A review of routing strategies for optical burst switched networks

INTERNATIONAL JOURNAL OF COMMUNICATION SYSTEMSInt. J. Commun. Syst. 2013; 26:315–336Published online 22 September 2011 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/dac.1345

A review of routing strategies for optical burst switched networks

C. Yahaya1,*,†, M. S. Abd Latiff1 and A. B. Mohamed2

1Faculty of Computer Science and Information Systems, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia2Faculty of Electrical Engineering, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia

SUMMARY

The exact contribution of this paper is a review of existing state-of-the-art routing strategies for opticalburst switched networks developed by researchers to deal with burst contention before it happens. Routingschemes are implemented in space domain, which make them simple and cost effective. Additionally, thepaper points out the importance of routing as an effective way to deal with burst contention compared to othersolutions. It also underlines the main differences between contention avoidance schemes and contention res-olution techniques for optical burst switched networks. We believe that this review will help different opticalburst switched researchers involved in the development of route optimization algorithms to control burstcontention. Copyright © 2011 John Wiley & Sons, Ltd.

Received 27 July 2010; Revised 17 May 2011; Accepted 26 July 2011

KEY WORDS: optical burst switched; burst contention; routing strategies; routing algorithms; AntNet; antcolony optimization

1. INTRODUCTION

In recent years, the field of networking has been growing at a remarkable rate. The fast expansionof the Internet and the ever-increasing number of bandwidth-greedy applications such as multime-dia applications (e.g., global grid computing, bulk data storage, medical imaging through e-healthsystems, and digital cinema [1]), m-commerce applications (e.g., flight reservations, bank trans-actions, which are currently using 3G technologies requiring more bandwidth for better efficiencyand performance [2]), Internet Protocol Television (IPTV), and the like [3], are proving the limita-tions of the current computer and telecommunication infrastructures, in terms of bandwidth. Typicalbandwidth requirements for these applications vary from 100 Mb/s to 2 Gb/s [4] Thus, there is animmediate need for the development of new high-capacity networks that are capable of supportingthese greedy multimedia applications.

Dense wavelength division multiplexing (DWDM) technology lends itself a considerable impor-tance as a means to cope with this rapid growth of the Internet and multimedia applications.Wavelength division multiplexing (WDM) is a multiplexing technology that enables various opticalsignals to be transmitted by a single fiber. Its principle is basically the same as frequency divisionmultiplexing (FDM), in which several signals are transmitted using different carriers, occupyingnonoverlapping parts of a frequency spectrum. In the case of WDM, the two wavelengths widelyused are 1300 nm and 1500 nm. These are two wavelength windows at which optical fibers havevery low signal loss. The total available bandwidth for these two areas is estimated at 50 Tb/s [5].However, electronic processing speed limits the actual usable bandwidth. At channel spacing of 2.5GHz and a capacity of 2.5 Gb/s per channel, a total of 100 wavelengths can be multiplexed in one

*Correspondence to: C. Yahaya, Faculty of Computer Science and Information Systems, Universiti Teknologi Malaysia,81310 Skudai, Johor, Malaysia.

†E-mail: [email protected]

Copyright © 2011 John Wiley & Sons, Ltd.

316 C. YAHAYA, M. S. A. LATIFF AND A. B. MOHAMED

single fiber cable of producing a bandwidth of 250 Gb/s. The density of optical channels in theWDM system depends on two parameters: the bandwidth of the channel and the frequency spac-ing [1] Using a channel spacing of 12.5 GHz and 12.4 Gb/s per channel, researchers in [6] havedemonstrated the transmission of 2.5 Tb/s (256 � 12.4 Gb/s) on a standard single mode fiber over200 km.

Three switching paradigms have been developed based on WDM, namely optical circuit switching(OCS) [7], optical packet switching (OPS) [8–11] and optical burst switching (OBS) [12–16].

In OCS networks, also called wavelength-routed WDM, circuit switching techniques are used inwhich all-optical wavelength paths (i.e., light-paths) are established between pairs of source anddestination nodes. The establishment of light-paths involves several tasks, such as topology andresource discovery, routing, wavelength assignment, signaling, and resource reservation. Figure 1shows an example of the OCS node.

The static attribute of OCS makes it unsuitable for Internet traffic, which is bursty in nature.Therefore, other switching technologies have been proposed to overcome the shortcomings of OCS.

Optical packet switching technologies (Figure 2) offer a new capability to process packets directlyin optical domain for the future optical Internet and by doing so, they eliminate the need foroptical-to-electrical-to-optical conversion. OPS networks can be categorized in a number of ways[17]: synchronous versus asynchronous packet switching, fixed-length versus variable-length packetswitching, and store-and-forward versus cut-through packet switching. Also, optical packet multi-plexing schemes differ and can use time division multiplexing (TDM), WDM, and optical codedivision multiplexing (OCDM) as proposed in [17].

Fast switching times are required to make photonic packet switching practical. However, switch-ing times for MEM-based switches are currently in the order of 1 to 10 ms, while semiconductoroptical amplifier-based switches have switching times that are less than 1 ns [18]. Cost inefficiencyis the main problem with semiconductor optical amplifier switches [19].

Besides, the switch architectures require signals to pass through optical couplers. This results inadditional power losses. Although switching speeds are expected to improve in the near future, thecurrent technology is not mature enough to support photonic packet switching [19].

Synchronization is another challenge in OPS. Synchronization is desired in OPS to minimizecontention. Though difficult to realize, Blumenthal, et al. [20] and Cardakli and Willner [21] havemanaged to implement it in laboratory settings. The drawbacks of both OCS and OPS led to thedevelopment of OBS as an alternative optical switching technology. In the basic OBS, which isbased on just enough time (JET) [22, 23], a control packet known as burst control packet (BCP)

WDM-DEMUX

OXC

WDM-MUX

Input Fibers

Output Fibers

Connection Setup Request Signals

Figure 1. A typical optical circuit switching node.

Copyright © 2011 John Wiley & Sons, Ltd. Int. J. Commun. Syst. 2013; 26:315–336DOI: 10.1002/dac

ROUTING STRATEGIES FOR OPTICAL BURST SWITCHED NETWORKS 317

OXC

Electronic

Switch

Controller

OEO ConverterOEO Converter

WDM-MUX

FDL

Input Fibers

Output Fibers

WDM-DEMUX

Figure 2. A typical optical packet switching node.

is first sent in a dedicated channel termed as control channel to set up a connection by reservingappropriate resources and configuring the switches along a path. Then followed by a burst of dataafter waiting for a fixed time, known as offset time, enough to process the control and reserverequired resources at each OBS router. This belongs to the one-way signaling protocol, tell-and-go(TAG) as discussed in [24] and [25]. In TAG, the sending node does not wait for an acknowledge-ment for the connection establishment. Just-in-time (JIT) [26] another TAG signaling scheme, isused in OBS. Nevertheless, tell-and-wait (TAW) signaling algorithms have been implemented inOBS [27]. Figure 3 depicts an example of an OBS core node where signaling takes place.

