Optical Fiber Technology 17 (2011) 64–69

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Optical Fiber Technology

www.elsevier .com/locate /yof te

An efficient Quality of Service (QoS) scheme for all optical networks

Amit Kumar Garg a,⇑, R.S. Kaler b

a School of Electronics and Communication Engg, Shri Mata Vaishno Devi University, J&K, Indiab Department of Electronics and Communication Engineering, Thapar University, Patiala, Punjab, India

a r t i c l e i n f o a b s t r a c t

Article history:Received 13 May 2010Revised 4 October 2010Available online 11 November 2010

Keywords:WDMOptical burst switchingQuality of ServiceLatency

1068-5200/$ - see front matter � 2010 Elsevier Inc. Adoi:10.1016/j.yofte.2010.10.007

⇑ Corresponding author.E-mail address: garg_amit03@yahoo.co.in (A.K. Ga

This paper addresses the issue of providing Quality of Service (QoS) for optical burst switching (OBS) sys-tems. In this paper, an efficient QoS oriented integrated scheme based on Optical Burst/Circuit Switchingnetwork architecture has been proposed. The proposed scheme utilizes the modified Just-Enough-Time(JET) protocol and advanced reservation based on Linear Predictive Filter (LPF), which can provide differ-entiated services for optical burst switching (OBS) network. Simulation results show that the proposedscheme is reasonably better than a First Come First Served (FCFS) protocol in guaranteeing QoS of real-time traffic. Also, the results yield significant delay reduction for time-critical traffic, while maintainingthe bandwidth overhead within limits. As a result, the proposed scheme has achieved QoS differentiationin terms of burst loss rate in comparison to conventional schemes.

� 2010 Elsevier Inc. All rights reserved.

1. Introduction

Optical burst switching (OBS) is an emerging solution to achieveall-optical wavelength division multiplexed (WDM) networks. Itcombines the advantages of optical circuit switching and opticalpacket switching [1,2]. In the past few years, various solutions havebeen proposed and analyzed in an attempt to improve the perfor-mance of OBS networks. In OBS networks, the basic switchingentity is a burst. Ingress OBS node assembles packets (typically IPpackets) generated by peripheral networks and form a large sizedpacket named as burst (average burst size is 15 Kbytes) in relationto time out or burst length policy. In other words, packets are buf-fered in the source edge routers to form a burst (also known asflow). For TCP applications, the information is organized in flow(multiple bursts). An optical burst contains data of the same classand of the same flow. Per flow queuing is needed at the ingressof the assembly unit. Flow in such a way that every burst belongingto that flow is treated in the same way from source to destination.Prior to transmitting a burst, a control packet is created and imme-diately sent toward the destination in order to set up a buffer-lessoptical path for its corresponding burst. After an offset delay time,the data burst is transmitted without waiting for an acknowledg-ment from the destination node. The optical path exists only forthe duration of a burst. OBS provides a huge bandwidth whichcould alleviate the increasing demands of Internet traffic; however,challenges remain on how to provide Quality of Service (QoS) forInternet applications in such a network. For example, applicationssuch as Internet telephony and video-conferencing require a higher

ll rights reserved.

rg).

QoS than electronic mail and general web browsing. In an IP net-work, many methods have been proposed to implement QoS suchas fair queuing, weighted fair queuing, frame-based fair queuing,etc. However, all of these methods are based on employing buffersat the network nodes. To implement the existing QoS mechanismsto differentiate services, all intermediate nodes should have a cer-tain amount of buffer space. However, the use of electronic buffernecessitates O/E and E/O conversions which sacrifice the datatransparency. On the other hand, no optical buffer (RAM) is avail-able and the use of fiber-delay lines (FDLs), which can provide alimited delay, should also be avoided as much as possible in theoptical layer.

