Towards Efficient Optical Burst-Switched Networks without All-Optical Wavelength Converters

Transcription

1 Towards Effcent Optcal Burst-Swtched Networks wthout All-Optcal Wavelength Converters João Pedro 1,2, Paulo Montero 1,3, and João Pres 2 1 D1 Research, Semens Networks S.A., R. Irmãos Semens 1, Amadora, Portugal {oao.pedro, paulo.montero}@semens.com 2 Insttuto de Telecomuncações, Insttuto Superor Técnco, Av. Rovsco Pas 1, Lsboa, Portugal pres@lx.t.pt 3 Insttuto de Telecomuncações, Unversdade de Avero, Campus Unverstáro de Santago, Avero, Portugal Abstract. Optcal Burst Swtchng (OBS) s a promsng swtchng paradgm to effcently support Internet Protocol (IP) packets over optcal networks, under current and foreseeable lmtatons of optcal technology. The prospects of OBS networks would greatly beneft, n terms of cost and ease of mplementaton, from lmtng the wavelength converson capabltes at the network nodes. Ths paper presents a framework for contenton mnmzaton/resoluton combnng traffc engneerng n the wavelength doman to mnmze contenton n advance and optcal bufferng at the core nodes to resolve contenton. Smulaton results show that wth the proposed contenton mnmzaton/resoluton framework the large number of expensve all-optcal wavelength converters used at the core nodes of an OBS network can be replaced by a moderate number of shared optcal delay lnes wthout compromsng network performance. Keywords: optcal burst swtchng, traffc engneerng, wavelength contenton mnmzaton, wavelength contnuty constrant. 1 Introducton The Optcal Burst Swtchng paradgm [1] has attracted consderable nterest as an optcal networkng archtecture for effcently supportng IP packet traffc, whle explotng the huge transmsson capacty provded by optcal fbres and Wavelength Dvson Multplexng (WDM) technology [2]. OBS bandwdth utlzaton effcency and technologcal requrements are n between those of coarse-graned Optcal Crcut Swtchng (OCS) and fne-graned Optcal Packet Swtchng (OCS). At the OBS edge nodes, multple IP packets are assembled nto bursts, whch are the traffc unts routed and swtched nsde the OBS core network. Thus, OBS provdes sub-wavelength granularty, renderng hgher bandwdth utlzaton effcency than OCS n supportng IP traffc. Moreover, bandwdth for data burst transmsson s reserved n advance and usng out-of-band sgnallng, avodng complex optcal processng capabltes at the core nodes, whch are mandatory n OPS networks. I. Tomkos et al. (Eds.): ONDM 2007, LNCS 4534, pp , IFIP Internatonal Federaton for Informaton Processng 2007