Despite the fact that OBS is favored for the next-generation optical network and is seen as anintermediate step between the current OCS and the future OPS, it still suffers from high bursts

OXC

Electronic

Switch

Controller

OEO ConverterOEO Converter

WDM-DEMUX WDM-MUX

Input Fibers

Output Fibers

Data Channel

Control Channel

Figure 3. A typical optical burst switching core node.



loss because of contention [28]. Burst contention occurs when two or more bursts contend for thesame resource along the paths at the same time. Because there are no buffers at the core node, con-tending bursts are simply dropped. In OBS networks, a resource is interpreted as a data channel, awavelength converter, a fiber delay line or even switching resources at the switching matrix in thecore node. If contention because of resource usage occurs, the burst is said to be dropped or lost.Contention control to minimize burst loss probabilities has been a subject of intense research andthere are a significant number of proposals to implement low burst loss probability networks. Theseapproaches are grouped under two categories: contention avoidance techniques (CAT) and con-tention resolution techniques (CRT) as depicted in Figure 4. CAT approaches are proactive, that is,they control the network and make sure contention does not happen. Routing algorithms in general,the focus of this paper, fall in this category. CRT techniques are reactive; they deal with contentionafter its occurrence.

The rest of this paper is organized as follows: Section 2 goes through related works. In Section 3,the architectures of OBS are discussed. In Section 4, contention control techniques are elaborated.Section 5 discusses general routing strategies. Section 6 outlines the current routing strategies usedin OBS and addresses research directions in OBS routing. The paper is concluded in Section 7.

2. RELATED WORKS

Routing strategies have been used in OBS to solve the contention problem. The authors in [29] havepublished a survey of routing schemes used in OBS. According to this study, the routing strategiesproposed for OBS networks can be classified as either reactive or proactive. Deflection routing,

Contention Management Schemes in OBS

Contention Avoidance Contention Resolution

Space Domain Space DomainTime Domain OpticalDomain

Routeoptimization

and Loadbalancing

Algorithms

AssemblyTechniques

SignalingTechniques

Offset timeselectionschemes

Ex: FDL Ex: DeflectionRouting, Burstsegmentation,

schedulingtechniques

Hybridtechniques

Ex:Wavelengthconverters

Figure 4. Classification of contention management in WDM OBS networks.



which was proposed by Wang et al. [30] is a contention resolution scheme and thus belong to thereactive routing category. In such schemes, the routing path used by contended bursts can be changedat the node where contention occurs without taking into consideration the overall network conges-tion status in the downstream links of the new burst path. Theoretically, deflection routing achieveslocal load balancing. However, it may lead to endless loop in the network resulting in longer end-to-end delay. Performance analysis of deflection routing is elaborated in [30]. To minimize burstloss probability, proactive routing strategies make use of either network congestion information oranticipated traffic demands to optimize the set of paths and the distribution of traffic between ingressand egress network nodes. Teng and Rouskas [31] have discussed some of these techniques in moredetail. Furthermore, the work in [29] divided routing strategies into: alternative (deflection), multi-path (source-based), and single-path routing algorithms. Although the papers have reviewed quite agood number of routing strategies used in OBS, they did not review the works on ant-based routingalgorithms currently being used extensively in OBS to solve the routing and wavelength assignment(RWA) problem to reduce burst loss probability (BLP) — a common goal for all routing strategies.

The researchers in [32] presented a general review of OBS. However, the paper only mentioneddeflection routing as a means of contention resolution mechanism. Garcia et al. [33] discussed rout-ing strategies for OBS in terms of static and dynamic approaches. Additionally, they proposed newstatic and dynamic routing algorithms for OBS. These algorithms are discussed in Section 5 of thispaper.

The work in [34] studied the performance of some routing algorithms such as shortest path. Thepath-excluding (PE) algorithm was proposed in [35]. The bypass path (BP) was presented in [35].Multipath routing with dynamic variance (MRDV) was introduced in [36], and finally, adaptivemultipath OBS routing was presented in [37]. Based on their comparison results, the researchersconcluded that none of the above routing techniques improves significantly the performance ofOBS when compared with the simple shortest path approach for the 15 nodes National ScienceFoundation (NSF) network topology studied and traffic profile used.

In this paper, current routing strategies used in OBS are discussed with focus on ant-based routingschemes, which are missing in the above routing reviews.

3. OPTICAL BURST SWITCHED NETWORK ARCHITECTURES

An optical burst switched network consists of two types of nodes: core and edge nodes. These nodesare currently manufactured by the Matisse [38] and InTune Networks [39] companies. Each of thesenodes plays different and specific roles in the network.

Core nodes (CNs) are also known as optical cross connect performing routing (implementingwhat is often called ‘all optical domain switching’), scheduling and contention resolution. Edgenodes (ENs) perform other services, like burst assembly and disassembly, creation of the BCPs (andall its related tasks), signaling, and also the tasks that relate to routing the burst inside the OBS net-work. CNs are located inside the OBS network cloud, while ENs interface with the client networks.These client networks may be Internet protocol (IP) networks, Ethernet networks, ATM networks,and others. The control packet is generated by the EN, and the bursts are then multiplexed into theoutput fiber in the direction of a core node, or inversely, the control packet and the burst are demul-tiplexed from the input fiber and the burst is disassembled and its constituent packets are routed intothe client network. Therefore, a burst is a set of data units (e.g., IP packets, Ethernet frames, ATMcells, etc.) that are grouped together by the EN, according to burst assembly schemes adopted in theunderlying network. Figure 5 shows the functional diagram of OBS nodes while Figure 6 depicts atypical OBS network.

In the search for a practical solution for the contention issue in OBS, several variations to thebasic JET/JIT based OBS architecture have been proposed in the literature [40, 41].

Qiao [42] proposed labeled optical burst switching (LOBS) for IP over WDM integration witha GMPLS-based control. In LOBS, each OBS node is augmented with a GMPLS-based controllersimilar to the label switched router in the conventional Generalized Multi-Protocol Label Switching(GMPLS) networks. The control packet, which is sent as an IP packet with a label in the control



BCP Processing

Scheduling

Contention Resolution

Burst disassembly

Packet forwarding

Burst Assembly

Signaling

BCP Creation

RWA

Ingress Node

Core Node

Egress Node

Figure 5. Functional diagram of OBS nodes.

Figure 6. Typical OBS network.

plane over a pre-established label switched path (LSP), can share the same path with the corre-sponding data burst. Routes are set up according to the requirements of the traffic using the explicitand constraint-based routing capabilities of the GMPLS framework. At the edge LOBS router, labelstacking and LSP aggregation are performed along with the aggregation of IP packets so that multi-ple LSPs can be multiplexed over a single path. The LOBS architecture also supports the protectionof paths by either the 1 C 1 scheme or any other shared-path protection scheme by establishingredundant LSPs during the Burst Header Packet (BHP) routing phase.

