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OpticsOptikOptikOptik 121 (2010) 1355–1362

0030-4026/$ - se

doi:10.1016/j.ijl

�CorrespondE-mail addr

www.elsevier.de/ijleo

Burst dropping policies in optical burst switched network

Amit Kumar Garga,�, R.S. Kalerb

aSchool of Electronics and Communication Engineering, Shri Mata Vaishno Devi University, Jammu and Kashmir, IndiabDepartment of Electronics and Communication Engineering, Thapar University, Patiala, Punjab, India

Received 22 September 2008; accepted 15 January 2009

Abstract

Optical burst switching (OBS) has been proposed as a competitive switching technology to support the nextgeneration optical Internet. However, due to their one-way resource reservation mechanism, OBS networks experiencehigh bursts (thus packets) loss rate. In OBS networks, the contention is resolved either by dropping one of thecontending bursts or more efficiently by dropping from one of the contending bursts only the parts that overlap withthe other bursts. In both situations, only one data source will suffer the data loss in favor to the other. In this paper, anovel burst dropping policy based on even selection of burst (BDPES) has been proposed in conjunction with anappropriate mechanism to provide differentiated service in order to support the quality of service (QoS) requirementsof different applications. In the proposed burst dropping policy, the dropped segments are selected evenly from bothcontending bursts and the truncated bursts are guaranteed to be larger than the minimum burst-length allowed by thenetwork. Furthermore, the proposed policy is enhanced via a flow control mechanism. Simulation results show that theperformance of proposed policy is better than existing burst dropping mechanisms in terms of reducing burst (packets)loss rate.r 2009 Elsevier GmbH. All rights reserved.

1. Introduction

The explosive growth of the Internet demands a high-speed transmission technology to support rapidlyincreasing bandwidth requirements. Currently, densewavelength-division multiplexing (DWDM) technologyachieves multiplexing of 160–320 wavelengths in onefiber with 10–40Gb/s transmission rate per wavelength.In order to efficiently utilize the raw bandwidth inDWDM networks, an all-optical transport system,which can avoid optical buffering while handling burstytraffic and which can also support fast resourceprovisioning and asynchronous transmission of variable-

e front matter r 2009 Elsevier GmbH. All rights reserved.

eo.2009.01.021

ing author.

ess: garg_amit03@yahoo.co.in (A.K. Garg).

sized packets, must be developed. Optical burst switching(OBS) [1] is a switching technique that occupies themiddle of the spectrum between the well-known circuitswitching and packet switching paradigms, borrowingideas from both to deliver a completely new functionality(as shown in Table 1).

The most common signaling protocol in OBS is just-enough-time (JET). In JET, a control packet is sent tothe destination to reserve necessary channels at each ofthe intermediate core nodes along the path. After anoffset time, the data burst (DB) is transmitted all-optically through the core. Since JET is a one-wayreservation signaling protocol, it can only provide verylimited quality of service (QoS) guarantee to the upperlayer. Different signaling and scheduling mechanismsdescribing the manner in which connections are

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Table 1. Comparison of optical switching schemes.

Optical switching (paradigm) Bandwidth utilization Latency (set-up) Optical buffer Traffic adaptively