2 Towards Effcent Optcal Burst-Swtched Networks 349 OBS networks use one-way resource reservaton mechansms for settng up the resources for each burst transmsson [1]. Therefore, the burst s ngress node starts transmttng soon after the burst has been assembled, nstead of watng for an acknowledgment of successful resource reservaton n the entre burst path. Hence, two or more bursts may contend for the same resources at a core node. Gven that unsolved contenton leads to burst loss, degradng the network performance, t s clear that effcent contenton resoluton strateges are of paramount mportance n these networks. Wavelength converson was shown to be the most effectve contenton resoluton strategy for OBS/OPS networks [3] and, as a result, most OBS proposals and studes assume the use of full-range wavelength converters at the network core nodes to resolve contenton. However, because all-optcal wavelength converson devces are stll undergong research and development [4], they reman complex and expensve. Consequently, avodng wavelength converson would greatly reduce the complexty/cost of mplementng OBS networks n the near future. Recently, strateges for mnmzng wavelength contenton n advance have been nvestgated [5], [6], and even tested n an OBS network demonstrator [7], wth the am of reducng burst loss n the absence of wavelength converson and optcal bufferng. Stll, despte the reported performance mprovements, an OBS network n these condtons only acheves low burst blockng probabltes at the expense of very small offered traffc loads [6], thus becomng a less attractve networkng soluton. In ths paper, we propose a novel framework for contenton mnmzaton and resoluton n OBS networks wthout all-optcal wavelength converters at the network nodes. Smulaton results show that the proposed framework enables the desgn of effcent OBS networks employng a moderate number of shared Fber Delay Lne (FDL) buffers nstead of a large number of expensve wavelength converters. The remander of the paper s organzed as follows. Secton 2 outlnes the OBS contenton resoluton strateges and correspondng node archtectures. A contenton mnmzaton/resoluton framework talored for OBS networks wthout wavelength converters s descrbed n secton 3, whereas secton 4 assesses the performance of ths framework and compares ts network resource requrements wth those of a network wth wavelength converson, for the same obectve performance. Fnally, secton 5 presents some concludng remarks. 2 OBS Contenton Resoluton and Node Archtecture The performance of OBS networks s manly hampered by contenton at the network nodes, whch arses whenever two or more data bursts, overlappng n tme, are drected to the same wavelength on the same output fbre. Contenton can be resolved usng strateges actng n one or several of three domans: wavelength, tme, and space. Thus, contendng bursts can be converted to other wavelengths avalable at the output fbre, delayed usng optcal buffers, or deflected to other output fbres/lnks of the node. Wavelength converson requres all-optcal wavelength converters, whereas optcal bufferng demands FDLs, whch unlke Random-Access Memory (RAM) only provde fxed delays. Deflecton routng does not requre addtonal hardware, but ts effectveness s heavly dependent on network topology and traffc pattern.

3 350 J. Pedro, P. Montero, and J. Pres The complexty of the OBS network nodes depends on the strateges employed for contenton resoluton n the wavelength and tme domans. Fg. 1 (a) shows the key blocks of the smplest node archtecture, comprsng a fast space swtch matrx wth a number of transt ports gven by two tmes MW, where M s the number of nput and output fbres and W s the number of wavelengths per fbre. Fg. 1 (b) depcts the most common OBS node archtecture, whch ncludes a set of MW dedcated wavelength converters, one per output transt port. It has been shown n [8] that sgnfcant savngs n the number of converters can be acheved, wthout notceable performance degradaton, by sharng the converters among all output transt ports, as llustrated n Fg. 1 (c). However, ths s realzed at the expense of ncreasng the sze of the space swtch matrx by addng 2C transt ports, where C s the number of shared converters. Fg. 1 (d) depcts a node usng F shared FDL buffers for resolvng contenton n the tme doman, whch also requres a larger space swtch matrx. Node archtectures combnng wavelength converters wth FDL buffers can also be devsed [3], [8]. (a) wthout wavelength converters and buffers (b) wth dedcated wavelength converters (c) wth shared wavelength converters Fg. 1. OBS network node archtectures. (d) wth shared FDL buffers