Another variant of OBS is wavelength routed OBS (WROBS) [43]. In this architecture, the advan-tages of the two-way reservation mechanism in circuit-switched networks and those of JET-basedOBS architecture are combined to avoid the contention losses. A centralized scheduler is used toallocate wavelengths to requests originating at the ingress nodes. Moreover, a centralized RWA is



used to identify an optimal light-path. The end-to-end delay and loss can be guaranteed in thisframework.

The difference between WROBS and OCS [7] is that the reservation of resources in WROBSis done only for the duration of the burst. This improves the network utilization compared to theOCS networks. The major drawback of this architecture is that it is not scalable because of thedependence on the centralized scheduler for each burst transmitted.

Dual-header OBS (DOBS) [44] is designed to reduce the delay incurred by the BHP at each nodeby decoupling the resource reservation and the scheduling operations. In the basic OBS, each BHPincurs a variable processing delay at each node, which reduces the offset time as the burst travelsthrough the path. Because of the variation in the offset time, there is unfairness to the bursts travelingshorter paths. To maintain a fixed offset time for all the bursts traveling through the paths with dif-ferent lengths, the inventors of DOBS proposed to use two different control packets for each burst.The first one is the service request packet (SRP). This packet contains the information necessary toschedule the bursts: offset time, length of the burst, and class information. The second one is theresource allocation packet (RAP) and it is used to configure the switch. In DOBS, the burst is notscheduled as soon as the control packet arrives. Rather, the SRP is processed first to determine theresources required for a burst and then it is forwarded to the next node without waiting for the burstto be scheduled. The burst is scheduled after a certain time just before the data burst arrives at thenode. After scheduling the burst, the RAP is sent to the downstream node to indicate the wavelengthselected for the burst. As soon as the RAP is received from an upstream node, the burst is scheduledusing the information of both RAP and SRP.

In [45], Barakat and Sargent reported that the main benefit of the DOBS architecture is the con-stant scheduling offset (CSO) where each link may be assigned an independent CSO value to ensurethat all the bursts can be scheduled on a first-come-first-serve basis. CSO-based DOBS has a lowerscheduling complexity compared to the JET/JIT-based scheduling and has better network utiliza-tion compared to the JIT signaling. DOBS achieves better throughput-delay performance comparedto either JIT or JET signaling because the resource reservation and scheduling are parallelized.Furthermore, DOBS has better fairness compared to the JET signaling because voids are notcreated during the scheduling, which reduces unfairness to bursts of larger length. It also avoidspath length-based unfairness because it maintains a constant offset time for all the bursts travelingalong the path.

In [46] Luo et al. proposed what they called the reliable optical burst switching (ROBS). In thisarchitecture, congestion control and retransmission mechanisms of Transmission Control Protocol(TCP) are added to the OBS layer. The objective of the new architecture is to provide reliable burststransfer in the OBS domain. To do that, the researchers created a TCP-like control plane for OBSnetworks. This new control plane incorporates congestion control and retransmission mechanismsof TCP in the BCP. Two new types of signaling messages (burst acknowledgment and dropping) aredefined. Edge and core nodes are modified to include several new modules to support the function-alities of the proposed architecture. The architecture looks attractive; however, the scientists haveonly implemented and evaluated the burst retransmission part of the architecture and concluded thatTCP goodputs using ROBS is much better than the traditional OBS scheme. However, we believethat ROBS should be implemented in full before a proper conclusion can be made.

In [33] Garcia proposed a new OBS architecture called common control channel optical burstswitched networks (C3-OBS). In this architecture, the information carried in each BCP in each nodeis used and shared by the underlying network nodes for better signaling purposes. The architectureis based on the concept of network-state awareness.

Another class of OBS is the time-variant OBS. In time-variant OBS, bursts are switched in thetime domain instead of wavelength domain. The motivation for time-based OBS proposals is toavoid the use of wavelength converters to resolve contention at the core node. Although the useof such converters does improve the network performance, wavelength converters are still at theirinfancy stage and are not cost effective. At the time of this writing, four types of time-variant OBShave been proposed and studied: time slice OBS (TSOBS) [47], slotted OBS [48], time synchronizedOBS (SynOBS) [49] and hierarchical time sliced OBS [50]. Because of space limitation, interestedreaders are referred to the cited reference for more details on time-variant architectures.



4. CONTENTION CONTROL TECHNIQUES IN OBS

Although our focus is on routing strategies as a way of contention control and avoidance, in thissection, and because of space limitation, we describe briefly the main burst contention resolutiontechniques.

Burst contention is the main problem of OBS networks because of the lack of a buffer at the corenetwork. Contention occurs when two or more bursts contend for the same out port at the sametime. There are the two main approaches when dealing with contention in OBS: contention avoid-ance schemes (CAS) and contention resolution schemes (CRS). CAS deals with contention before ithappens and it is, generally, implemented in space domain. The advantage of CAS techniques is thatthey do not require extra equipment. Route optimization schemes belong to CAS. Different routeoptimization techniques related to CAS will be discussed in Section 5. In this section, the focus ison CRS.

Contention resolution techniques are reactive because they deal with contention after it hasalready occurred. In OBS, contention can be resolved in three domains: (i) time domain byusing fiber delay lines (FDLs); (ii) optical domain by using wavelength converters; and (iii) inspace domain by applying deflection routing. These techniques can be implemented separately orcombined with other contention management techniques.

To resolve burst contention in time domain, buffering is necessary, which requires the use ofmemory. Because there is no optical memory as of this writing, FDLs [51] have been proposed inthe literature for this purpose. However, FDL technology is still at its infancy [41]. An FDL is aspecial type of fiber that allows a limited fixed delayed transmission of the optical signal. This delayis proportional to the nature of the fiber and to its length. For example, a 200-km standard FDL isneeded to delay a single burst for 1 ms [19].

In optical domain, wavelength converters are used to resolve contention in the core node. Themotivation for this is that, in WDM/DWDM technologies, several wavelengths run, in parallel, ona single fiber cable that connects two optical switches. It is expected that, there will be as many as160–320 wavelengths per fiber [19]. Wavelength conversion is the process of converting the wave-length of an incoming channel to another wavelength at the outgoing channel, thereby increasingwavelength reuse, that is, the same wavelength may be spatially reused to carry different connec-tions in different fiber links in the network. Wavelength converters offer 10–40% increase in reusevalues when wavelength availability is small [52].

Despite these advantages of wavelength conversion and the fact that, optical wavelength convert-ers have been demonstrated in laboratory environments, the technology is not yet mature, and therange of possible conversions are somewhat limited [41].

Contention in OBS is resolved in space domain by applying deflection routing where contentedbursts are routed to an output port other than the intended output port. Deflection routing is notfavored because of potential looping, out-of-sequence delivery of packets and more latency. Deflec-tion routing was used in [30, 53] and was combined with FDL in [54] for contention resolution. In[55] Levesque et al. proposed a new deflection scheme: adaptive hybrid deflection and retransmis-sion, which combines deflection and retransmission approaches based on network conditions suchas burst loss rate and link utilization. In [56], a novel deflection routing algorithm called AIMD-NBCP was proposed to deal with contention resolution. The scheme uses the number of the BCP atthe core node to estimate the average load and then uses this information to adjust the burst trans-mitting rate of the edge node. The authors concluded that their proposed AIMD-NBCP scheme isefficient at keeping the network stable and enhancing the overall performance of deflection routing.

