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Performance Comparison of Optical Burst-Switched Networkswith Flexible Burst Scheduling at the Ingress Edge Nodes

João Pedro 1,2*, Paulo Monteiro 1,3 , João Pires 2 1 Nokia Siemens Networks S.A., R. Irmãos Siemens 1, 2720-093 Amadora, Portugal

2 Instituto de Telecomunicações, Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisboa, Portugal 3 Instituto de Telecomunicações, Universidade de Aveiro, Campus de Santiago, 3810-193 Aveiro, Portugal * Tel: +351 21 4167179, Fax: +351 21 4242082, e-mail: joao.pedro@nsn.com

ABSTRACTThe prospects of Optical Burst Switching (OBS) as a cost-effective optical networking paradigm are related withthe capability (and the associated complexity) of resolving contention in these networks. The lack of RAM-likeoptical buffers greatly limits the effectiveness of contention resolution in OBS networks. However, the electronic

buffers available at the ingress edge nodes can be exploited, not only for shaping burst traffic using appropriate burst assembly strategies, but also for minimizing/resolving contention by adjusting the bursts departure timefrom these nodes. This paper describes OBS network architectures which benefit from this flexible ingress burstscheduling and compares their performance and complexity. It is shown that JET-based OBS networks usingtraffic engineering in the wavelength domain achieve a comparable burst loss to that of centralized scheduling.

Keywords : burst scheduling, electronic buffering, optical burst-switching, traffic engineering.

1. INTRODUCTIONOptical burst switching has been promoted as a networking architecture that provides sub-wavelength granularityin high capacity WDM networks, enabling the efficient support of data traffic over an all-optical network, whileusing only moderately complex optical technology [1]. The current and foreseeable lack of an optical deviceequivalent to electronic RAM and the asynchronous nature of burst transmission limit the bandwidth utilizationefficiency of these networks. In particular, multiple data bursts may contend for the same transmission resource,which requires dropping some of them in case contention cannot be fully resolved. Contention occurs when twoor more bursts, overlapping in time, are directed to the same wavelength of a given fiber or when the number of overlapping data bursts directed to the same output fiber exceeds the number of wavelengths. Moreover, giventhat the formation of unusable voids between consecutive bursts scheduled on the same wavelength cannot beavoided, the wavelengths capacity is fragmented, further reducing the bandwidth utilization efficiency.

OBS networks can use one-way resource reservation, such as the Just Enough Time (JET) protocol [1], or centralized resource reservation, such as the Wavelength Routed-OBS (WR-OBS) network [2]. In the former,

burst scheduling is performed link-by-link without knowledge of the wavelengths availability in the remaininglinks of the burst path, requiring the use of wavelength converters to resolve contention. On the other hand, withWR-OBS a single scheduler knows the wavelengths availability on all of the links and determines the path andwavelength that will be used by every data burst. Although it avoids the use of expensive wavelength converters,this network architecture lacks scalability and robustness. Still, an interesting feature of WR-OBS is its ability toadjust the bursts departure time from their ingress nodes as to avoid contention at the core nodes [2]. The BurstOverlap Reduction Algorithm (BORA) extended the benefits of judiciously delaying bursts at the ingress nodesto JET-based networks [3]. The rationale is that if bursts depart these nodes using the least number of differentwavelengths, the chances of contention at a core node due to an excessive number of overlapping bursts directedto the same fiber are reduced. Recently, a more advanced strategy, combining Traffic Engineering (TE) in thewavelength domain with Delayed Burst Scheduling (DBS), was proposed [4]. It accumulates the merit of BORAalgorithms with the capability to reduce the fragmentation of the wavelengths capacity by serializing bursts atthe ingress nodes and isolating as much as possible burst traffic of overlapping paths on different wavelengths.

This work describes three OBS network architectures that exploit the inexpensive electronic buffers availableat the ingress edge nodes to minimize/resolve contention, namely a centralized burst scheduling architecture, andthe BORA and TE-DBS strategies for JET-based networks. The complexity of their scheduling algorithms isanalysed and their network performance is compared, showing that in a metropolitan ring network the TE-DBSstrategy can achieve the same loss performance of the less scalable centralized scheduling architecture.