4 Towards Effcent Optcal Burst-Swtched Networks 351 In vew of the above smplfed node archtectures, the complexty and cost of the OBS network depends of: () the swtch matrces sze, () the number of wavelength converters, and () the number of FDL buffers. Moreover, snce a FDL buffer s bascally an optcal fbre wth a length desgned to provde a specfc delay, t s smpler than an all-optcal wavelength converter. Ths fact motvated us to nvestgate the feasblty of an OBS network usng only shared FDL buffers to resolve contenton. In the followng, we descrbe a framework for contenton mnmzaton and resoluton talored for networks based on ths archtecture and show evdence that, usng a moderate number of FDLs, t enables the same bandwdth utlzaton effcency of an OBS network usng dedcated full-range wavelength converters. 3 Contenton Mnmzaton and Resoluton Framework for OBS Networks under the Wavelength Contnuty Constrant In all-optcal networks, the absence of wavelength converson at the network nodes mposes the wavelength contnuty constrant on the data path, that s, the same wavelength must be used n all lnks of the path, from the ngress node to the egress node. The resultng wavelength assgnment problem has been extensvely studed n the context of OCS networks [9], where wavelength avalablty n the entre path s known and exploted by the wavelength assgnment strategy. However, due to the one-way nature of resource reservaton n OBS networks, the ngress node knows the wavelengths avalablty on ts output lnks, but t s not aware of ther avalablty on the downstream lnks of the burst path. Thus, bursts gong through overlappng paths can be assgned the same wavelength by ther ngress nodes, resultng n contenton for that wavelength at some common lnk. The probablty of wavelength contenton can be mnmzed n advance by explotng the followng prncple [5]: f two or more burst paths share one or more network lnks, contenton on those lnks wll be reduced f each burst path preferably uses wavelengths dfferent from those preferred by the other overlappng burst paths. In practce, ths s acheved by mantanng at the burst path s ngress node an optmzed prorty-based orderng of the wavelengths and usng t to search for an avalable wavelength (on ts output fbre lnk) for transmttng data bursts towards the egress node. The work n [5] ntroduced a strategy for optmzng the wavelength orderngs usng nformaton of network and traffc condtons that usually reman unchanged over relatvely long tme scales, such as network topology, routng paths, and average offered traffc load between nodes. Recently, we proposed a new strategy [6] that uses the same nput nformaton and was shown to be sgnfcantly more effectve n mnmzng wavelength contenton than that of [5]. In the followng, we descrbe a strategy smpler than that of [6], but that was found to attan smlar performance. Consder an OBS network wth N nodes, L undrectonal lnks, and W wavelengths per lnk. Let Π denote the set of paths used to transmt bursts, and let E denote the set of lnks traversed by path π Π. Let also γ denote the average traffc load offered to burst path π. The extent of wavelength contenton from bursts of a path π on bursts of a path π s expected to ncrease wth both the average traffc load offered to the paths and the number of common lnks. Defne the nterference level of π on π as

5 352 J. Pedro, P. Montero, and J. Pres I( π,π ) = γ E E,, (1) where E E denotes the number of lnks shared by both paths. Let 1 P(π,λ ) W denote the prorty of wavelength λ on path π, that s, the ngress node of π only assgns λ to a burst on ths path f the frst W P(π,λ ) wavelengths, ordered by decreasng prorty, are not avalable. Based on the problem nputs, the strategy determnes the nterference level of each path on every other path and reorders the paths of Π such that f < the followng condton holds I( π,π k ) > I(π,π k ) or I π,π k ) = π Π k π Π k π Π k ( I(π,π ) and E > E. (2) π Π k k Intally, no prortes are assgned to the wavelengths, that s, P(π,λ )=0 for all π and λ. The followng steps are executed for every prorty 1 p W n decreasng order, and for every ordered path π Π: (S1) Let = { λ : P(π, λ ) = 0, 1 W } Λ denote the ntal set of canddate wavelengths contanng all wavelengths that were not assgned a prorty on π. If Λ =1 go to (S7). PΛ = pk : πl, l, P(πl, λ ) = pk, El E > 0, λ Λ denote the set of prortes already assgned to canddate wavelengths on paths that overlap wth π. ρ = mn λ max{ P(πl, λ ) : l, El E > 0} Λ be the lowest prorty from the set of the hghest prortes assgned to canddate wavelengths on the paths that use lnks of π. Update the set of canddate wavelengths as follows (S2) Let { } (S3) Let { } { λ : π, l, P(π, λ ) > ρ, E E > 0, λ Λ} Λ Λ \. (3) l l If Λ =1 go to (S7). C ( em, λ ) = γl : El em, El E > 0, P(πl, λ ) = ρ be the cost assocated wth wavelength λ on lnk em E. Thus, the mnmum cost among the hghest costs assocated wth the canddate wavelengths on the lnks of (S4) Let { } path π s mn { max { C( e, λ ) : e E }} αe λ m m l = Λ. Update the set of canddate wavelengths as follows { λ : e, C( e, λ ) > α, e E, λ Λ} Λ Λ \. (4) m m If Λ =1 go to (S7). (S5) Let C ( π, λ ) = C( em, λ ) be the cost assocated wth wavelength λ m E e on π. Thus, α = mn C(π, λ ) s the mnmum cost among the costs π λ Λ assocated wth the canddate wavelengths on π. Update the set of canddate wavelengths as follows If Λ =1 go to (S7). { λ : (π, λ ) > α, Λ} m Λ Λ \ C π λ. (5)