Another method for contention resolution is burst segmentation [57, 58]. In burst segmentation,instead of dropping the entire contents of a contending burst, only the overlapping segments of theburst are dropped. This is because, in this contention resolution scheme, the burst is divided intosegments. These segments define partitioning points of a burst when in the optical network. Each ofthese segments may consist of a single packet or multiple packets. Additional control information isattached to each segment, such as its length, checksum, etc. The burst header packet consists of thelength of the burst, the offset time and QoS information. One obvious problem with burst segmen-tation despite its contribution to the reduction of burst loss probability is the overhead that it adds to



the network, the complexity of the scheme and the low utilization of the bandwidth because of thenumerous control packets [59] Table I summarizes the advantages and disadvantages of the abovecontention resolution techniques.

5. ROUTING STRATEGIES

Routing is defined as the act of moving information across an internetwork from a source (s) to adestination (d ). Along the way, at least one intermediate node typically is encountered. It is alsoreferred to as the process of choosing a path over which to send the packets and it takes place atlayer 3 (network layer) of the Open System Interconnections (OSI) reference model. Routing algo-rithms are the software at layer 3 responsible for deciding the next intermediate node for the packet,whereas routing protocols use metrics to evaluate what path will be the best for a packet to travel. Ametric is basically a variable used to rank routes from best to worst or from most preferred to leastpreferred [60] such as path bandwidth, reliability, delay, current load on that path, etc. Metrics areused by routing algorithms to determine the best path to a destination. To aid the process of pathdetermination, routing algorithms may initialize and maintain routing tables, which contain routeinformation. Route information varies depending on the routing algorithm used.

The following design parameters are very important while designing a routing protocol:

Performance Criteria: Link cost, number of hops, delay, throughput, etc.;Decision Time: Per packet basis (datagram) or per session (virtual-circuit) basis;Decision Place: Each node (distributed), central node (centralized), originated node (source);Network Information Source: None, local, adjacent node, nodes along route, all nodes;Network Information Update Timing: Continuous, periodic, major load change, topologychange;

Routing algorithms can be classified as follows:

� Static versus adaptive;� Single-path versus multipath;� Intradomain versus interdomain;� Flat versus hierarchical;� Link-state versus distance vector;� Host-intelligent versus router-intelligent.

5.1. Static versus adaptive

This class of routing algorithm describes how and when the routing tables are set up and how theyare modified. The set-up and the modification of the routing table can either be static or dynamic.

In static routing algorithms, paths are computed before packets arrive and do not change unlessthe network administrator alters them. Algorithms that use static routes are simple to design andwork well in small networks where network traffic is relatively predictable and network design isrelatively simple. The drawback of these algorithms is that routing decisions are not based on current

Table I. Comparison of different contention resolution techniques [2].

Scheme Advantages Disadvantages

Optical domain: wavelength Low burst loss Immature and technologyconversionTime domain: FDL buffer Simple and maturing technology Bulky, expensive, high power

loss and noiseSpace domain: deflection No extra hardware required Out of order arrivals, possiblerouting instability, high latencyBurst segmentation High efficiency, lower burst loss Immature, high complexity, extra

delay and increased signaling.



topology or traffic. Because static routing systems cannot react to network changes, they are gener-ally considered unsuitable for large and constantly changing networks. Thus, most of the dominantrouting algorithms today are dynamic routing algorithms.

Adaptive or dynamic routing algorithms adjust to changing network circumstances by analyzingincoming routing update messages and are more beneficial in medium and large size networks [57].Dynamic routing algorithms can be supplemented with static routes where appropriate.

5.2. Single-path versus multipath

The classification here is based upon the number of paths a router stores for a single destination.Single path algorithms are where only a single path (or rather single next hop) is stored in therouting table. In multipath (load sharing), the routing protocols support multiple paths to the samedestination. Unlike single-path algorithms, these multipath algorithms permit traffic multiplexingover multiple lines. The advantages of multipath algorithms over single-path are better throughputand high reliability.

5.3. Intradomain versus interdomain

The Internet is composed of autonomous systems that define the administrative authority and therouting policies of different organizations. Autonomous systems are made up of local routers thatrun Interior gateway protocols such as routing information protocol, enhanced interior gatewayrouting protocol, open shortest path first (OSPF), and intermediate system-to-intermediate sys-tem within their boundaries. Such algorithms and protocols are known as intradomain routingalgorithms/protocols.

On the other hand, interdomain algorithms work within and between domains. The externalGateway Protocol version 4 (BGP-4) defined in RFC 1771 [61] is the current Internet interdomainprotocol.

Because of the fact that the nature of these two algorithms is different, an optimal intradomain-routing algorithm would not necessarily be an optimal interdomain-routing algorithm.

5.4. Flat versus hierarchical

Some routing algorithms are flat space, while others use routing hierarchies. In a flat routing scheme,the routers are peers of all others. In a hierarchical routing system, some routers form what is knownas a routing backbone. Packets from nonbackbone routers travel to the backbone routers, where theyare sent through the backbone until they reach the general area of the destination. At this point,they travel from the last backbone router through one or more nonbackbone routers to the finaldestination.

Routing systems often designate logical groups of nodes called domains, autonomous systems,or areas. In hierarchical systems, some routers in a domain can communicate with routers in otherdomains, while others can communicate only with routers within their domain. In very large net-works, additional hierarchical levels may exist, with routers at the highest hierarchical level formingthe routing backbone. The primary advantage of hierarchical routing is that it mimics the orga-nization of most companies and therefore supports their traffic patterns well. Most network com-munication occurs within small company groups (domains). Because intradomain routers need toknow only about other routers within their domain, their routing algorithms can be simplified, and,depending on the routing algorithm being used, routing update traffic can be reduced accordingly.

5.5. Link-state versus distance vector

This category is based on the way the routing tables are updated.

� The key features of the distance vector routing are as follows:

ı The routers share the knowledge of the entire autonomous system;ı Sharing of information takes place only with the neighbors;ı Sharing of information takes place at fixed regular intervals, say every 30 s.



� The main features of the link-state algorithms are:

ı The routers share the knowledge only about their neighbors compared to all the routers inthe autonomous system;

ı Sharing of information takes place only with all the routers in the Internet by sending smallupdates using flooding compared to sending larger updates to their neighbors;

ı Sharing of information takes place only when there is a change, which leads to lesser Internettraffic compared to distance vector routing.

Because convergence takes place more quickly in link-state algorithms, these are somewhat lessprone to routing loops than distance vector algorithms. On the other hand, link-state algorithmsrequire more processing power and memory than distance vector algorithms. Link-state algorithms,therefore, can be more expensive to implement and support. Link-state protocols are generally morescalable than distance vector protocols.

5.6. Host-intelligent versus router-intelligent

Here, the basis of classification is whether the source knows about the entire route or just about thenext hop where to forward the packet.

