Network SurvivabilityAnother key concern in WDM optical transport net-

works is network survivability, which is referred to as thecapability of continuous data transmission upon occur-rence of failure. In the context of a WDM optical transportnetwork, a duct cut implies loss of data at terabits persecond. Due to its paramount importance, substantial ef-forts have been made to find efficient methods to achieveoptical network survivability in the literature [40-48]. Pro-tection and restoration are two schemes for survivable net-works. In both cases, data transmission is switched from aworking channel to a backup channel upon occurrence offailure. A protection mechanism reserves some spare capac-ity before the occurrence of failure. A restoration schemesearches for backup resources after failure happens, andthus has a longer restoration time than the protectionscheme. In a WDM optical network, there is no clearboundary between these two schemes. We will use protec-tion and restoration interchangeably in this paper. Path-based protection/restoration refers to the schemes that usea backup path, which is link-disjoint with the workingpath, to recover the traffic from failure. Link-based protec-tion/restoration recovers from a link failure by routing thedata on a detour between the two end nodes on the linkdisrupted. Compared with a path-based scheme, link-based scheme has a faster restoration speed, but at theexpense of more spare capacity. We can also classify theprotection/restoration schemes as 1�1, 1:1, and 1:N. 1�1path-based protection uses dedicated network resource onboth the working and backup paths to transfer data simul-taneously. The destination node will choose one of thembased on the signal quality. 1:1 scheme pre-selects sparecapacity, but allows lower priority users to occupy it undernormal conditions. Upon a failure, it will preempt the net-work resources from the low priority user and switch thetransmission to backup channels. 1:N scheme allows Nusers share a backup resource. In case of failure, only one ofthe users can obtain the backup resources. Both 1:1 and1:N schemes need additional signaling effort to configurenetwork resources upon a failure.

The above discussion can be found in most of the pa-pers related to optical network survivability in the literature[40-48]. Actually, some of these approaches were originallydeveloped for ATM or SONET networks, where a surviv-able network is also called a self-healing networks. Theseconcepts can be applied directly to optical networks, be-


cause the survivable network design appears to be a logicalproblem. However, the unique properties of an all-opticalcore network require additional effort and impose chal-lenges when leveraging existing schemes. Challenges in-clude the difficulties in real-time fault detection at the op-tical layer, the requirements for physically diverse lightpathcomputation, the difficulty in achieving fast restorationspeed, etc. In Section 4.1, we give a brief introduction tothe distributed restoration schemes in self-healing net-works and indicate the possibility for them to be used inGMPLS-based optical networks. The sections followingSection 4.1 reviews some challenges in designing survivableall-optical networks.

4.1 Self-healing networkTwo key concerns in designing a self-healing network

are: (i) Efficient control with fast restoration procedure;(ii) Economic spare capacity assignment. Given the trafficmatrix, the latter one can be formulated as a linear pro-gramming problem, where the objective is to minimize thetotal spare capacity under a set of constraints [49–51].This can be implemented off-line at a central controller.But, the centralized restoration experiences a restorationspeed of minutes and is not suitable for a network with un-predictable traffic [56]. Distributed restoration algorithm(DRA), which can be classified into path-DRA and link-DRA, has been studied for many years [52,53,56]. As in-dicated by its name, the spare capacity is found by distrib-uted messages sent by the end node upon a failure. Thesimulation results in [52] showed that path-DRA can re-store failure within two seconds and achieve a near optimalspare assignment. However, 2-second restoration still maynot satisfy the fast restoration requirement for optical do-main. The idea of distributed preplanning is to preplan thespare capacity for expected failure, whose working proce-dure is similar to DRA, except that it is executed before theoccurrence of any failure [56]. Distributed preplanningwith fast restoration signaling protocol provides a solutionfor efficient network restoration, in which the spare capac-ity is effectively found by flooding messages in the wholenetwork before any failure and using a fast restoration pro-tocol to implement notification and switching upon a fail-ure. By this way, a fast restoration is implemented, becausethe relatively slow planning phase has finished before fail-ure happens. However, the unpredictability is not adesirable property of DRA for transport carriers.