6 Towards Effcent Optcal Burst-Swtched Networks 353 (S6) Update the set of prortes assgned to the canddate wavelengths as follows P Λ { p : p ρ p P } P, Λ \ k k k Λ. (6) If P Λ >0 go to (S3). Else, randomly select a canddate wavelength λ Λ. (S7) Assgn prorty p to the canddate wavelength λ Λ on path π, that s (, λ ) p P π =. (7) Each executon of the above steps, selects the wavelength that wll be used wth prorty p on path π. Therefore, these steps are executed W Π tmes to determne all wavelength orderngs. The hghest prorty wavelengths are the frst to be selected and preference n wavelength selecton s gven to the burst paths wth larger nterference level over the other paths. The wavelength selecton s made by frst defnng a set of canddate wavelengths, and then successvely reducng ther number untl there s only one. The crterons used to reduce ths set sze are such that the canddate wavelength selected mnmzes the prorty nterference on the lnks of the path. In other words, the wavelength selected s one that has preferably been assgned low prortes on paths that share lnks wth the burst path π. In the followng, ths greedy strategy s named Heurstc Mnmum Prorty Interference (HMPI). Upon assemblng a data burst, drected to a gven egress node, the ngress node wll search for an avalable wavelength usng the wavelength orderng optmzed for the correspondng path. Snce a burst transmtted over a hgh prorty wavelength wll have greater chances of avodng wavelength contenton, we use a proper schedulng algorthm at the ngress node to ncrease the amount of traffc carred on these wavelengths. The ngress burst schedulng algorthm s allowed to delay assembled bursts at the electronc buffer of the ngress node by any amount of tme bounded to D ngress. Hence, the algorthm wll frst try to assgn the hghest prorty wavelength to the burst f ths wavelength s avalable durng the requred tme for burst transmsson startng at some tme n the future not exceedng the addtonal D ngress delay. If ths wavelength s stll not avalable, the algorthm wll proceed n the same manner for the second hghest prorty wavelength and so on, untl ether an avalable wavelength s found or all wavelengths have been searched. In vew of the wavelength contnuty constrant, contenton at a transt node can only be resolved by delayng contendng bursts at the shared FDL buffers. Several FDL buffer confguratons have been presented [3]. Here, we adopt a pragmatc approach to optcal bufferng. Thus, only sngle-wavelength FDL buffers are used, whch cannot accommodate bursts overlappng n tme usng dfferent wavelengths, but avod a larger swtch fabrc and extra multplexng and demultplexng equpment requred by mult-wavelength buffers. Moreover, snce a larger delay demands ether a longer fbre or buffer recrculaton, ncreasng optcal sgnal degradaton and the amount of extra hardware for compensatng t, we mpose a lower bound D mn and an upper bound D max on the FDL delay, rrespectve of the number of buffers per node, and nhbt buffer recrculaton. Hence, n a node wth F shared FDL buffers, the FDLs delay s unformly dstrbuted between D mn and D max. Contenton s resolved by delayng the contendng burst usng the smallest avalable FDL buffer that enables the burst to be transmtted through the next lnk of ts path usng the same wavelength.