Some routing algorithms assume that the source node will determine the entire route for a givensource pair. This is referred to as source routing or host intelligent routing, because the sourcerouter is the one that makes routing decision. In such systems, intermediate routers merely act asstore-and-forward devices, and continue to send the packet to the next stop.

In router intelligent routing systems, hosts know nothing about routes. Routing decisions are madeintermediate routers based on their own strategy.

In the following section, routing algorithms in OBS are discussed under two categories: static anddynamic schemes.

6. ROUTING ALGORITHMS FOR OPTICAL BURST SWITCHED NETWORKS

In this section, we review different routing algorithms used in OBS to avoid contention and mini-mize burst loss probability. The discussion is divided into two categories: static routing and dynamicrouting.

6.1. Static routing algorithms

In OBS, static routing is based on the following algorithms [62]: shortest path, Dijkstra algo-rithm, fixed alternative routing, extended Dijkstra routing, and travel agency algorithm. The lasttwo schemes were proposed by Garcia [62].

The authors in [63] proposed a technique called OBS-aware policy in which they required thateach light-path is ‘filled’ within the first m hops, and do not allow any new connections to join thesame light-path later. This means that, light-paths are allowed to clash only within the m first hopsfrom each connection’s point of view. They then assumed that there is no wavelength conversion,which is a good assumption, even though similar reasoning should be applicable also for networkshaving wavelength conversion capable nodes.

The authors claimed the followings improvements:

� Fairness among links because of the fact that the flows compete on common resources at mostm times;� Overall efficiency is improved because bursts are not being blocked within the last hops. The

later the blocking occurs, the more resources have been wasted;� The NAK packets will arrive earlier at the source allowing a faster retransmission cycle if

applicable.

However, the researchers stated clearly that their resulting mixed-integer linear programming(MILP) solution is likely to be intractable when the problem size increases, for example, the number



of network nodes and wavelength channels grows. Therefore, they suggested that in practice one stillneeds to come up with further simplifications for the routing problem, probably using some heuristicrouting algorithm resembling the ideas behind their proposed OBS-aware routing policy, which willresult in a reasonably good solution with a manageable computational effort and scalable. Addition-ally, the use of Erlang B Formula overestimate the burst loss as it does not take into account thestreamline effect.

In [64], the authors approached the contention avoidance issue in OBS from a traffic engineeringperspective. They used integer linear programming and simple integer relaxation heuristic to resolveand avoid burst loss in OBS based on multipath and source routing. Their objective was to balancethe load across the network. They assumed full wavelength conversion and did not consider thestreamline effect. Therefore, the obtained results lack practicality and more realistic techniques areneeded to deal with burst contention.

Klinkowski et al. [65] considered multipath source routing and used network optimization theoryto improve it. Given the fact that, an overall BLP has a nonlinear character, the scientists used bothlinear programming formulations with piecewise linear approximations of this function and non-linear optimization gradient methods. In their nonlinear optimization problem, they assumed thatthere is a pre-established virtual path topology consisting of a limited number of paths betweeneach pair of source–destination nodes (static routing). Using a gradient optimization method, theycalculated a traffic splitting vector that determines the distribution of traffic over these paths. Theycompared their results with shortest path (SP) and showed that theirs is more efficient. Streamlineeffect was not considered and nothing was mentioned about the use of wavelength converters orFDL. Therefore, the results need to be improved.

The authors in [66] used a linear programming approach to deal with route optimization in OBSaiming at burst loss reduction similar to what all other researchers have done. Their approach isbased on source routing and predefined multipath. To calculate BLP in a link, they considered twomodels: nonreduced link load and reduced link load. In nonreduced link load, the traffic offeredto a link is the sum of the traffic offered to all the paths that traverse the link under consideration,while in reduced link load, the traffic offered to a link is the sum of the traffic offered to all thepaths that traverse that link minus the traffic loss in preceding links along these paths. The resultswere compared with both OSPF and a bypass routing algorithm. The two models outperformed bothOSPF and bypass routing techniques. However, they did not consider streamline effect in their losscalculation. Therefore, their optimization can be misleading.

The researchers in [67] dealt with route optimization by considering two scenarios. The firstscenario concerns a network in a normal working situation while the second scenario considers anetwork in a failure state where some links are not working. They developed MILP formulations tosolve these two problems and used heuristic algorithm for the actual evaluation of their proposedsolution because MILP is computationally complex and intense. They assumed full wavelength con-version and considered streamline effect when calculating the BLP. This is among the few papersthat considered streamline effect in computing BLP, so their result is near to optimal. However,because they assumed full wavelength conversion and fixed routing, this solution is not scalable andthe use of wavelength converters makes it impractical.

The work in [68] is similar to that in [67] and suffers from the same limitations, which are the useof MILP, wavelength conversion, and static nature of the algorithm.

As mentioned earlier, Garcia [62] proposed two new static routing algorithms for OBS: extendedDijkstra algorithm [69] and travel agency algorithm. According to them, the extended versionof Dijkstra generates the shortest paths and therefore may allow a faster convergence to rout-ing table equilibrium in dynamically routed meshed networks. The travel agency algorithm is themain routing algorithm for the new architecture of OBS proposed by Garcia, known as C3-OBS[33]. This algorithm is based on fixed source routing algorithm. However, it differs from suchalgorithms in that each burst request by an ingress node receives a response with a route that is cal-culated based on its requirements. If no path can be found, it receives a negative acknowledgement.Also, in the TAA, no assumption is made about the resource reservation protocol utilized in thenetwork; therefore, it is able to handle bursts requests according to the chosen resource reservationprotocol. Another key feature of TAA is that the routing table is assessed for network resource



availability each time a burst ingress request is received. This makes the algorithm static fromthe burst point of view, because the full path for the burst is completely predefined in the ingressnode, and dynamic from the source–destination node pair point of view. This is because the pathdefined for a given source–destination pair may be different each time a burst transmissionis requested.

The work in [70] discussed the performance of two static routing strategies based on load bal-ancing approaches. The first strategy is path selection based on the minimization of the maximumcongested link and the other one is path selection based on the minimization of the maximum end-to-end congested path. The researchers compared the results of these two schemes with SP algorithmand demonstrated that both the schemes perform better than SP in terms of BLP. The drawback oftheir approach is that it assumes full wavelength conversion.

Barradas and Medeiros [71] tackled contention issues in OBS via a traffic engineering approachfor path selection. They proposed and evaluated two streamline-based preplanned routing (SBPR)strategies. The first technique uses K predetermined paths for each pair of nodes and it is calledSBPR-PP. The second technique does not require pre-determined path requirements SBPR-nPP.The main idea is to balance the traffic across the network to prevent congestion at the core network.The advantage of these two techniques is that they can work with/without wavelength convertersand according to the results obtained by the researchers, they outdo SP in performance. However,these techniques are not scalable because they are based on the linear programming approach. Thework proposed in [72] is similar to that in [71] and suffers from the same limitations.