p-Cycle, which is a link-based restoration technique inmesh networks, is proposed to solve both capacity andspeed problems in restoration [54]. It is shown to havea mesh-like capacity and ring-like restoration speed. In thep-Cycle scheme, a number of cycles are prepared for recov-ering from every link failure in the network. Optimalp-Cycle coverage discovery is an integer-programmingproblem. Distributed Cycle PreConfiguration (DCPC)[54] has been introduced as an approach to implementa self-organizing p-Cycle, where p-Cycle coverage is pre-

planned in a distributed manner automatically. DCPCworks as follows [54]: A cycler node is chosen in a round-robin fashion among all the nodes in the network and othernodes are regarded as Tandem nodes; The cycler nodefloods out messages to all the Tandem nodes; Tandemnodes will keep broadcasting messages until they return tothe cycler node, forming several cycles on the cycler; Somerules are applied to ensure that only simple cycles areformed; Based on the metrics of each cycle collected bythe flooding messages, the cycler node can make a local de-cision to choose the best cycle to serve as the p-Cycle; Aftereach node serves as a cycler node, the last cycler node willinitiate a global construction of best p-Cycle coverage; Theconcept of DCPC is similar to DRA preplanning, but theobjective is to find optimal cycle coverage; The simulationresults in [54] showed that DCPC can have a mesh-like ef-ficiency in terms of spare capacity assignment and a ring-like restoration speed (50-150 ms).

All the schemes discussed above are mainly designedin the context of SONET/SDH networks, where in-bandsignaling is easy to implement in the overhead bytes. It ispossible to map the concepts into WDM optical net-works if the control channel is terminated at the intelligentOXCs (However, the wavelength continuity constraint willbring complication to the distributed resource assignmentin optical networks). GMPLS routing protocols may be ex-tended to support DRA or DCPC. The challenges in map-ping these schemes into optical networks are addressed inthe following sections.

4.2 Restoration speed at the optical layerThe optical layer is located at the bottom in a network

from a layered perspective. It serves as the medium for thedata transfer from upper layers. If the restoration speed isnot fast enough at the optical layer, all the upper layers willbe affected by the disruption at the optical layer. As statedin [57], a 2-second outage will cause all the circuit-switched connection, private-line, and dial-up services tobe disconnected. So, two seconds becomes the connection-dropping threshold. Besides, some real-time services askfor a more critical restoration speed on the order of tens ofmilliseconds. On the other hand, the layers above the opti-cal level may have their own restoration schemes. Con-tention for network resources may happen when the opti-cal level restoration is not fast enough to avoid failuredetection at the upper layer. Escalation is proposed as thesolution for multi-layer efficient restoration, which willbring more complex design for the all-optical network[39]. The simplest and most efficient solution is to providefast restoration at the optical layer. Since SONET auto-matic switching protection (ASP) provides 50 ms restora-tion speed and has been proven to have a good effect inpractice, network designers aim at SONET-competitivesurvivability at the optical level.

DRA is shown to have a restoration speed of less thantwo seconds in mesh networks, but it is far greater than the

requirement of fast restoration (�50 ms) at the opticallayer. Pre-configured p-Cycle has a fast restoration speed(50-150 ms), but DCPC needs a sampling period of about1/3 second as stated in [54]. As a result, the convergencetime for DCPC may be on the order of seconds. If light-path connection requests arrive frequently, which is verylikely to happen in the IP-over-WDM context, the speedof DCPC in calculating new p-Cycles is not fast enoughto achieve real-time provisioning. The same problemalso remains for the DRA preplanning schemes.

Emulation experiments in fast restoration usingGMPLS have been conducted on the GMPLS prototypecontrol plane at AT&T labs [55]. The experimental resultsshowed a worst-case restoration speed under 200ms usingpath-based restoration, in which backup lightpaths are pre-planed and RSVP signaling is used to reserve wavelengthand configure OXCs upon detection of failure. Although alink-based restoration is faster than a path-based scheme, itis not practical in an all-optical network due to the difficultyin detecting failure at intermediate nodes. We will discussfault detection in the next section.