7 354 J. Pedro, P. Montero, and J. Pres 4 Results and Dscusson The performance of OBS networks usng dedcated wavelength converters, shared wavelength converters, and shared FDL buffers, s evaluated n ths secton usng network smulaton [10]. A 10-node rng topology wth a unform traffc pattern s used n the performance studes. Moreover, the network employs Just Enough Tme (JET) resource reservaton [1] and bursts are routed through the shortest paths. In all cases, each of the L=20 undrectonal lnks has W=32 wavelengths, a wavelength capacty of B=10 Gb/s, a swtch fabrc confguraton tme of 10 µs, and an average burst sze of 100 kb, whch gves an average burst duraton of T=80 µs. A negatve exponental dstrbuton s used for both burst sze and burst nterarrval tme. The average offered traffc load normalzed to the network capacty s gven by γ E Π Γ = π. (8) L W B The OBS rng network usng shared wavelength converters (SWC) or shared FDL buffers (SFB) s desgned to match the bandwdth utlzaton effcency of usng dedcated wavelength converters (DWC). Ths s acheved by determnng the number of shared converters or FDL buffers requred to attan the same average burst blockng probablty at the expense of the same average offered traffc load than that of usng dedcated converters. In ths case, we set the obectve average burst blockng probablty to 10-3 and 10-4, whch s obtaned wth dedcated wavelength converters for an average offered traffc load of Γ=0.40 and Γ=0.47, respectvely. Fg. 2 and Fg. 3 plot the average burst blockng probablty of the 10-node rng network as a functon of the number of shared wavelength converters and the number of shared FDL buffers, respectvely. In the latter case, the mnmum and maxmum FDL delays are set to D mn =T and D max =10T. The average burst blockng probabltes were obtaned by smulatng 20 ndependent burst traces for Γ=0.40 and Γ= E SWC Average burst blockng probablty 1.0E E E E-4 10 Γ=0.40 Γ=0.47 DWC 1.0E Number of shared wavelength converters per node Fg. 2. Network performance usng shared wavelength converters.

8 Towards Effcent Optcal Burst-Swtched Networks E SFB Average burst blockng probablty 1.0E E E E-4 10 Γ=0.40 Γ=0.47 DWC 1.0E Number of shared FDL buffers per node Fg. 3. Network performance usng shared FDL buffers. The curves n these plots show that as the number of shared wavelength converters or shared FDL buffers s ncreased, the average burst blockng probablty s reduced and eventually the bandwdth utlzaton effcency of usng dedcated wavelength converters s acheved. The results also suggest that larger numbers of ether shared converters or FDL buffers are needed when the obectve burst blockng probablty s hgher. Moreover, n both cases the curves tend to a lower boundary of burst loss performance. In the SWC node archtecture, ths boundary corresponds exactly to the performance of DWC, whereas n the SFB node archtecture the lower boundary s a result of mposng a maxmum FDL delay rrespectve of the number of FDL buffers. Fg. 4 plots the performance of the 10-node rng network usng shared FDL buffers and the HMPI strategy to optmze the wavelength orderngs of each burst path. The mnmum and maxmum FDL delays are also set to D mn =T and D max =10T, whereas the maxmum ngress delay s gven by D ngress =T. 1.0E SFB - HMPI Average burst blockng probablty 1.0E E E E-4 10 Γ=0.40 Γ=0.47 DWC 1.0E Number of shared FDL buffers per node Fg. 4. Network performance usng shared FDL buffers and the HMPI strategy.

9 356 J. Pedro, P. Montero, and J. Pres These curves exhbt a decrease of several orders of magntude n terms of average burst blockng probablty, as compared to the ones of Fg. 3. More to the pont, they show a remarkable decrease on the number of shared FDL buffers requred to match the bandwdth utlzaton effcency of usng dedcated wavelength converters. For nstance, for an obectve average burst blockng probablty of 10-4 the SFB node archtecture needs around 19 shared FDL buffers, whereas wth the use of wavelength orderngs optmzed wth the HMPI strategy, ths number s reduced to only 5. Thus, buffer requrements are reduced by almost a factor of 4. The reported FDL buffer savngs are due to the effectveness of the prorty-based wavelength assgnment and burst schedulng strategy n mnmzng contenton n the wavelength doman. In order to gan further nsght on the complexty of each of the node archtectures under study, Fg. 5 presents the number of transt swtchng ports, the number of wavelength converters, and the number of FDL buffers per node that s needed to acheve an average burst blockng probablty of 10-4 when the average offered traffc load s Number of unts per network node Transt swtchng ports Wavelength converters FDL buffers 20 0 DWC SWC SFB SFB-HMPI Fg. 5. Node resource requrements for an average burst blockng probablty of 10-4 and an average offered traffc load of The node resource requrements plotted n Fg. 5 show that the use of dedcated wavelength converters requres a very large number of converters, although wth the beneft of avodng an ncrease on the number of transt swtchng ports. On the other hand, sharng the wavelength converters allows reducng ther number to less than half, but demands a sgnfcant ncrease on the number of transt swtchng ports. The node archtecture wth shared FDL buffers needs slghtly less transt swtchng ports and successfully replaces the expensve wavelength converters by smpler FDL buffers. However, the best compromse between the expanson of the space swtch matrx and the number of ether wavelength converters or FDL buffers s clearly gven by usng the prorty-based wavelength assgnment and burst schedulng strategy for mnmzng wavelength contenton combned wth shared FDL buffers to resolve contenton n the tme doman.