Researchers in [73] were the first to propose an ant-based routing algorithm for OBS. In this algo-rithm, the ants are treated as control packets and are named as BCP-REQ and BCP-ACK to representforward and backward ants, respectively. Both BCPs are transmitted by the control channel. BCP-REQ is used to reserve network resources while BCP-ACK is used only to indicate successful bursttransmission. Thus, the algorithm still uses one-way signaling protocol for resource reservation.Although the algorithm performs better than the shortest path routing, it assumes full wavelengthconversion and uses FDL in the core node for contention resolution.

All the above routing techniques are static and they are either based on shortest path routingalgorithms or its derivatives and extensions, fixed alternative routing and in most of the cases usingMILP. Because RWA in OBS is a hard NP problem, the MILP solution tends to be more complexand nonscalable. Also, static routing algorithms are less efficient in term of bandwidth utilizationand result in high BLP. Besides, nonadaptive routing schemes are not scalable. Therefore, moredynamic routing algorithms are being proposed in the literature to deal with high BLP in OBS.Specifically, researchers are now considering the use of swarm intelligent algorithms [74, 75] tosolve the problem. Ant colony optimization (ACO) [76, 77] and its derivatives for communicationnetworks, such as AntNet [78] is frequently used for that purpose because it is adaptive, multipath,survivable (fault tolerant), proactive, and scalable.

6.2. Dynamic routing algorithms

The researchers in [79] dealt with contention reduction in OBS networks by proposing three newadaptive routing schemes based on the following important criteria: BLP, resource utilization, sys-tem complexity, and fairness. Their adaptive routing schemes take into account routing and thewavelength reservation problem at the same time. Additionally, by exposing a larger proportion ofunderutilized network resources to longer-path bursts, the researchers managed to improve fairnessas well as overall burst loss performance. The three adaptive routing techniques are: hop-by-hoprouting using forward channel reservation, hop-by-hop routing using link connectivity, and hop-by-hop routing using neighborhood forward channel reservation. JET was used as signaling protocoland latest available unscheduled channel was used for channel scheduling. On the basis of the sim-ulation results, the researchers claimed that the three forward channel reservation-based dynamicrouting schemes perform better than their counterparts in terms of BLP at light and moderateloads. This is attributed to the fact that dynamic routing can maximize the link utilization as highas possible. However, because hop-by-hop routing using link connectivity tries to distribute databursts to the candidate nodes having a higher connectivity index, it performs slightly better than



hop-by-hop routing using forward channel reservation. Among the three proposed algorithms, hop-by-hop routing using neighborhood forward channel reservation performs the best because it utilizesmore link state information than others.

The streamline effect was not considered in any of the three algorithms and that is the mainshortcoming of the algorithms.

In [80] Yang and Rouskas proposed a suite of dynamic routing algorithms based on different routeselection metrics: weighted bottleneck link utilization strategy (WBLU), weighted link congestion(WLC) strategy, and end-to-end path priority-based (EPP) strategy. In the first algorithm, routes areranked using link utilization information. The aim is to reduce or avoid contention by routing burststhrough less utilized links. The second scheme routes bursts along the path with high probability ofa successful transmission. To achieve this goal, Burst Loss Ratio (BLR) information on each path,based on a link-state protocol, is used to rank different available paths. In the last technique, EPP,although BLR is used as path selection metric, the technique does not rely on individual link conges-tion information as in WLC. Rather, it asks this probability directly from the ingress node based onthe feedback messages it receives from the network. These three routing strategies were comparedwith SP and all of them performed better. Based on the obtained results, the researchers also saidthat,the three path switching strategies are similar in performance. However, they concluded that,WBLU performs the best at low loads, EPP is the best strategy at high loads, and WLC has moder-ate BLR between the values of WBLU and WLC. The outstanding performance of WBLU over theother techniques at low load is attributed to the fact that, it uses link utilization as route selectionmetric. At low network loads, most links have low utilization, and avoiding the few highly utilizedlinks can significantly improve the burst drop probability. On the other hand, at high load, EPPoutperforms both WBLU and WLC strategies. This is because in EPP, path priorities are updatedimmediately upon the receipt of feedback messages from the network, whereas WBLU and WLCupdate their routing decisions periodically (i.e., once they receive the most recent information onlink utilization or congestion). The period of update for WBLU and WLC is independent of thenetwork load.

In [62] Garcia proposed a novel dynamic routing called the next available neighbor (NAN) routingalgorithm. This algorithm is based on the fact that in a dynamic environment, continuous informa-tion sharing increases traffic intensity. Such fluctuation in network traffic requires efficient transportand routing algorithms. In the NAN scheme, in the event of a packet or burst drop possibility, insteadof dropping the burst, the current node will forward it to the closest node to the destination node.The advantage of this algorithm is that it is complementary to existing routing strategies as it can beused to manage burst or packet loss. In an OBS environment, when the core node receives a BCPand discovers that it cannot guarantee the reservation of the resources requested by the BCP, insteadof sending a negative acknowledgement (NAK) BCP back to the ingress node, it will manufacturea NAN-BCP destined to a NAN node. When the burst arrives at that node, it will not be dropped;rather, it will be routed to a NAN node. If one is available or, eventually, if the node allows for thisprocedure, the burst will be Optical to Electrical (O/E) converted, buffered, and later reinserted inthe network.

Yoshikawa et al. [81] proposed a centralized routing and wavelength assignment algorithm forOBS networks. The algorithm estimates the expected total blocking time in the network and usesiterative local optimization to minimize the estimated blocking time. The researchers claimed thatthe proposed algorithm attains much smaller blocking probability than conventional distributed con-trol algorithms. They also said that with the introduction of optical buffers and burst retransmission,the proposed method realizes low burst loss rates (<10�6) acceptable for most applications. Thelimitation of the solution is the use of FDL.

The researchers in [37] proposed a new adaptive routing for OBS networks, which aims at bal-ancing network load and reducing burst loss. To do that, they assigned, dynamically, a cost to eachpossible path between any source–destination pair. Their algorithm initially uses equal cost multi-path technique on each node with the number of hops metric. Also, periodically, all output linkscosts are recalculated in every node for each source–destination pair and the load balancing is doneaccording to the costs obtained on each link. They also used Erlang B to estimate BLP, but does notaccurately estimate it making the proposed solution inefficient.



In [82] the researchers studied the behavior of manycasting over OBS based on multiple QoSconstraints, for example, physical layer impairments, transmission delay, and reliability of thelink. Additionally, they have developed a mathematical model based on lattice algebra for thismulticonstraint problem. To minimize request blocking for the multiconstrained manycast (MCM)problem, they proposed two routing algorithms: MCM-shortest path tree (MCM-SPT) and MCM-dynamic membership (MCM-DM). To avoid the shortcomings of centralized algorithms (i.e., singlefailure and poor performance), the researchers based their algorithms on distributed routing tech-niques, where each node individually maintains the network state information and executes thealgorithm. The functionality of these algorithms is described as follows:

(1) Handle multiple constraints with the help of link-state information available locally.(2) Service differentiated provisioning of manycast sessions.(3) Find the best possible destinations in terms of service requirements for the manycast sessions.

