Delayed reservation decision in optical burst switching networks with optical buffers. Title. Li, GM; Li, VOK; Li, CY; Wai, PKA

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1 Tile Delayed reservaion decision in opical burs swiching neworks wih opical buffers Auhor(s) Li, GM; Li, VOK; Li, CY; Wai, PKA Ciaion The 3rd nernaional Conference on Communicaions and Neworking in China (ChinaCom 2008), Hangzhou, China, Augus n Conference Proceedings, 2008, p ssued Dae 2008 URL hp://hdl.handle.ne/10722/61990 Righs nernaional Conference on Communicaions and Neworking in China Proceedings. Copyrigh EEE.

2 Delayed reservaion decision in opical burs swiching neworks wih opical buffers G.M. Li*, Vicor O.K. Li+ *School of nformaion Engineering SHANDONG Universiy a WEHA, China +Deparmen of Elecrical and Elecronic Engineering The Universiy ofhong Kong, Hong Kong, China C.Y. Li, P.K.A. Wai Phoonics Research Cener and Deparmen ofelecronic and nformaion Engineering The Hong Kong Polyechnic Universiy Hong Kong, China Absrac-The proposed delayed reservaion decision scheme offers he same node complexiy as ha in general OBS neworks wih opical buffers. Simulaions show he proposed scheme has beer performance han exising schemes in erms of burs loss probabiliy. Keywords- opical burs swiching; Opical Buffer. NTRODUCTON The massive explosion in nerne raffic has driven he developmen of wavelengh division muliplexing (WDM) echnology. Typically, he nework is circui-swiched, wih a lighpah (circui) se up beween he source and desinaion nodes, and an appropriae wavelengh along he roue is chosen. No opical buffer is needed a inermediae nodes. However, each fiber can only conain a limied number of wavelenghs, and if wo differen lighpahs aemp o share one wavelengh in a fiber, one of hem mus be blocked. Alhough wavelengh converers can be used o decrease he blocking probabiliies, he problem of wavelengh allocaion and rouing is NP-hard [1]. Anoher drawback of wavelengh rouing is ha i is no efficien for bursy daa raffic since once a lighpah is se up, he bandwidh resource is dedicaed o his lighpah, even if here is no raffic. To improve he bandwidh efficiency, P over WDM using opical packe swiching (OPS) becomes an ideal choice of he nex generaion neworks. deally, all funcions inside each node ofan DPS nework are performed in he opical domain [2], i.e., all-opical packe swiching. However, opical processing capabiliies are very limied and opical random access memory (RAM) is no available a presen. n order o accommodae curren echnology, opical burs swiching (OBS) was proposed in he lae 1990s [3]. An OBS nework consiss of edge nodes a he periphery of he nework, and core nodes inside he nework. Edge nodes aggregae packes from upper layer ino opical daa burss (DBs) and keep hem in he opical domain. Each opical burs has an associaed conrol packe (CP). CP is sen in a separae conrol channel and processed elecronically [4,5] a each node, in an aempo schedule is corresponding daaburs. n general OBS neworks, nodes processing conrol packes (CPs) usually follow he jirs-come-firs-served discipline; hey can only know abou previously scheduled daa burss (DBs), bu canno predic he infonnaion of incoming DBs. As a consequence, his leads o inefficien resource uilizaion [6]. Similarly, alhough exising OBS neworks wih opical buffers [7-10] have buffered he CPs wih he same delay of he DBs using fiber delay lines (FDLs) in order no o aler he offse beween he CPs and he DBs, scheduling he DBs is sill based on he arrival sequence of CPs. Thus, resource allocaion is inefficien. n his paper, we propose he delayed reservaion decision (DRD) scheme in OBS neworks wih FDLs o improve he nework performance. The remainder of his paper is organized as follows. n he second secion, we briefly review opical burs conenion resoluion using FDLs and hen presen he proposed scheme. n he hird secion, we give numerical evaluaions and we find ha he proposed scheme has beer performance han exising schemes. Finally, we