Optical burst switching (OBS) provides a feasible solution toIP-Over-WDM systems, which support multiple types of traffic,such as audio and data, each with different QoS requirements. Itbecomes increasingly important to design an OBS system thatguarantees QoS provisioning for different classes of traffic.Whereas more bandwidth is being provided in Gigabit optical net-works, one must address the latency issue in the next generationnetwork. There have been numerous proposals in the literaturefocusing on the latency reduction issue in OBS systems. For exam-ple, a typical OBS system features one-way reservation thatlowers the round-trip delay for signaling transmission. In [3], aJust-In-Time (JIT) protocol has been proposed to reduce burst delaydue to light-path setup. The optimal switching architectures of thecore routers to process control headers has been discussed in [4].All these strategies are focused on reducing the latency in the corenetwork. We observe that the bandwidth at the core network(OC192 and upper) is much higher than that in the edge network(OC3–OC48). The time for burst assembly therefore has a signifi-cant impact on the end-to-end burst delay. This is especially the

A.K. Garg, R.S. Kaler / Optical Fiber Technology 17 (2011) 64–69 65

case for applications with strict delay constraints (e.g., Internettelephony and video-conferencing). Hence, reducing burst delayat the edge routers will be greatly beneficial to latency reductionand QoS provisioning.

In this paper, a novel integrated QoS supporting scheme hasbeen proposed that can support the multiple bursts transmission.The scheme has advanced resource reservation based on linearpredictive filter for latency reduction at the edges of an OBS sys-tem. The scheme is further extended by using modified-JET proto-col to achieve controllable QoS differentiation on burst delay fordifferent classes of traffic. Two classes of services, real-time andnon-real-time, are considered here. The bursts in the real-timeclass have a strict bound on delay and delay-jitter, thus requiringa guaranteed low blocking probability. On the other hand, thebursts in the non-real-time class can tolerate delay but require reli-able delivery which can be accomplished by buffering and retrans-missions. In this paper, it is assumed that no buffers are used in theoptical layer, which is highly desirable in all optical networks.However, buffering is used in the electrical layer for control pack-ets in OBS nodes. Simulation results show that the proposedscheme substantially reduces the burst delay caused by the burstassembly at edge nodes, while maintaining the bandwidth wastageof the system within limits and also achieves a QoS differentiationin terms of burst loss probability and flow loss probability.

2. State of the art in QoS issues for OBS

QoS provisioning in OBS networks is usually based on the differ-entiated services approach, which is mainly applied in three differ-ent ways such as (i) exploiting an extra offset-time (OT) in so calledOffset Time Differentiation (OTD) scheme (ii) assigning priority tothe higher traffic class (iii) varying the burst aggregation parame-ters [5].

In the first case, an extra OT is assigned to high priority burstswhat favors them during resource reservation process in the net-work. In the second, QoS is provided in the core switch by pre-empting low priority bursts (with or without segmentation) orrescheduling low priority control packets. Finally, varying the burstaggregation parameters can help in minimization the end-to-enddelay of high-priority packets. The pre-emption introduces highsignaling overhead and complexity, while in OTD, a sensitivity ofthe high priority class to burst length characteristics has beenobserved, which further extend pre-transmission delay that maynot be tolerated by some real-time applications. A segmentationtechnique, which first was introduced in order to decrease burstloss probability in OBS networks and then to assist the QoS [6], alsointroduces high complexity. Therefore, the problem is still opened.The Just-Enough-Time (JET) protocol presented in [7] has two fea-tures; one is delayed reservation, which reserves the bandwidth oneach link for data burst duration. The other is postponing of thearrival of data burst. By providing a delay to the data burst at thelocal node, JET helps to increase the usage of bandwidth and reducethe number of retransmission. In [8], the authors conclude that JEThas the best performance compared with other burst reservationmechanisms in a single-class optical burst switching network.Moreover, [8] evaluates JET by simulations and an approximateanalysis in a two-class OBS node. Pros and cons of JET are alsogiven in such a network. In [9], the authors propose a prioritizedOBS protocol based on JET which can provide QoS in buffer-lessWDM optical networks. The authors, in [9], analyze the lowerand upper bounds on the blocking probability of each traffic classand evaluate the performance of the proposed prioritized OBSprotocol. They conclude that real-time traffic can achieve a signif-icantly reduced blocking probability by using a reasonable amountof additional time. In addition, the overall blocking probability and