Circuit Low High Not required Low

Packet/cell High Low Required HighBurst High Low Not required High

A.K. Garg, R.S. Kaler / Optik 121 (2010) 1355–13621356

established and resources are reserved and releasedhave been proposed for OBS. First-fit, horizon [2],latest available unscheduled channel (LAUC) and latestavailable unscheduled channel with void filling (LAUC-VF)[3], are among the proposed scheduling algorithms. Inboth LAUC and LAUC-VF scheduling algorithms, aburst chooses the unused channel that becomes availableat the latest time. When void filling (VF) is allowed, gapsbetween two scheduled data bursts can also be utilized.In these schemes, the data-burst reservation time startsat the beginning of the actual burst arrival and lasts untilthe end of the burst. A major concern in OBS networksis contention and burst loss. Typically, there are twomain sources of burst loss: contention on the outgoingdata channels and contention on the outgoing controlchannel. Contention is aggravated when the trafficbecomes bursty and when the data-burst duration variesand becomes longer. Contention and loss may bereduced by implementing contention resolution policies.There are different types of contention resolutiontechniques, such as time deflection (using buffering)[4], space deflection (using deflection routing) [5],wavelength conversion (using wavelength converters)and soft contention resolution (using different conten-tion resolution algorithms) [6]. Clearly, a combinationof such techniques can be very effective. Using bufferingin the core switches may not be viable, since thehardware complexity and high cost of such devicesmake them less attractive and limits their practicality.Space deflection can potentially result in inefficientrouting and a high number of collisions. Furthermore, itresults in high end-to-end delay and possible packetreordering, neither of which may be acceptable for manyapplications. Wavelength conversion on output ports isa very efficient approach for resolving contention andadds an additional dimension (in addition to time andspace) to contention resolution. When a contentioncannot be resolved by any one of these techniques, oneor more bursts must be dropped. The policy for selectingwhich bursts to drop is referred to as the soft contentionresolution policy. A soft contention resolution algo-rithm may be utilized in conjunction with a schedulingalgorithm to reduce the overall burst loss rate (BLR)and consequently, enhancing link utilization. Thus, thecontention resolution algorithm is invoked only whenno available unscheduled channel can be found for a

burst header packet (BHP) request. QoS support is animportant issue in OBS networks. Applications withdiverse QoS requirements urge the Internet to guaranteeQoS. There are two models for providing servicedifferentiation: relative and absolute. In the relativeQoS model, traffic is classified into classes. Performanceof each class is not defined quantitatively in absoluteterms based on loss, delay and bandwidth. Instead, theQoS of one class is defined relatively to other classes.For example, class of high priority is guaranteed toreceive lower loss than class of lower priority. However,no upper bound on the loss is guaranteed for the high-priority class. The absolute QoS model aims to provideworst-case guarantee on the loss, delay and bandwidthto applications. This type of hard guarantee is essentialfor the classes of delay and loss sensitive applications,which include multimedia and mission-critical applica-tions. Efficient admission control and resource provi-sioning mechanisms are needed to support the absoluteQoS model. Several schemes have been proposed tosupport the relative service differentiation in OBS. In [7],a proportional QoS scheme based on per-hop informa-tion was proposed to support burst loss probability anddelay differentiation. Also, an additional offset schemethat provides relative burst loss probability differentia-tion was proposed in [8]. Absolute QoS in OBS networkis primarily to guarantee burst loss probability for theprioritized traffic. For the delay and bandwidth QoSmetrics, since core nodes are bufferless and data burstsare transmitted all-optically, the delay is mainly propa-gation delay, while the bandwidth is a direct function ofloss probability. Though it has been well accepted thatabsolute QoS support is important, there is no scheme inthe literature to provide absolute QoS in OBS.

In this paper, a novel burst dropping policy based oneven selection of burst (BDPES) has been proposed inconjunction with an appropriate mechanism for QoSoriented OBS network absolute model with output datachannel contention. In the proposed dropping policy,the dropped segments selected evenly from bothcontending bursts and the truncated bursts, are guar-anteed to be larger than the minimum burst-lengthallowed by the network. Additionally, congestioncontrol mechanism has been used to improve theperformance of the proposed QoS-based burst droppingtechnique.

ARTICLE IN PRESSA.K. Garg, R.S. Kaler / Optik 121 (2010) 1355–1362 1357

2. Burst drop policies for contention resolution

2.1. Latest arrival drop policy (LP)

The simplest soft contention resolution policy is thelatest arrival drop policy. In LP, the algorithm searchesfor an available unscheduled channel (as in LAUC-VF)and if no such channel is found, the latest incoming databurst will be discarded. Although the processing speedof BHPs in the LP scheme is attractive, the maindisadvantage of this technique is that it has relativelypoor performance with respect to data loss when nobuffers are utilized. Inherently, LP is not capable ofdifferentiating packets with different priority types. Anovel scheme proposed by Yoo and Qiao [9] suggeststhat giving extra offset time to high-priority data burstscan ensure their early reservations. This approach isknown as offset-time-based QoS. The extra offset timemust be large enough to ensure that the blocking ofhigh-priority bursts by any lower priority burst isminimized. Therefore, offset-time-based QoS is a trade-off between guaranteeing lower loss for high-prioritydata bursts and increasing their end-to-end delay.