2. OBS NETWORK ARCHITECTURESConsider an OBS network with N nodes, L unidirectional fiber links, and W wavelengths per link. The nodes areassumed to have the functionalities of both edge and core nodes. Let Π denote the set of routing paths π i Π used to transmit bursts between each pair of ingress and egress nodes. Moreover, let t p

l (π i) be the cumulative

propagation delay until the l th

link of pathπ

i and t g denote the minimum guard time between consecutive bursts,which accounts, for instance, for the switch fabric configuration time. In the following, two burst schedulingstrategies for OBS networks using the JET protocol and one centralized burst scheduling strategy are described.

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3. COMPLEXITY ANALYSIS

The complexity of the burst scheduling strategies is assessed through a simplified analysis. Let M denote themaximum number of time intervals on which a wavelength is available that may have to be known at any giveninstant. Note that M depends on the burst duration and the maximum ingress burst delay. In JET-based networksit also may depend on the offset time, whereas in the case of centralized burst scheduling it may depend on theround trip time values between the ingress nodes and the scheduler.

The BORA-FS and TE-DBS strategies have similar complexity. Using a straightforward implementation, thecomplexity of scheduling an ingress or transit burst is O(W .M ), because there are at most W .M time intervals thatneed to be checked (eventually, more elaborated implementations may reduce this complexity [5]). The number of bursts that the scheduler of a node must handle depends on the number of routing paths that use the outputlinks of the node and on the traffic load on these paths. In a ring network, assuming full mesh connectivity andshortest path routing, the number of routing paths going through a link is proportional to N 2 [7]. Hence, under theassumption that the same average traffic load is offered to all routing paths, the complexity of burst scheduling in

both BORA-FS and TE-DBS strategies is given by O( N 2.W .M ).The CDBS-FF strategy searches for a wavelength that is available in all the links of the burst path. For each

wavelength, the scheduler starts by checking for a suitable time interval on a given link and after determining therequired ingress delay it checks the availability of the wavelength in all the remaining links, keeping a pointer tothe latest time interval checked in each link. If it finds there is a link where the wavelength is busy it updates therequired ingress delay and repeats the search until either it finds a delay value Δ ≤ Δ max for which there is anavailable time interval on each link or the burst cannot be scheduled in this wavelength. The length of this search

process is proportional to M times the number of links. The number of links in a path grows with N , and thus thecomplexity of searching the availability of a wavelength is O( N .M ). Since all W wavelengths may have to besearched, the complexity of scheduling a burst is O( N .W .M ). Other implementations of centralized schedulingcan be found in [8]. With full mesh connectivity, the number of routing paths is N .( N -1). Hence, the complexityof burst scheduling in CDBS-FF is given by O( N 3.W .M ), which is N times that of BORA-FS and TE-DBS.

4. RESULTS AND DISCUSSION

The performance of BORA-FS, TE-DBS and CDBS-FF is evaluated on a 10-node metropolitan ring network using the network simulator of [9]. The length of the ring links is 25 km, resulting in a network scenario where,eventually, the centralized scheduling approach is feasible, despite its limitations in terms of scalability and datatransfer delay. Each link has the same number wavelengths and all wavelengths have a capacity μ = 10 Gb/s. Theguard time between consecutive bursts is t g = 1.6 μs. Moreover, shortest path routing and a uniform traffic

pattern are assumed, that is, data bursts assembled at an ingress node are randomly destined to one of the other nodes and routed on the shortest path. Both the burst size and the burst interarrival time are negative-exponentially distributed and the average burst size is 100 kB, which results in an average burst duration of 80 μs. The average offered traffic load normalized to the network capacity is given by

μ

γΓ

π=

∑W L

hi

iiΠ (1)

where h i is the number of hops of path π i Π . Figure 1 plots the network performance of the burst schedulingstrategies as a function of the maximum ingress burst delay.