throughput can be maintained regardless of the additional offsettime used. Furthermore, another study [10] extends [9] to supportan arbitrary number of classes in IP over WDM networks. In[11–13], another QoS performance of optical burst switching inIP over WDM networks is presented. However, it is based on usinglimited number of FDLs.

Further, in [14], the authors have proposed an integrated net-work architecture called Polymorphous, Agile and TransparentOptical Networks (PATON). Unlike prior efforts in using hybridswitching whereby both optical burst switching (OBS) and opticalcircuit switching (OCS) are applied using separate signaling proto-cols as well as separate switching fabrics, PATON uses Polymor-phous OBS (POBS), which extends OBS and GMPLS signaling toseamlessly combine different switching and reservation schemesto support a variety of applications and services requiring variousbandwidth granularities, transmission schedules and QoS levels.The unique features of PATON are that, by using a single yet highlyflexible and versatile framework based on POBS, PATON can pro-vide data transparency and low latency by eliminating intermedi-ated processing in the core networks, while achieving statisticalmultiplexing gains among bursts or sub-wavelength Time DivisionMultiplexed (TDM) circuits. This will reduce not only the numberof transponders, wavelengths, and types of core networks required,but also the size of the switches and routers in the network. In [15],the authors address the problem of routing optimization in opticalburst switching (OBS) networks. A simplified analytical model ofOBS network has been used with an overall burst loss probabilityas the primary metric of interest. Since the objective function ofoptimization problem is nonlinear, they propose two solutionsbased on a non-reduced link load (NR-LL) model and a reduced linkload (R-LL) model. In order to find partial derivatives of the costfunction, a calculation considered previously for circuit-switchednetworks is applied. Exact partial derivatives of NR-LL model hasbeen derived along with approximation of partial derivatives ofR-LL model. Simulation results demonstrate that obtained solu-tions effectively reduce the overall burst loss probability over theshortest path routing. Moreover, in many cases, they over-performan alternative routing. Also, in [16], the authors proposes a newmodel for priority assignment to the incoming call connectionrequest for all optical WDM communication, adopting standardqueuing theory concept. The model has been proposed for threedifferent levels of call connection priorities, categorizing them intothree types of signals as type0, type1 and type2 according toincreasing priority and generation probabilities of P0, P1 and P2respectively. The traffic at the node will be serviced according tothe priority leveled on the header of the assigned packets. A trafficthroughput model has been developed to evaluate the blockingprobability of three types of connection requests. It is envisagedthat proper priority control and dynamic allocation of availablechannels in different priority category opens a scope for lowerblocking probability.

3. Proposed efficient QoS scheme

The proposed scheme comprises of (i) Modified-JET (ii) LinearPredictive Filter based Resource Reservation (LPFRR) (iii) QoSdifferentiation mechanism. In the proposed scheme, at each edgerouter, the aggregate packet arrival process is superimposed byindependent ON/OFF source. ON and OFF periods are exponentiallydistributed and the minimum length of ON period is 1 and that ofOFF period is 0. Bursts arrival during ON period, is defined as flow.Thus, the length of ON period indicates the number of bursts in aflow. Also, in the proposed scheme, the bursts are of fixed length.

In the proposed scheme, the incoming traffic is first classifiedinto high priority and low priority. The corresponding control

66 A.K. Garg, R.S. Kaler / Optical Fiber Technology 17 (2011) 64–69

headers are transmitted before the bursts are assembled to reduceprocessing time. The burst lengths announced in the control head-er are predicted based on an N-order LPF, i.e. based on the length ofthe last N bursts with some safety-margin. Control header for highpriority traffic is being processed prior to header for low prioritytraffic. Also, flow of bursts is formed by considering same or differ-ent destinations. In order to achieve QoS differentiation, for groupof continuous bursts destined to the same destination node, flow-reservation for multiple continuous bursts is made.