2.2. Look-ahead window contention resolution

(LCR)

The look-ahead contention resolution algorithm takesadvantage of the separation between the data bursts andthe burst header packets. By receiving BHPs one offsettime (d) prior to their corresponding data bursts, it ispossible to construct a look-ahead window (LAW) witha size of W time units. Having such a collective view ofmultiple BHPs, results in more efficient decisions withregard to which incoming bursts should be discarded orreserved. On the other hand, at each hop, the BHPsmust be stored for duration of W time units before theyare retransmitted (thus requiring dXW). Clearly, oneway to maintain the original offset time is to delay databursts by W time units by using fiber delay lines (FDLs)on each hop. Once the burst arrival times within theburst window are determined, the contending slots canbe found. Using the LCR algorithm, it is possible toidentify which bursts should be discarded and whichshould be scheduled. However, the data bursts areactually dropped or scheduled only when the startingtime of a burst is equal to the start of the burst window.After the LCR process is completed for the look-aheadwindow, the starting time of the window is advanced tothe next slot and may include new BHPs. Scheduledrequests are irreversible and cannot be changed by thefuture requests. Several advantages can be attributed tothe QoS-enabled LCR. For instance, it can supportunlimited number of classes of service without requiringextra offset time. The LCR mechanism can offer

absolute as well as proportional differentiation. Inabsolute differentiation, the possibility of a high-priorityburst being blocked by any lower priority burst iseliminated. On the other hand, in proportional differ-entiation the dropping criteria will be based on therelative length and priority level of data bursts. In such ascheme, it is possible that between a short duration high-priority burst and a long duration low-priority burst, theone with higher priority will be discarded. Clearly, interms of complexity, minimal additional steps arerequired to enable service differentiation in LCR.

2.3. Shortest burst drop policy (SBP)

A less complex version of the LCR algorithm, calledLCR with shortest drop, is known as the shortest burstdrop policy. In this case, contention regions aredetermined within window sizes of W ¼ Lmax. Then, ineach region, the bursts with the shortest duration andlatest arrival time will preferentially be dropped. Inorder to reduce the end-to-end data-burst delay, theLCR with shortest drop algorithm can be modified suchthat the window size is reduced to a single slot and thecontending burst with the shortest duration in each slotwill be discarded. In this case, BHPs are processedand transmitted as soon as they are received. Onedrawback of SBP scheme is its potential over-reservingof resources, since some earlier reservations may beeliminated later. In terms of supporting class differ-entiation, SBP can support unlimited number of prioritylevels and requires no extra offset assignments for burstswith higher service requirements. It also guaranteescomplete class isolation. In addition, SBP offersproportional differentiation.

2.4. Segmentation drop policy (SP)

The basic assumption in this scheme is that eachtransmitted data burst consists of individual indepen-dent segments such as slots. Therefore, if contentionoccurs, only the segments of the lower priority burstinvolved in the contention will be removed. Details ofthis mechanism, known as segmentation, along with itsvariations are described in [10]. Although the QoS-enabled segmentation algorithm appears to be straight-forward, the hardware implementation in terms of burstassembly and disassembly, as well as overhead insertionand extraction, can be complex.

3. QoS support in OBS networks

A great effort has been directed to the QoSprovisioning in the Internet, where many QoS mechan-isms are introduced. However, the mechanisms that

ARTICLE IN PRESSA.K. Garg, R.S. Kaler / Optik 121 (2010) 1355–13621358

work for electronic packet switched networks has notfound the same success in optical networks, because thelack of efficient optical buffer, which reduces thescheduling capabilities. The main QoS provisioningstrategies proposed for OBS networks are as follows.