Through simulation results, the authors claimed that MCM-SPT algorithm performs better thanMCM-DM for delay constrained services and real-time service, whereas MCM-DM algorithmperforms better for nonreal-time traffic.

Galdino et al. [83] proposed and investigated the performance of a dynamic routing and wave-length code assignment algorithm using hybrid technology with WDM/OCDM for OBS network.They used an OCDM-based multigranularity optical cross-connect to enable the better granu-larity in optical path switching than the wavelength-based path. They claimed that they werethe first to combine random and first fit heuristics for the assignment of wavelength and opticalcodes in WDM/OCDM network. The combined techniques are known as: random–random (R–R),random–first fit (R–FF), first fit–random (FF–R), first fit–first fit (FF–FF).

On the basis of their results, they concluded that random–random heuristic has a better perfor-mance in terms of BLP. Moreover, they confirmed that WDM/OCDM technology increases thecapacity and scalability in the optical network.

Levesque and Elbiaze [84] introduced a new routing mechanism for JET-based OBS networks,called graphical probabilistic routing model that selects less utilized links, on a hop-by-hop basisby using a Bayesian network. The algorithm does not use both wavelength converters and FDL atthe core nodes of the OBS. According to their simulation results, the proposed adaptive routingalgorithm reduces the BLR compared to static approaches.

To reduce the negative impact of cascaded wavelength conversions constraints in OBS, Gao et al.[85] proposed a novel proactive routing scheme that considers the instantaneous link congestionat the moment when the bursts arrive. The researchers claimed that this routing algorithm has thefollowing advantages: (i) it utilizes the same offset times for the same node pairs while providingdynamic routing without using any FDL; (ii) it decreases BLP to a great extent; and (iii) it mitigatesunfairness among the bursts with different hop counts. On the basis of the obtained results, theyconcluded that their method not only effectively improves burst loss performance but also yieldsbetter fairness.

Garlick and Barr [86] were the first to propose an ant-based algorithm to solve the dynamic RWA(DRWA) problem in WDM networks. The proposed algorithm works as follows: when a connectionrequest arrives at a core node, a number of ants are launched from the source to the destination tosearch for paths. Once the ant reaches the destination, it reports a path between the source and thatdestination. The reported paths are scored, separately, based on their length and congestion infor-mation. After all the ants have reached the destination, the best path is selected from the reportedpaths to establish the requested connection. In comparison with conventional approaches, this algo-rithm achieves good performance in terms of blocking probability; however, a connection requestwill suffer from a high setup delay because of the need to wait for all ants to complete their searchbefore a path can be chosen. Actually, the ants launched from one node do not cooperate with theants from other nodes.

Ngo et al. [87] also proposed an ant-based dynamic RWA algorithm for general optical WDMnetworks with irregular topology. The algorithm differs from the one proposed in [86] as the coop-erative ants are used to continuously update the routing table on each node in a way that the currentnetwork state is reflected in the routing tables. This allows fast path determination of a connection



request and guarantees a low blocking probability. However, the authors did not mention anythingabout wavelength continuity or FDL.

A fault-tolerant dynamic RWA algorithm based on the ACO framework was proposed in [88]. Inthis work, the authors developed a dynamic ant-based reliable routing algorithm to minimize BLP.Ant colony algorithm was used as a platform to dynamically determine a protection cycle and estab-lish a dependable light-path with backup paths sharing, which is defined as NP-complete. For betteroperation of the algorithm, the researchers proposed the use of a routing table and a pheromonetable. The routing table contains a set of feasible protection cycles between source and destinationnodes while the pheromone table consists of pheromone intensity status. Through extensive simula-tions, the scientists claimed that their proposed survivability routing algorithm outperforms the oneproposed in [89]. However, the algorithm needs more scrupulous tests by varying parameters suchas the number of agents or pheromone parameters to improve the performance of this survivablerouting algorithm.

Triay and Cervello-Pastor in [28] proposed the use of ACO algorithm to support the routing andwavelength assignment in OBS. From the simulation results the authors claimed that their new pro-tocol responds effectively to congestion while providing better performance in comparison with theshortest path routing with random wavelength assignment. The main difference between this algo-rithm and the previous is that it assumed wavelength continuity constraint. However, in the vent ofcontention, FDL is used to temporally store the contending bursts. Because FDL is a technologythat is still at its infancy, the obtained results may not be realistic.

In [75] Yan-lei et al., developed a mathematical model for ACO and proved through simu-lation that ACO does converge faster than the Dijkstra traditional routing algorithm. They alsodemonstrated that ACO has lower blocking probability compared with Dijkstra.

The researchers in [90] evaluated an ant-based architecture for all-optical networks and concludedthat ACO algorithms perform better in optical networks compared with traditional routing schemes.This is due to the fact that ACO algorithms are more scalable because the routing table growsaccording to the network size. In other routing algorithms, the routing table grows quadraticallywith the number of nodes in the network. Also, ant-based routing algorithms tend to balance thenetwork load.

Aragon et al. [91] also used ACO to solve the RWA problem in WDM networks. They have shownthat ACO outperforms other distributed routing algorithms applied in WDM networks. They alsodemonstrated that the proposed algorithm is 30% faster than non-ACO-based algorithms. Thoughthe results of the algorithm outperformed other routing schemes used in optical networks, its imple-mentation could pose a problem because of the use of FDL. Also, the algorithm presents highblocking probability under heavy load because of the lack of proper resource management. In theabsence of such measures, resource shortage is the major cause of packet dropping, which results inhigh blocking probability.

Pedro et al. [92] described a distributed framework for routing path optimization OBS basedon the ACO algorithm. The proposed algorithm consists of additional data structures stored atthe nodes and special control packets (ants) that traverse the network to estimate the goodness oftheir paths and update accordingly the data structures of the nodes. The simulation results revealthat the algorithm does improve the performance of OBS networks by reducing data loss andthat it attains a performance comparable to that of the centralized algorithms. The results alsodemonstrate that the framework is robust to changes in its parameters. Another important uniquefeature of the study is that it implements the algorithm in both egress and ingress of the network.This makes it efficient in dealing with load balancing at the expense of increasing the size of therouting tables.

Hassan et al. [93] have examined the problem of DRWA in WDM networks with the wavelengthcontinuity constraint applied and have shown that traditional search algorithms like integer lin-ear programming and graph coloring are inefficient for solving optimization problems like DRWAbecause of the complexity of the task. Therefore, they proposed a particle swarm optimizationinspired by swarm intelligence to solve DRWA. The simulation results obtained show that the pro-posed solution outperforms other swarm intelligence schemes like genetic algorithms for DRWA interms of low blocking probability.



In [94] the researchers proposed, for the first time, an ant-based route optimization that workswithout FDL and wavelength converters, and considers streamline effect. Also, the proposed algo-rithm implements the concept of hierarchical time sliced OBS for efficient use of the bandwidthutilization as proposed in [50]. The authors implemented a novel contention resolution techniquecalled time slot aggregation. In time slot aggregation, when contention occurs, contending burststime slots are assembled and sent as a jumbo burst to the desired destination.