For the high priority traffic, a burst header is transmitted beforethe burst has been assembled. For the low priority traffic, when aburst has been assembled, the node waits to see whether anotherburst is arriving so that it transmits a flow-reservation. Accord-ingly, bursts are transmitted as per one-way reservation. Depend-ing upon the traffic classification, the source node decides when totransmit a flow header and when to transmit a single burst header.

The following is the detailed description of the proposedscheme.

3.1. Modified-JET

The modified protocol is based on JET. Two classes of service areconsidered: class 0 and class 1. Class 0 corresponds to the best ef-fort or non-real-time services for applications such as transportingplain data, while class 1 corresponds to guaranteed or real-timeservices for applications involving audio and video communica-tions. Since class 1 traffic should be delivered with a strict boundon delay and requires a low blocking probability, it is given a high-er priority for bandwidth reservation. The queuing mechanismsthat can be used in optical networks are limited because of thetechnical limitations of deploying buffers in the optical layer. How-ever, the OBS network allows the possibility to use buffers for con-trol packets in the electrical layer. The control packet contains theinformation about its corresponding burst and is electronicallyprocessed by the ingress OBS node and all the subsequent nodesalong the path to the destination user. Therefore, the control pack-ets cannot be transported transparently in an OBS network. It isfeasible to buffer the control packets in the electrical layer at theOBS nodes. The proposed Modified-JET protocol is based on theusage of queues for control packets in the electrical layer at theOBS nodes. Two queues are added in JET protocol, q0 for class 0 con-trol packets and q1 for class 1 control packets. The size of q0 and q1

is limited by the memory resource in the OBS node. Moreover, atime-window Dt is associated with these two queues. The controlpackets will be in a particular window [t1, t1 + Dt], if the controlpacket arrives in that interval. All incoming control packets inthe particular window are buffered and are kept in the correspond-ing queues. There is offset_time between the control packet and itscorresponding burst. It is seen that class 1 is always scheduled be-fore class 0 in time-window Dt, which indicates that class 1 hasmore priority than class 0 in reserving bandwidth.

3.2. Linear Predictive Filter based Resource Reservation (LPFRR)

The brief working of LPFRR is described as follows.

3.2.1. PredictionBefore a data burst begins to assemble, the burstification con-

trol unit (BCU) predicts the reservation length for the incomingdata burst. This estimation is derived from an LPF-based method.

3.2.2. Pre-transmissionAs soon as a burst begins to assemble at an edge node, i.e., when

the first bit of the first packet in a burst arrives at the burst assem-bly queue at time, the BCU fills the information necessary for path

setup, including the reservation length, into a BHP. The BHP is thensent into the core network.

3.2.3. ExaminationWhen the burst assembly finishes, the actual burst length is

compared with the reservation length in the pre-transmittedBHP. One of the following cases may occur:

(i) If the actual burst length is less than or equal to the pre-reserved length, i.e., the BHP has reserved enough band-width for the data payload, the BHP pre-transmission isdeemed a success.

(ii) If the actual burst duration exceeds the reservation length,the BHP pre-transmission is deemed a failure. The BHP hasto be re-transmitted for this burst at a later time with theactual burst size and the data payload lags behind by the off-set time.