3.1. Offset-based QoS scheme

Offset-based QoS [11] scheme adds an extra offsettime to the basic offset between BCP and its corre-sponding DB. The additional offset time called QoSoffset is to compensate for the processing time of theBCP. The duration of such a QoS offset is varied,depending on the priority of the service class. Thisoffset-based QoS scheme is proposed for JET, wherebyhigher priority classes have a larger offset. With thisscheme if a low-priority DB with no additional QoSoffset time and a higher priority DB with a QoS offsettry to make a reservation of the network resources, then,the DB with the larger offset will be able to reserveresources in advance and before the low-priority DB. Ingeneral, this results in a lower burst loss probability ofhigh-priority classes than that of lower priority classes.Although the offset-based QoS scheme does provide anacceptable service differentiation, it is faced with somechallenges that cannot be ignored. For example, DBs ofhigh-priority classes suffer longer waiting time (delay)than the data bursts of low-priority classes. Further-more, the scheme is nonpreemptive i.e., as long as low-priority DBs can block optical paths, no completeisolation is achieved. Yet, starvation of low-priorityclasses is possible if the offered traffic load of high-priority bursts is high and not controlled.

3.2. Active dropping-based QoS scheme

In this scheme, a burst dropper is implemented infront of every core node [12]. Dependent on a droppingpolicy, some BCPs and their corresponding DBs aredropped before reaching the reservation unit. Therefore,the admission to the outgoing wavelengths is controlled,enabling the core nodes to locally control the offeredload of certain service class to maintain networkresources for other service classes. Active dropping-based QoS scheme intervene before congestion occurs,as the selective dropping of DBs is initiated according tothe data traffic profile to guarantee that the higherpriority classes have higher chances to make successfulreservations. However, this scheme suffers of a majordisadvantage that is the absence of feedback from thecore nodes to the edge nodes and thus traffic volume ofdifferent classes cannot be controlled. Furthermore,isolation between different traffic classes is not guaran-teed. If the offered traffic load of a low-priority class issignificantly augmented, which increases the overall

burst loss probability; burst loss probabilities of allclasses are increased. Therefore, an additional trafficcontrol mechanism is required.

3.3. Segmented-based QoS scheme

In segmentation-based scheme [13], each data burst isdivided into several independent segments. If DBscontend for the same network resources, the contentionis resolved by discarding or deflecting some segments ofone of the contending bursts. The remaining part of theburst (truncated burst) will then be forwarded to thedownstream nodes where it will experience more short-ening, be dropped, or be delivered to the egress node.Unfortunately, this scheme is implemented at the cost ofincreasing the size of the control packets, since the BCPshould at least contain the segment number, the burstlength and the routing information. Furthermore, theimplementation of burst segmentation strategies is facedby some challenges and practical issues such as segmentdelineation, data-burst size, etc.

4. Proposed burst dropping policy with even

selection of burst (BDPES)

In this mechanism, the QoS requirements of the upperlayer packets are defined based on their class. Packets ofthe same class and destination are assembled into thesame data segment (DS), which will be labeled with apriority number accordingly. A data burst may containdata segments of the same or different priorities. Usingan adaptive burst assembly algorithm (ABA), the datasegments are assembled into data bursts; in such a waythat the lower priority data segments envelop the higherpriority data segments as shown in Fig. 1. To realize theproposed scheme, both the edge and core nodes mustco-operate using proposed burst assembly algorithm inthe edge nodes and BDPES in core nodes. To realize anefficient OBS implementation and achieve high-linkutilization, the data-burst transmission time i.e., burstlength/channel speed, should be larger than the switchfabric configuration time. The length of the data burst isentirely overlooked by the resource-allocation schemesbased on the burst segmentation scheme, since no policyrelated to the size of the truncated burst are implemen-ted, during the burst segmentation process in the corenodes. Furthermore, there is no fairness in allocating thenetwork resources to the contending data burst; all thesegments are simply discarded from only one burst toresolve the contention, which will increase the likelihoodof having bursts shorter than the minimum burstlength (MBL). A better solution is to select evenly(fairly) the segments to be dropped from the contendingdata bursts. Likewise, the truncated burst should be

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Data burst Segment

Lower priority data segments

Higher priority data segments

Fig. 1. Burst (data segment) envelope format.