10 Towards Effcent Optcal Burst-Swtched Networks Conclusons In vew of the current lmtatons of all-optcal wavelength converson technology, OBS networks would greatly beneft, n terms of cost and ease of mplementaton, from avodng the use of wavelength converters at the network nodes. Ths paper has presented a framework from contenton mnmzaton/resoluton combnng traffc engneerng n the wavelength doman to mnmze contenton n advance and optcal bufferng at the core nodes to resolve contenton. Moreover, a performance study wth node archtectures that use ether dedcated wavelength converters, shared wavelength converters, or shared FDL buffers to resolve contenton was presented. Smulaton results show that the proposed contenton mnmzaton/resoluton framework only requres a moderate number of shared FDL buffers and a small ncrease n the space swtch matrx to acheve the same bandwdth utlzaton effcency of a network usng dedcated full-range wavelength converters. Therefore, t can contrbute to lower the complexty/cost of deployng OBS networks n the near future. Acknowledgments. The authors acknowledge the fnancal support from Semens Networks S.A. and Fundação para a Cênca e a Tecnologa (FCT), Portugal, through research grant SFRH/BDE/15584/2006. References 1. C. Qao and M. Yoo, Optcal Burst Swtchng (OBS) A new paradgm for an optcal Internet, Journal of Hgh Speed Networks, vol. 8, no. 1, pp , January R. Ramaswam and K. Svaraan, Optcal Networks: A Practcal Perspectve, 2 nd edton. San Francsco, CA: Morgan Kaufmann, S. Yao, B. Mukheree, S. Yoo and S. Dxt, A unfed study of contenton-resoluton schemes n optcal packet-swtched networks, IEEE Journal of Lghtwave Technology, vol. 21, no. 3, pp , March A. Pouste, Semconductor devces for all-optcal sgnal processng, n Proc. of ECOC 2005, paper We3.5.1, Glasgow, Scotland. 5. J. Teng and G. Rouskas, Wavelength selecton n OBS networks usng traffc engneerng and prorty-based concepts, IEEE Journal of Selected Areas n Communcatons, vol. 23, no. 8, pp , August J. Pedro, P. Montero, and J. Pres, Wavelength contenton mnmzaton strateges for optcal burst-swtched networks, n Proc. of IEEE GLOBECOM 2006, paper OPNp1-5, San Francsco, USA. 7. Y. Sun, T. Hashguch, V. Mnh, X. Wang, H. Morkawa, and T. Aoyama, Desgn and mplementaton of an optcal burst-swtched network testbed, IEEE Communcatons Magazne, vol. 43, no. 11, pp. s48-s55, November C. Gauger, Performance of converter pools for contenton resoluton n optcal burst swtchng, n Proc. of SPIE OptComm 2002, pp , Boston, USA. 9. H. Zang, J. Jue, and B. Mukheree, A revew of routng and wavelength assgnment approaches for wavelength-routed optcal WDM networks, Optcal Networks Magazne, vol 1, no. 1, pp , January J. Pedro, J. Castro, P. Montero, and J. Pres, On the modellng and performance evaluaton of optcal burst-swtched networks, n Proc. of IEEE CAMAD 2006, pp , Trento, Italy.