4.3 Fault detectionA SONET network uses in-band overhead informa-

tion to implement performance monitoring electrically atintermediate nodes. In all-optical networks, fault detec-tion becomes a challenge for bypass lightpaths at interme-diate nodes without electrical processing. So, link-basedrestoration is not practical until a solution for fast faultdetection at intermediate nodes in the optical domaincomes out. Measuring bit error rate (BER) is a popularmethod for performance monitoring in the electrical do-main, while the power measurement and optical spec-trum analysis are candidates for optical fault detection inoptical domain. We list some optical performance moni-toring techniques [38] and corresponding challenges fortheir deployment in an all-optical network:• Power Detection

Power detection measures the power against an expectedvalue over a wide band. However, it will take a relativelylong time to detect a slight decrease in power. In addi-tion, power detection is insensitive to any power reduc-tion, which is uniformly distributed over all bits.

• Optical Spectrum Analysis (OSA)The OSA scheme relies on the analysis of the opticalspectrum. Spectrum filters are used to isolate the spec-trum of signal and noise. As a result, optical signal-to-noise ratio (OSNR) can be measured and used as thethreshold to detect fault conditions. However, OSA is aslow procedure mainly used by operators. Additionalprogramming effort is needed in order to implement afast and automated OSA procedure.

• Pilot tones [58]Pilot tones are signals, which travel along the samewavelengths (channels) and nodes as the payload, butare distinguishable from payload. This is achieved by

modulating a low-frequency pilot tone (KHz) ontoeach wavelength of ongoing lightpaths. The frequenciesof the tones and the low-speed digital informationcarried by them can easily be monitored at variouspoints in the network using optical taps and inexpensivelow-speed detectors followed by narrow-band electricalfilters [59]. The introduction of a pilot control systemintroduces a new design criterion. The tone frequenciesmust be low enough so that they will not affect high-frequency payload, and must be high enough to not beaffected by slow gain dynamics in the EDFA. In adynamic environment, each pilot tone monitoring pointmust know the pilot tone frequency on each channel,since different frequencies may be used for pilot tones oneach wavelength. Thus, the pilot tone frequency for eachwavelength should be carried in the signaling messagesbefore a lightpath is established.

4.4 Physically diverse lightpathcomputation

A backup lightpath is required to be physically diversefrom its working lightpath. Two lightpaths are physicallydiverse if they are not subject to a single point of failure.The physical diversity ensures that the working and backuppaths will not be affected under a common failure, such asa conduit cut. A simple assumption is that the network isonly protected from a single link failure at the physicallayer. So, the failure of one of the physically diverse pathswill not affect the other. Since lightpath computation hap-pens at the OXC layer, knowledge of the underlying fiberconfiguration is required at the OXC layer.

An optical transport system may be connected by asequence of fiber cables, which are placed in a sequence ofconduits. The layer containing conduits is called the fiberspan layer. Two lightpaths, which are disjoint at the OXClayer, may be placed in the same conduit and thus are notphysically disjoint. Conduits are buried underground alonga right of way (ROW), which is normally obtained fromcompanies operating railroad, pipeline, etc. So, even if twofibers are located in different conduits, they may be buriedin the same pipeline, causing a single point of failure.

An example of physically diverse lightpath computa-tion is shown in Figure 3. At the OXC layer, for the light-path from node A to node E, there exist three choices forthe route: A-D-E, A-B-E, and A-C-E. Those three light-paths are node-disjoint from OXC layer perspective. How-ever, A-D-E and A-B-E share a common fiber span a-g,whose failure will cause both A-D-E and A-B-E to fail. So,A-D-E and A-B-E are not physically diverse paths. A-D-Eand A-C-E are a pair of physically diverse paths, which canbe used as working and backup paths for the connectionfrom A to E.

For the purpose of increasing network utilization, thelightpath computation is aimed at finding a pair of theshortest physically disjoint paths, whose cost is defined asthe sum of the cost on all the links of the two paths.