= −

A.K. Garg, R.S. Kaler / Optik 121 (2010) 1355–1362 1359

monitored at the core nodes and guaranteed to be largerthan the MBL, which is the minimum length allowedinto the network to avoid congestion in the controlchannels as well as inefficient bandwidth utilization.

Fig. 2. Proposed BDPES algorithm.

4.1. Burst dropping policy with even selection of

burst

In OBS network, the contention is resolved either bydropping one of the contending bursts or moreefficiently by dropping only the overlapping part ofone of the contending bursts. In both situations, onlyone data source will suffer the data loss in favor to theother and the size of the burst is not monitored.However, in the proposed BDPES, the droppedsegments are selected evenly from both contendingbursts and the truncated data bursts are guaranteed tobe larger than the minimum burst-length allowed by thenetwork.

Notations:

�

SB: scheduled data burst with arrival time tsa andleaving timetsl. � CB: contending data burst with arrival time tca and

leaving time tcl.

� TB: truncated data burst. � X,Y: number of segments in SB and CB, respectively. � DSL: data segment with length. � SBL: scheduled data-burst length i.e., SBL ¼ tsl�

tsa ¼ X*DSL.

� CBL: contending data-burst length i.e., CBL ¼ tcl�

tca ¼ Y*DSL.

� EDS: expected number of segments to be dropped

from each data burst in case of contention i.e.,EDS ¼ |tca�tsl|/2DSL.

� TS: time of switching. � Offset-time condition: tOffsetXtmax-burst, where

tmax-burst is the maximum size of the burst.

� Upper bound of the maximum burst size: tmax-burstp

tpropagationa/N, where tpropagation, a and N are the meanpropagation delay, the ratio of FDL delay to thepropagation delay and the number of nodes, respec-tively. The delay ratio (a) affects the increment ofEnd-to-End (ETE) delay: the ETE delay incrementfrom FDL can be reduced by lowering the delayratio (a).

The proposed burst dropping policy with evenselection of burst is based on four main events:contention-detection, truncated-burst, resource-alloca-tion (even) and service differentiation. The detailedBDPES algorithm is shown in Fig. 2.

To realize the services differentiation scheme, both theedge nodes and core nodes must co-operate. The edgenodes should use an appropriate assembly algorithmthat complements the aforementioned burst droppingpolicy with even selection of burst deployed in the corenodes.

4.2. Proposed adaptive burst assembly algorithm

In this scheme, a small amount of traffic has beenstored at the edge router and the output traffic isdetermined by the amount of stored traffic.

Notations:

�

Tedgemax has been considered as the preset maximum

tolerant value for the edge buffering delay (here, edgebuffering delay refers to the period a packet spentwaiting at the edge node).

� At the initial phase of assembly, the arriving packets

will be stored at the edge node until the timer exceedsTedge

max.Then, edge router sends traffic out at an averagerate in the timescale ½0;Tedge

max �.

� At time t, the rate of the output traffic is set to the

average rate in ½t; tþ Tedgemax �.

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Fig. 3. Adaptive burst assembly algorithm (ABA).

Explicit-signaling DSL

0

0

1 11

1 1 1

0 1 1

FCRB

Length of DB= 2* DSL

One DS is dropped

Burst control packet (BCP)

Flag 1

Fig. 4. The burst control packet (BCP) format.

A.K. Garg, R.S. Kaler / Optik 121 (2010) 1355–13621360

�

When, the bursts are sent out periodically, the burstsize is set to the average number of packets arrivingduring assembly period Tb. � Let, each edge router has M queues to sort the

arriving packet. Also, it is assumed that the timer ofqueue Q[i] be denoted by Ti and the length of Q[i] bedenoted by L[i].