The proposed QoS scheme requires a priori knowledge of theburst length before it is fully assembled. This is made possible withan N-order LPF. Let Ld(k) be the length (in the time scale) of the kthburst, then the length of the next incoming burst is predictedaccording to the lengths of the previous N bursts by:

eLdðkþ 1Þ ¼XN

i¼1

ðwðiÞÞ � Ldðk� iþ 1Þ ð1Þ

where wðiÞ; i 2 1; . . . ;N are the coefficients of the adaptive filter andare updated by the normalized Least Mean Square (LMS) algorithm[17]. A control header makes an advance resource reservationaccording to the predicted value. The advance reservation length,denoted as Ly[k + 1], if optimal, should be equal to the actual burstlength. Due to the imperfection of a predictor, however, an esti-mated length may turn out to be smaller or larger than the actualburst duration. Suppose the reservation length is set to be equalto the predicted length, a smaller prediction of burst length

eðkþ 1Þ ¼ Ldðkþ 1Þ � eLdðkþ 1Þ� �

> 0� �

will result in an insuffi-

cient reservation of path holding time for the data burst. Thisrequires the BHP to be re-transmitted after the burst assemblyfinishes, thus degrading the LPFRR latency reduction performance.This problem is compensated by some modifications in the

reservation method. Instead of making Lyðkþ 1Þ ¼ eLdðkþ 1Þ, the

reservation length has been redefined as Lyðkþ 1Þ ¼ eLdðkþ 1Þ þ d,where d is a small margin of correction. d may be any of the multipleof the sample Root Mean Square (RMS) of the LPF, defined as

d ¼ m �

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiPN

iþ1e2ðk�iþ1Þ

qN , where m is a real value and is determined

according to the tradeoff consideration between the bandwidthand the successful BHP pre-transmission probability.

3.3. QoS differentiation mechanism

The proposed QoS differentiation scheme supports multiplebursts transmission. To cope with this requirement, OBS/OCS(optical circuit switching) integrated network architecture hasbeen deployed. In the proposed scheme, group of continuousbursts bound for the same destination has been defined as a ‘‘flow”.Firstly, a head burst of the flow tries to establish a path. It has beengiven a high priority by adding an extra offset for constructing thepath certainly even when the destination is far. At the source node,the head burst first searches an available wavelength from all thewavelengths and chooses one wavelength randomly. At the inter-mediate nodes, the head burst confirms only the availability ofan active wavelength, since no wavelength converter is used atthe nodes. Other bursts first searches the available path con-

A.K. Garg, R.S. Kaler / Optical Fiber Technology 17 (2011) 64–69 67

structed by the head burst and if the available path exist, bursts aretransmitted by using the path. Therefore, if the temporary path issuccessfully constructed by the head burst, following multiplebursts are assuredly transmitted. Otherwise, all bursts are trans-mitted respectively according to one-way reservation like a normalOBS network. If the flow uses the path, the final burst of the flowwould release it. Trigger of the path construction is an importantissue in the proposed scheme. Flow size is an essential conditionfor setting up the path. To reserve and lease a path, flow must becomposed of not less than two bursts. The system metrics suchas traffic, QoS and distance have been taken into consideration.Especially, distance is considered as an important trigger, sincetransmissions to the farther destinations are difficult in OBS net-work. In this paper, the two (high and low) classes in terms ofthe burst loss probability and flow loss probability are consideredand use the path for high-class flows. Another important elementthat influences QoS performance is a scheduling algorithm. It per-forms during the resources reservation process and is responsiblefor wavelength assignment for a burst. Several scheduling algo-rithms have been proposed for OBS, e.g. the simplest one is Horizon[18] that is based on the knowledge of as the latest time at whichthe channel (wavelength) is currently scheduled. The Latest Avail-able Unused Channel with Void Filling (LAUC-VF) [3] algorithmwith a number of its variations extends and improves the Horizonscheme. LAUC-VF keeps track of the latest unused resources(instead of the latest unscheduled resources like in the Horizon),thus allows putting short bursts into a time gaps before the arrivalof a future scheduled burst. The drawback of the algorithm is itshigh processing complexity. In the proposed scheme,LAUC-VF_MIN-SV (minimum starting void) scheduling strategyhas been considered; this algorithm is derived from the LAUC-VFand aims in minimization voids introduced between new sched-uled bursts and a preceding one. The detailed description of sched-uling algorithms is found in [19,20].