Because OBS continues to be the favored technology for future communication networks to meetthe increasing need of large-bandwidth applications, routing strategies are seen as a major approachto solve the main problem of OBS (i.e., high burst loss because of contention). This is becauserouting is an important aspect of any network. If properly designed, it is able to enhance networkperformance by reducing congestion and thus avoids burst contention in the case of OBS. As dis-cussed earlier, many routing algorithms have been proposed in the literature for OBS. Routing isbeing used to control burst contention in two ways: first, as a contention resolution mechanism,and second, as a contention avoidance technique. Deflection routing algorithms belong to the firstcategory, while route optimization algorithms are contention avoidance schemes. In Table II, therouting strategies are categorized into two main categories: MILP optimization-based routing andheuristic-based routing. Table II also states the status of FDL, wavelength converters and streamlineeffect for each routing algorithm.

On one hand, mathematical-based routing algorithms use MILP to solve an optimization functionin the search for the best path(s) that minimize congestion and avoid contention.

On the other hand, heuristic algorithms follow some well-defined steps to route the burst betweensource and destination in a way that network congestion is reduced, which logically leads tominimum contention, and high throughput.

Table II indicates that the tendency in developing routing strategies for OBS is towards heuris-tic algorithms. Of the routing algorithms discussed, 70% (21/30) are heuristics, while only 30% ofthem are based on MILP. Of the heuristic algorithms, 40% are based on ACO. This high preferencefor heuristic approaches over MILP is due to the fact that RWA in OBS is an NP-complete problemand cannot be solved efficiently with MILP. Also, heuristic algorithms are less complex and scalablewhile MILP algorithms are complex and not scalable.

ACO and AntNet [78] based algorithms are gaining the attention of OBS researchers simplybecause they are adaptive multipath routings, which converge faster than MILP and have inher-ent desire for failure recovery. Table III shows an approximate performance of ant-based routingcompared with SP [28].

From Table III, it is clear that ant-based routing algorithm outperforms SP especially at low load.Although at high load the difference in BLP is somehow low, ant-based routing algorithm still scoreslower BLP.

7. CONCLUSION AND FUTURE WORKS

OBS is the most likely near future solution to greedy multimedia applications. However, this favoredtechnology suffers from burst contention. Solving burst contention is indispensable for makingOBS a feasible solution. In this paper, it has been shown that there are two approaches in deal-ing with burst contention: CAS and CRT. CAT solutions are less complex and more cost effectivethan CRT-oriented techniques.

Moreover, up to this writing, most of the route optimization techniques used in OBS, assume theuse of FDL and/or wavelength converters. These two technologies are still in their infancy stage[41]. Thus, they are not cost effective.

This paper shows that (Table II), heuristic algorithms, especially ant-based algorithms, are themost appropriate solution for RWA in OBS networks. Additionally, contention avoidance solu-tions are more preferable than contention resolution approaches because of their cost effectiveness.However, the two solutions can coexist.

On the basis of the above findings, the authors are currently implementing a ant-based survivableroute, wavelength, and time slot allocation algorithm for time-variant OBS, particularly, hierarchical



time-sliced OBS [50] not only to reduce burst contention but also to study the viability of this archi-tecture in WDM mesh networks environment, a work never done before, to our best knowledge.Hierarchical time-sliced OBS is an enhancement of the work in [47] denoted as time sliced OBS.The proposed algorithm considers streamline effect and does not use wavelength converters forcontention resolution. The use of FDL is made optional. The objective is to reduce network imple-mentation cost while supporting QoS provisioning and maintaining an acceptable level of networkperformance in terms of BLP, delay, and throughput.

Table II. Classification of routing algorithms in WDM optical networks.

Routing type Comparison features

Heuristic

No. Ref. ACO (AntNet) Non-ACO MILP FDL WC SLE Year

1 [30]p

� � � 20002 [85]

p p� � 2002

3 [62]p

�p

� 20044 [63]

p�

p� 2005

5 [72]p p p

� 20056 [67]

p�

p p2006

7 [79]p

� � � 20068 [86]

p2006

9 [87]p

� � � 200610 [64]

p� � � 2007

11 [65]p

� � � 200712 [68]

p� � � 2007

13 [61]p p

� � 200714 [37]

p� � � 2007

15 [62]p

� � � 200816 [66]

p�

p p2008

17 [69]p

�p

� 200818 [78]

p� � � 2008

19 [84]p

� � � 200920 [55]

p� � � 2009

21 [83]p

� � � 200922 [74]

p� � � 2009

23 [91]p

� � � 200924 [92]

p� � � 2009

25 [93]p

� �p

200926 [70]

p� � � 2010

27 [71]p

� � � 201028 [81]

p� � � 2010

29 [82]p

� � � 201030 [28]

p� � � 2010

Sub total 9 13 9Total 21 9 30

WC, wavelength converters; SLE, streamline effect;p

, YES; �, NO; — not specified.

Table III. BLP comparison of ant-based and SP routing algorithms.

Average BLP

Load% SP routing Ant-based routing

20 0.012 0.001640 0.014 0.01160 0.016 0.01380 0.018 0.014100 0.019 0.016



ACKNOWLEDGMENT

The first author wishes to thank the Islamic Development Bank for sponsoring his study.

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AUTHORS’ BIOGRAPHIES

Yahaya Coulibaly received his B.E. degree in Computer and Information Engineeringfrom the International Islamic University Malaysia (IIUM), Malaysia in 1999. Hereceived his M.Sc. in Networks and Communication Engineering from UniversitiPutra Malaysia (UPM) in 2002, Malaysia. He worked at AGETIC, (formerly known asMINTI) an agency of Ministry of Communication and New Technology (currently knownas Ministry of Posts and New Technologies) Bamako, Mali form 2004 to 2008. During thisperiod, he occupied the post of Director of Infrastructure and Development from 2006 to2008. He is now (2011) working towards his Ph.D. degree in Computer Science. His mainresearch areas of interest are but not limited to: Optical Networking, wireless and mobilenetworks, Grid Computing.

Muhammad Shafie Bin Abd Latiff received his Ph.D. from Bradford University, UnitedKingdom. His research interest is mainly in computer network focusing on grid computingand visualization technology. He holds the CCNA Professional and CCAI Certification. Heis an Associate Professor and currently the Deputy Dean of Academics at the Faculty ofComputer Science and Information System, Universiti Teknologi Malaysia (UTM).

Abu Bakar bin Mohammad obtained his Bachelor’s degree from the University ofStrathclyde, Scotland in 1985, Master’s degree from Hatfield Polytechnic (now knownas Hertfordshire University) and Ph.D. degree from Bradford University in 1995.Presently he is a professor and executive director of the Infocomm Research Alliance(ICRA) Universiti Teknologi Malaysia. He is an author of more than 100 technicalpapers published in journals and submitted at international and national conferences.He owns patents for seven products. His field of research includes MEMS, photoniccommunications system design, photonics switching and photonics devices.


A review of routing strategies for optical burst switched networks

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