� The threshold for generating a burst is Lth[i]. When

the value of the queue length L[i] is smaller thanLth[i], all packets in Q[i] will be assembled into aburst. Otherwise, a burst is generated with the lengthof Lth[i] and the other packets are left in Q[i].

The proposed scheme is thus implemented using theABA algorithm as shown in Fig. 3.

In the proposed scheme, the threshold Lth[i] iscalculated by the following equation:

Lth½i� ¼ ð1þ aÞ � Lk½i��

where ðLk½i�XL½i�=b

ð1þ aÞ � L½i�=b�

where ðLk½i�oL½i�=b

where Lk½i�, the mean length of the recent k bursts ofQ[i], is used to predict the average size of the arrivingbursts, a is the parameter of the redundancy degree forthe prediction of the arriving burst size and b is definedas b ¼ Tedge

max=Tb.

4.3. Flow control using explicit congestion control

technique

To effectively implement the proposed burst droppingpolicy, it is noted that dividing each data burst into datasegments will not be sufficient and representing each

DS’s control information in the BCP is not feasible(which is traditionally done). In this paper, it isproposed that each segment may range from one toseveral packets and its length should agree withminimum and maximum length requirements (forefficiency). Additionally, the length of each DS shouldbe explicitly reflected in the BCP. Therefore, a suitableBCP format is proposed. The proposed BCP’s formatprovides constant transmission overhead and makes theBCP scalable to higher speeds, as it uses the flow control

and reservation bits (FCRB) as the segments’ lengthindicator instead of flags [14]. In the proposed burstcontrol format as shown in Fig. 4, an explicit signaling isused. The bits of FCRB are used to indicate explicitlythe amount of data sent and arrived. This explicitsignaling works in two modes as follows:

(1)

Forward direction: In this, it notifies the egress nodethat congestion procedures should be initiated whereapplicable for traffic in the opposite direction of thereceived bursts. It indicates the number of the droppedsegments and that the received burst has encounteredcongested resources. This information could be sentback to the source node and the end system willexercise flow control on the traffic at the higher layers.

(2)

Backward direction: In this, it notifies the egress nodethat congestion procedures should be initiated whereapplicable for traffic in the same direction as the sentbursts. It indicates the number of the droppedsegments and that the sent burst has encounteredcongested resources. The ingress node will then lowerthe number of data segments sent in each data burst tobe equal to the number of data segments that couldget through the network to the destination. Then thenumber of data segments is augmented progressivelyuntil the maximum size of the data burst is reached oruntil the FCRB field reports congestion.

4.4. Analytical model

The well-known Erlang-B formula has been used toobtain the burst loss probability:

Pðk;TLÞ ¼TLk=k!

Pk0TLm=m!

(1)

ARTICLE IN PRESSA.K. Garg, R.S. Kaler / Optik 121 (2010) 1355–1362 1361

where TL is the traffic load and k is the number ofwavelengths available at each output port. In this paper,a queuing model M/G/N has been used to evaluate theperformance of the proposed scheme BDPES. With nobuffering, the technique can be modeled as M/G/Nqueue system with infinity of imaginary servers besidesthe available n servers (i.e., number of wavelengths).With the number of busy servers equal to (n+k), twocases arises:

(1)

kp0; no contention (number of busy server is pn). (2) k40; all the n servers are busy and there are k

imaginary active servers for k upcoming DBsattempting to be switched.

s R

ate

1e-2

1e-1

1e+0Latest arrival drop policy(LP)Proposed burst drop policy (BDPES)Segmentation drop policy(SP)

For case (2), there are k DBs lost for every n

data segments transmitted. As soon as the contentionis resolved, the contending burst is moved fromthe imaginary server to be served by an originalserver. The packet loss probability P(PLB) can beobtained by