conclude.. OBS NETWORKS WTH OPTCAL BUFFERS A. General OBS neworks wih opical buffers Alhough opical buffers are no mandaory in OBS neworks, sudies show ha using FDLs as opical buffers can effecively improve he nework performance [7, 10]. n an OBS nework wih FDLs, when a conrol packe canno successfully reserve a wavelengh a an oupu por for is corresponding daa burs, i will ry o reserve he available FDL wih he shores delay insead. There are wo differen node srucures in OBS neworks wih FDLs: one is ha each link has a dedicaed FDL module [9], and we call i FDL share per link; he oher is ha each node has only one FDL module o be shared by all links [7], and we call i FDL share per node. The difference beween FDL share per link and FDL share per node mainly lies in he hardware cos and he efficiency of FDL uilizaion. FDL share per node needs much less single FDL elemens, and much less wavelengh converers. Bu he radeoff is ha i needs more pors in he opical swich. Since wavelengh converers are expensive, in enns of minimizing hardware cos, FDL share per node is a beer choice since i is less bulky and uilizes FDLs more efficienly han FDL share per link. To explain he operaion of resolving conenions by FDLs, we consider a scenario in which each node has one oupu por wih a single wavelengh and a single FDL wih fixed delay T d,

3 lljr.q-'b.ilnnel as shown in Figure 1. Suppose a new conrol packe CP x arrives a he curren node i. CP x conains he informaion ofis offse and is corresponding daa burs lengh informaion T DB. Based on hese parameers, he expeced daa burs arriving ime s and daa burs deparing ime en (en = s + T DB ) can be obained. Afer CP x finishes is processing a ime j, i aemps o reserve he wavelengh a he oupu por from s o en' The reservaion reques for he inerval (s, en) fails since i is blocked by an already scheduled daa burs DBo, shown in Figure 1. Then, CP x will check wheher conenion occurs in he daa channel afer delay T d and wheher he daa burs can be accommodaed in he FDL wih he delay T d f boh condiions are saisfied, CP x will reserve he FDL for he inerval (Sb en) a a cerain wavelengh and reserve he daa channel for he inerval ('s, 'en) simulaneously. When he daa burs arrives a node i a ime Sh i will go hrough he FDL. Thus he daa burs can be buffered for ime T d and he daa burs is sill kep in he opical domain and ransmied o he downsream nodes. s - _._!- T-:-- -:-- --_.- li 1 s 's, i , _--...-, o Tep TOB... C_QnrQlb_anol DBx Figure 1. Chronological sequence ofa sraighforward approach in OBS neworks wih FDLs; conenion occurs a node i, he updaed conrol packe is sen ou immediaely afer processing in his scenario. When FDLs are used as opical buffers o resolve burs conenions in OBS neworks, a sraighforward opion is o send he updaed conrol packe o he downsream nodes immediaely afer he conrol packe successfully reserves resources in FDLs and daa channel for is corresponding daa burs. As shown in Figure 1, afer CP x successfully resolves conenion using FDL a node i, CP x is updaed wih he new informaion of he delayed daa burs and sen o he downsream node immediaely. The offses a all downsream nodes will be changed according o he FDL delay a node i. Previous work [4, 7-9] noe ha a burs wih longer offse ime has lower dropping probabiliy. FDLs help o improve performance in he exended-offse based qualiy of service (QoS) provision schemes. Mos exising work in OBS neworks focus on he general scheme as shown in Figure 2. n order o keep he burs prioriy unchanged, he corresponding burs offse value mus be he same as in he scenario wihou FDLs. This implies ha, afer CP x successfully resolves conenion using FDL a node i, he conrol packe mus be elecronically buffered wihhe same delay as ha ofhe suiable FDL. Under he general reservaion scheme in OBS neworks wih FDLs [7, 8], he decision of he nework resource allocaion is made immediaely afer processinghe CP. Forhe example shown in DB'x Daa channel _.._ '"---_Q _._ ky!