1.0E-5

1.0E-4

1.0E-3

1.0E-2

1.0E-1

1.0E+0

0 40 80 120 160 200 240 280 320 360 400

Maximum burst ingress delay [ μ s]

A v e r a g e

b u r s

t b l o c

k i n g p r o

b a

b i l i t y

CWR-FF

BORA-FSTE-DBS

10 -1

10 -3

10 -4

10 -2

10 0

10 -5

Γ =0.80

Γ =0.60

Γ =0.70

1.0E-5

1.0E-4

1.0E-3

1.0E-2

1.0E-1

1.0E+0

0 40 80 120 160 200 240 280 320 360 400

Maximum burst ingress delay [ μ s]

A v e r a g e

b u r s

t b l o c

k i n g p r o

b a

b i l i t y

CWR-FF

BORA-FS

TE-DBS10 -1

10 -3

10 -4

10 -2

10 0

10 -5

W =64

W =32

(a) Impact of average offered traffic load with W=32 (b) Impact of number of wavelengths per link with Γ =0.80

Figure 1. Average burst blocking probability for the three burst scheduling strategies.

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The performance results show that the three burst scheduling strategies benefit from increasing the maximumingress burst delay. From Fig. 1(a), it is also noticeable that the decrease rate of the average burst blocking

probability is sharper for smaller average offered traffic load values. Moreover, it is clear that with TE-DBS andCDBS-FF there is a significantly larger reduction on the burst blocking probability than that with BORA-FS,whereas the use of centralized burst scheduling can only outperform the TE-DBS strategy for large maximumingress delay values.

The simulation results of Fig. 1(b) suggest that the strategy that benefits the most from having a larger number of wavelengths per link is TE-DBS. In fact, for 64 wavelengths per link, the TE-DBS strategyoutperforms the CDBS-FF strategy for all the maximum ingress burst delay values plotted. This is because anincrease on the number of wavelengths per link also increases the effectiveness of isolating burst traffic of overlapping paths on different wavelengths. Consequently, the serialized data bursts have higher chances of

being kept in the same wavelength as they go through the core nodes. Since this reduces the utilization of thewavelength converters deployed at the core nodes, it may also enable to reduce their number, withoutcompromising the burst loss performance, if they are deployed in a shared configuration. This feature of TE-DBS has been discussed in [10].

It should be noticed that although the same additional ingress delay is considered for all three strategies, thisdelay does not include neither the offset time required by JET-based networks neither the round trip time

between the ingress node and the node housing the scheduler demanded by centralized burst scheduling. Sincethe offset time is smaller than the round trip time, the data transfer delay with CDBS-FF will still be larger than

that observed with BORA-FS and TE-DBS.

5. CONCLUSIONSThis paper has shown that JET-based networks can match the loss performance of a less scalable and less robustcentralized burst scheduling approach by using traffic engineering in the wavelength domain and judiciouslydelaying bursts at their ingress nodes.

ACKNOWLEDGEMENTSThe authors acknowledge the support of Nokia Siemens Networks S.A., Portugal, and Fundação para a Ciênciae Tecnologia (FCT), Portugal, under research grant SFRH/BDE/15584/2006.

REFERENCES

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[5] J. Xu, C. Qiao, J. Li, G. Xu: Efficient channel scheduling algorithms in optical burst switched networks,in Proc. of IEEE INFOCOM 2003 , San Francisco, USA, vol. 3, pp. 2268-2278.

[6] J. Pedro, P. Monteiro, J. Pires: Wavelength contention minimization strategies for optical-burst switchednetworks, in Proc. of IEEE GLOBECOM 2006 , San Francisco, USA, paper OPNp1-5.

[7] D. Hunter, D. Marcenac: Optimal mesh routing in four-fibre WDM rings, Electronics Letters , vol. 34,no. 8, pp. 796-797, April 1998.

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[9] J. Pedro, J. Castro, P. Monteiro, J. Pires: On the modelling and performance evaluation of optical burst-switched networks, in Proc. of IEEE CAMAD 2006 , Trento, Italy, pp. 30-37.

[10] J. Pedro, P. Monteiro, J. Pires: Bandwidth-efficient optical burst-switched networks using only a minimumnumber of shared wavelength converters, in Proc. of OFC 2008 , San Diego, USA, paper JWA89.