PðPLBÞ ¼ TL�1X1

k¼1

iPðnþ kÞ (2)

where TL�1 and P(n+k) are, respectively, the trafficload and the probability that (n+k) servers are busy.Since the number of busy servers in M/G/N model hasthe Poisson distribution [15], P(n+k) can be obtained asfollows:

Pðnþ kÞ ¼ TLnþke�TL=ðnþ kÞ!; k ¼ 1; 2; 3; 4; . . . (3)

Offered Load (Erlangs)

0.2

Bur

st L

os

1e-5

1e-4

1e-3

1.00.80.60.4

Fig. 5. Overall burst loss rate (BLR) performance usingdifferent burst drop schemes.

Traffic load

0.0

Pack

et d

rop

rate

0.0

0.1

0.2

0.3

0.4

0.5Priority 0:(Proposed BDPES)Priority 0:(Conventional BP)Priority 1:(Proposed BDPES)Priority 1: (Conentional BP)

1.00.80.60.40.2

Fig. 6. Packet drop rate vs. traffic load (without congestioncontrol).

5. Performance evaluation

The simulation has been implemented on the 12-nodeNSF network [16]. The paths between all of thesource–destination pairs are calculated using Dijkstrashortest path algorithm. Packets arrive at ingress nodesfollowing the Poisson distribution with rate l and areassembled into bursts. Each burst is composed of 5segments and each segment is composed of 10 packets.Packet size is exponentially distributed with average sizeof 10,000 bits. The links used are of 10Gbps. Tosimplify the simulation, the switching time has not beenconsidered. There are no wavelength converters andoptical buffers at the core nodes. Offsets between BHPsand their associated data bursts are fixed. Only twotraffic classes are assumed with the same traffic load.First class with Priority-1 is the higher priority whereasPriority-0 is for the second class, which is the lowerpriority. 95% confidence interval (batch mean method)is used.

6. Simulation results

The performance metric used for the simulation isburst (packet) loss rate. The burst loss rate is thepercentage of burst that are sent by the source but neverreceived by the destination. Fig. 5 shows the overallperformance of proposed BDPES compared to SP andLP algorithms. As expected, SP and LP provide theupper and lower bounds on performance, respectively.On average, the BDPES performs about 25% betterthan LP.

Firstly, the simulation has been performed withoutthe use of congestion control mechanism as shown inFig. 6. Secondly, the proposed congestion controlmechanism (only backward signaling) has been usedand the result is shown in Fig. 7. Through simulation, ithas been observed that the lower priority data segmentsexperience more dropping rate. The higher priority datasegments are given a higher quality of service and thereduction on the dropping rate is particularly by usingproposed congestion control mechanism. Fig. 8 showsthat the comparable result that has been achievedthrough proposed BDPES-based OBS and the analyticalmethod. Further improvement is expected, if theproposed OBS system is designed to support deflectionrouting with fiber delay lines or with wavelengthconversion capabilities.

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Traffic load

0.0

Pack

et d

rop

rate

0.0

0.1

0.2

0.3

0.4

0.5Priority 0:(Conventional BP)Priority 0:(Proposed BDPES)Priority 1:(Conventional BP)Priority 1: (Proposed BDPES)

1.00.80.60.40.2

Fig. 7. Packet drop rate vs. traffic load (with proposedcongestion control).

Traffic load per Fiber

0.05

Pack

et lo

ss p

roba

bilit

y

1e-6

1e-5

1e-4

1e-3

1e-2

1e-1Analytical-OBSProposed BDPES-OBS

0.300.250.200.150.10

Fig. 8. Packet loss probability vs. normalized traffic load.