Given a directed graph, Suurballe’s algorithm [26,27] canbe used to find a pair of shortest link/node-disjoint paths.However, a pair of disjoint paths at the OXC layer does notimply that they are also physically disjoint. The work in[28] showed how to change Suurballe’s algorithm slightlyto find a pair of physically disjoint paths in the networkwith three kinds of fiber span configurations, includingfork, express, and standard configurations. The fiber spantopology is then limited to these three configuration typesin order to use the algorithm; however the types of fiberspan configuration in reality may be different.

A capacity-efficient distributed routing scheme hasbeen proposed in [60]. It tries to extend the currentOSPF protocol to implement a distributed method forfinding link-disjoint lightpaths and achieving an efficientspare capacity assignment at the same time. However, thescheme only finds the link-disjoint lightpaths at the OXClayer. Further studies are necessary to design a distributedrestoration scheme, which can find a physically diversebackup lightpath taking into consideration the sparecapacity assignment, wavelength usage, routing conver-gence time, and flooding message size [61].

The concept of Shared Risk Link Group (SRLG)was proposed to deal with the diverse routing problem[29]. Each link in the network is mapped to a set ofSRLGs, each of which represents an instance of possiblefailure at the optical fiber span layer. Two lightpaths thatdo not share SRLGs are regarded as SRLG diverse pathsand can be used as a pair of working and backup paths.The definition of SRLGs in a network is determined atthe installation time and will not change often. How todefine SRLGs in a network to achieve a fair survivabilityand network utilization remains a topic of further study.

5 ConclusionGMPLS appears to be a promising technology paving

the way toward an intelligent optical core. Some relevanttopics, such as RWA problem, routing and signaling,and network restoration, have been studied for many

years. However, solutions for those problems remain atthe theoretical level. More effort is needed to find practi-cal solutions, taking into account the unique character-istics of all-optical networks. We have discussed theissues and challenges in implementing an optical corenetwork from the automatic provisioning and the fastrestoration perspectives. Although most of the issues arelogical problems, some challenges, such as the fault detec-tion in optical domain, still await a good solution fromthe lower level (e.g., hardware devices). For carriers, thecost is a key concern for upgrading today’s transport net-work into an intelligent optical core. Besides technical is-sues, whether or not the solution can bring increased rev-enue becomes a very important criterion. We believe thatall the challenges presented in this paper, plus the costissue, need to be addressed in the design of the next-gen-eration GMPLS-based optical core.

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Lu Shen

[email protected]

Lu Shen is currently a Ph.D. candidatein the Department of Computer Science andEngineering at the University of Nebraska-Lincoln. His research interests include con-trol and management in WDM optical net-works, IP over WDM, and MPLS/GMPLSprotocol design. Before he joined the University of Nebraska-Lincoln,he had been with Nortel Networks and Motorola Software Center asa software engineer.

Byrav Ramamurthy

[email protected]

Byrav Ramamurthy received his B.Tech.degree in Computer Science and Engineer-ing from Indian Institute of Technology,Madras (India) in 1993. He received hisM.S. and Ph.D. degrees in Computer Sci-ence from the University of California (UC), Davis in 1995 and1998, respectively.

Since August 1998, he has been an assistant professor in theDepartment of Computer Science and Engineering at the Universityof Nebraska-Lincoln (UNL). He is the founding co-director of


the Advanced Networking and Distributed Experimental Systems(ANDES) Laboratory at UNL. He is the Feature Editor ofThesis Review for Optical Networks Magazine. He served as aguest co-editor for a special issue of IEEE Network magazine onOptical Communication Networks. He served as a member of thetechnical program committees for the IEEE INFOCOM, IEEEGLOBECOM, Opticomm, ICC and ICCCN conferences. He is au-thor of the textbook (Design of Optical WDM Networks — LAN,MAN and WAN Architectures) published by Kluwer Academic

Publishers in 2000. He serves as the secretary of the IEEE ComSocOptical Networking Technical Committee (ONTC).

Prof. Ramamurthy was a recipient of the Indian NationalTalent Search scholarship and was a fellow of the Professors for theFuture program at UC Davis. He is a recipient of the UNL Re-search Council Grant-in-Aid award for 1999 and the College ofEngineering and Technology Faculty Research Award for 2000. Hisresearch areas include optical networks, distributed systems, andtelecommunications.

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