4. Performance evaluation

The performance of the proposed scheme has been evaluatedusing NS-2 simulator [21]. The topology used is NSF14-Nodes

Input Load0.0 0.1 0.2 0.3 0.4 0.5

Bur

st L

oss

Rat

e

0.01

0.1

1

Fig. 1. Burst loss rat

network. It is assumed that each node is composed of both an edgerouter and a core router. In each network, bidirectional links withone fiber in each direction has been considered. Each link has datachannel and one control channel .The transmission rate on eachchannel is 10 Gbps. The bursts are of fixed length, i.e. 15,000 bytes.ON period is composed of four bursts on average and at a maxi-mum, 20 bursts are included in it.

Some of the simulation parameters used are wavelengths(8 per fiber), Control burst processing time (4 ls), switching time(12 l, propagation delay on a link (0.2–1 ms), time-window(Dt = 0.5 s), safety-margin (d = 4). The system performance hasbeen analyzed with metrics such as: burst loss rate (BLR), flow lossrate (FLR), latency reduction improvement, bandwidth overheadand blocking probability of the network. BLR is defined as theamount of dropped bursts against the amount of arrival bursts. Ifone or more bursts in the flow are discarded, it is assumed as theflow loss. FLR is defined as the amount of irregular flows againstthe amount of generated flows. Extra-offset based QoS provisionscheme as a conventional scheme has been considered for makingcomparison with the proposed scheme. In this scheme, extra-offsettime is added to all the bursts of the high priority flow and all thebursts are transmitted respectively according to one-way reserva-tion without using the path. The value of a given extra-offset timeis equal to the average burst length. The offered load ratio of class 1to class 0 is set at 1:9 in the simulations. To compare the perfor-mance of proposed Modified-JET protocol with another protocol,the straightforward FCFS protocol is also implemented.

5. Simulation results

Fig. 1 shows BLR versus input load respectively. The perfor-mance measure is estimated by varying an input load from 0.1 to0.7 with an increment of 0.05. As shown in Fig. 1, it is seen thata QoS differentiation can be achieved in terms of BLR as well asthe conventional scheme (the similar results are also obtained interms of FLR).

Figs. 2 and 3 show the results of comparison of the Modified-JET(proposed) and the FCFS protocol. In all, there are four types of traf-fic: FCFS class 0, FCFS class 1, Modified-JET class 0 and Modified-JET

0.6 0.7 0.8

low priority traffic flow (conventional-OBS)low priority traffic flow ( proposed scheme)Average flow (conventional-OBS)Average flow (proposed scheme)High priority traffic flow (conventional-OBS)High priority traffic flow (proposed scheme)

e vs. input load.

Load (in Erlang)20 40 60 80 100

Blo

ckin

g Pr

obab

ility

0.0001

0.001

0.01

0.1

Proposed with Class (C0)FCFS with Class (C0)FCFS with Class (C1)Proposed with Class (C1)

Fig. 2. Comparison of blocking probability of different traffic.

Load (in Erlang)20 40 60 80 100

Blo

ckin

g Pr

obab

ility

0.00

0.01

0.02

0.03

0.04 Proposed schemeFCFS

Fig. 3. Blocking probability in proposed and FCFS (time-window = 0.5 s, offsettime = 0.2 s).

Advance Reservation (*RMS)0.0 0.5 1.0 1.5 2.0 2.5 3.0Sy

stem

Res

erva

tion

Ove

rhea

d (%

)

0

5

10

15

20

25

30

ProposedConventional-OBS

Fig. 5. Bandwidth overhead vs. advance reservation.

Number of Wavelengths4 6 8 10 12 14

Burs

t blo

ckin

g (L

oss)

pro

babi

lity

1e-6

1e-5

1e-4

1e-3

1e-2

1e-1

1e+0

1e+1Conventional with Class (C0)Proposed with Class (C0)Conventioanl with Class (C1)Proposed with Class (C1)

Fig. 6. Burst blocking probability vs. number of wavelengths.