A.K. Garg, R.S. Kaler / Optik 121 (2010) 1355–13621362

7. Conclusions

In this paper, an overview of the optical burstswitching network and its current contention resolutiontechniques based on burst dropping policies is provided.As the size of the data bursts have a direct impact on theOBS control channels and the bandwidth utilizationtherefore, a new and effective implementation of burstdropping scheme (BDPES) has been presented. Withthis scheme, the dropped segments are evenly distributedbetween the contending bursts to achieve some kind offairness between traffic flows and to minimize thenumber of short data bursts. Furthermore, the proposedscheme enables the core nodes to monitor and managethe size (length) of the data bursts traveling within thenetwork backbone. The scheme is simple, practical andits implementation does not lead to any compromises onone of the main motivational reasons behind theemergence of the OBS paradigm, which is simplicity.The simulation results show that the proposed schemecan readily support service differentiation and offershigh overall performance with moderate complexity.Additionally, a new burst control format is proposed.With this format, the length of the data burst and datasegments can be shown, as well as, the number of thedropped segments and the forwarded segments usingonly a limited number of bits (flow control andreservation bits). As a result, it is clear that thefunctionality of FCRB field can be extended to provideflow and congestion control capabilities in the optical

domain. The proposed scheme can be modified toreduce the total end-to-end delay at the cost of slightlylowering the performance.

References

[1] C. Qiao, M. Yoo, Optical burst switching (OBS) – a newparadigm for an optical Internet, J. High Speed Networks8 (1999) 69–84.

[2] J.S. Turner, Terabit burst switching, J. High SpeedNetworks 8 (1999) 3–16.

[3] Y. Xiong, M. Vanderhoute, H.C. Cankaya, Control

architecture in optical burst-switched WDM networks,IEEE J. Selected Areas Commun. 18 (2000) 1838–1851.

[4] C. Gauger, Dimensioning of FDL buffers for optical

burst switching nodes, in: Proceedings, Optical NetworkDesign and Modeling (ONDM) 2002, Italy, 2002.

[5] X. Wang, H. Morikawa, T. Aoyama, A deflection routing

protocol for optical bursts in WDM networks, in:Proceedings, Fifth Optoelectronics and CommunicationsConference (OECC) 2000, Makuhari, Japan, pp. 94–95.

[6] S. Yao, B. Mukherjee, S.J.B. Yoo, S. Dixit, All-optical

packet switching for metropolitan area networks: oppor-tunities and challenges, IEEE Commun. Magazine 39(2001) 142–148.

[7] Q. Zhang, V. Vokkarane, J. Jue, B. Chen, Absolute QoSdifferentiation on optical burst-switched networks, IEEEJ. Selected Areas Commun. 22 (2004).

[8] Li Yang, Congestion control and quality-of-service (QoS)provisions for optical burst switched networks, Ph.D.Thesis, North Carolina State University, Raleigh, NC,

May 2006.[9] M. Yoo, C. Qiao, Supporting multiple classes of services

in IP over WDM networks, in: Proceedings, IEEEGLOBECOM 1999, pp. 1023–1027.

[10] V. Vokkarane, J. Jue, S. Sitaraman, Burst segmentation:an approach for reducing packet loss in optical burstswitched networks, in: Proceedings, IEEE International

Conference on Communications (ICC), vol. 5, 2002,pp. 2673–2677.

[11] P. Fan, C. Feng, Y. Wang, N. Ge, Investigation of the

time-offset based QoS support with optical burst switch-ing in WDM networks, in: Proceedings, IEEE Interna-tional Conference on Communications (ICC), vol. 5,2002, pp. 2682–2686.

[12] Y. Chen, M. Hamdi, D.H.K. Tsang, Proportional QoSover OBS networks, in: Proceedings of the IEEEGLOBECOM, 2001.

[13] V. Vokkarane, J.P. Jue, Prioritized routing and burstsegmentation for QoS in optical burst-switched networks,in: Proceeding of the OFC, 2002, pp. 221–222.

[14] Y. Mei, C. Qiao, Efficient distributed control protocolsfor WDM optical networks, in: Proceedings of theInternational Conference on Computer Communication

and Networks, 1997, pp. 150–153.[15] D. Bertsekas, R. Gallager, Data Networks, second ed.,

Prentice-Hall, Englewood Cliffs, NJ, 1992.[16] The Network Simulator: NS2. /http://www.isi.edu/

ns-nam/ns/S.