68 A.K. Garg, R.S. Kaler / Optical Fiber Technology 17 (2011) 64–69

class 1. Fig. 2 shows that class 1 traffic has better performance withthe Modified-JET protocol, especially when the load of the network(in Erlang) is greater than 70. Moreover, because class 1 trafficconsumes more bandwidth with Modified-JET, it also degradesthe performance of class 0 traffic.

However, Fig. 3 shows that blocking probability in the networkis almost the same, which indicates that Modified-JET does notdegrade the blocking probability of the network.

Further, the systems performance improvement g depends onparameters such as w (real-value) – the ratio of so (offset) over

Probabi0.0 0.2 0.4

Late

ncy

Red

uctio

n (%

)

0

20

40

60

80

Ψ=1Y=1.25Ψ=0.5Ψ=2Ψ=0.25Ψ=4Ψ=0.1

Fig. 4. Latency improvement vs. Ps (probabilit

sa (duration to assemble a burst), Ps – the probability that anadvance reservation succeeds.

Fig. 4 presents the latency reduction percentage vs. Ps, whenw ? varies. It shows that g increases as so approaches sa andreaches its maximum gain when the ratio is 1. Specifically, if theburst length can be predicted precisely such that the pre-transmis-sion of the BHP succeeds with a high probability (Ps � 100%), theproposed scheme reduces the latency for the high-class traffic rea-sonably when so = sa.

lity (PS)0.6 0.8 1.0

y that an advance reservation succeeds).

A.K. Garg, R.S. Kaler / Optical Fiber Technology 17 (2011) 64–69 69

Fig. 5 shows that the proposed scheme gains significant latencyreduction at the cost of very small bandwidth overhead in a systemscale.

One of the most important QoS parameter in optical burstswitching is the burst loss rate (probability). The main aim hereis to provide relative service differentiation with regards to burstloss probability. It is important that high priority class of databursts have low loss probability even under low network resourcesuch as available wavelengths and optical switch paths in anoptical burst switching node. Conventional schemes (such as off-set-time based and priority-based) are not flexible and affect thecorrectness and performance of some multimedia applications.

Fig. 6 shows burst blocking probability vs. number of wave-lengths. As expected, with the proposed scheme, the burst loss rateof high priority class decreases faster than lower class(Class (C0)).It means more resource (wavelength) benefits for high priorityclass. With priority, a high-class data burst (Class (C1)) will havelarger differential time (because each OBS node dynamicallychoose the early differentiation time) and thus lower loss rate.Thus, the proposed scheme adjusts the data burst loss rates fordifferent classes of bursts and provides better differentiated QoSrequirement with the available resources in comparison to conven-tional scheme such as extra-offset based QoS provision scheme.Therefore, when considering the entire network, with the proposedscheme lower number of wavelengths are needed to transport agiven traffic load and thus a fewer light-paths are required.

6. Conclusions

In this paper, an efficient integrated scheme supporting QoS hasbeen proposed to reduce the data burst delay at the edge nodes ofOBS systems. It is seen that the proposed scheme guarantees thelow-delay constraint of the real-time traffic and offer QoS differen-tiation in an OBS system. It dramatically reduces the burst delaydue to the burst assembly and the necessary a priori all-opticalpath setup time, while keeping the system bandwidth overheadwithin limits. The simulation results show that the real-time appli-cations which are denoted by class 1 traffic have a better perfor-mance using the modified-JET protocol than with FCFS protocol.Moreover, modified-JET protocol is also easily deployed in OBSnetworks. By simulations, the burst loss rate has been evaluated.The obtained results have shown that the proposed schemeachieve a QoS differentiation in terms of burst loss rate, achieves

lower latency at the cost of very small bandwidth overhead andmaintains reasonable blocking probability.

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