- so Y yy yi FDL _._ _._ _.._ ', j Figure 2, resource allocaion decision is done a j, bu CP x has o be elecronically buffered wih he same delay, T d, as he desired FDL; he updaed conrol packe, CP x ', is sen o he down sream node a 1 '. By comparison, he sraighforward scheme is more cos-efficien in hardware han he general scheme since i does no need exra elecronic buffer for he conrol packes. s drawback is ha i canno be implemened o provide qualiy of service (QoS) in he exended-offse scheme since changing offses will change he prioriy of he corresponding daa burss. S Tep T OB Daa channel f Bx DB'x FDL +-Td ". ". -" -,' _. f :'":< 1 ""-: -" _.. -i': 1 s '1.l's... i ':' o Figure 2. The chronological sequence ofburs conenion resoluion using FDLs in a node wih a single wavelengh and a single FDL; conenion occurs a node i, he updaed conrol packe is elecronically buffered wih T d afer processing in his scenario. B. Delayed reservaion scheme The firs-come-firs-served discipline leads o inefficien resource uilizaion in general OBS neworks [6,11]. n mos exising work on OBS nework wih FDLs, conrol packes are elecronically buffered wih he same delay as he daa burss using FDLs, and scheduling daa burss in FDLs and daa channels is sill based on he arrival sequence of conrol packes. Thus, he resource allocaion is sill inefficien. Figure 3 illusraes an example of he inefficien resource allocaion in he general OBS neworks wih FDLs due o he firs-come-firs-served discipline. We assume each node in a general OBS nework has a fiber wih a fixed delay T d o resolve burs conenions. We furher assume ha a node has received hree conrol packes, cl, c2, and c3, which reques he same oupu por 0 1 wih wavelengh channels Dl and D2. Their corresponding daa burss are marked Bl, B2, and B3 following heir corresponding conrol packes. cl, c2, and c3 finish heir processing a imes ], 2 and 3 respecively. We assume ha, due o he burs conenion a he oupu por, all hree daa burss, Bl, B2, and B3, need o be buffered by he FDL. According o he reservaion scheme in general OBS neworks wih FDLs, conrol packes cl, c2 and c3 need o be elecronically buffered unil 5, 6 and 7; hen heir updaed corresponding conrol packes cl', c2' and c3' are sen o he downsream nodes. The acual ime of arrival of Bl, B2, and B3 a he oupu por are 8, ll and ]O respecively. A new incoming conrol packe, c4, arrives a he node a 4 and is daa burs B4 arrival ime is 9 as shown in Figure 3(a). Follows he common reservaion scheme of he general OBS neworks wih FDLs, he scheduling decisions are made

4 as following: cl arrives a he node a ] and is daa burs encouners burs conenion. c1 finds ha B1 can be scheduled a ime 8 and here is no burs conenion a he desired FDL; hus, afer finishing processing e1 a, conrol packe e1 is buffered elecronically for an inerval = 5-1 and he updaed conrol packe e1' is sen o he downsream node a 5. The delayed daa burs is scheduled a 8. Conrol packes c2 and e3 are processed likewise. To minimize he idle periods beween daa burs ransmissions, B2 is scheduled on channel D1 wih saring ime 11 afer is conrol packe finishes processing and makes scheduling decision a 2. Consequenly, B3 is assigned channel D2 wih saring ime f]o and is scheduling decision is made a 3. When c4 arrives a he node, i finds is daa burs B4 can no be scheduled on any channel due o he burs conenion, as shown in Figure 3(b). Thus, FDLs or oher conenion resoluions have o be used; oherwise, daa burs B4 will be dropped. However, even afer delay T d, no daa channel is available o accommodae he daa burs B4, as shown in Figure 3(c). Ulimaely, B4 is dropped. cl cl' Bl... l!c2 i c2' i B2.!!!cJ i!cl' : BJ! % '" - i! i!!!c4 i! i! i i Y j j ii 9 (a) (b) B4 i i lo! i:'e packe, CP x ', i.e., a ime (]' -To), where To is he ransmission ime of he conrol packe. The ransmission ime To is hardware dependen. We can assume ha i is a consan value a each node, and i is usually negligible compared o he duraion of scheduling. The procedure ofhe proposed scheme is as follows: aferreceiving a conrol packe, he conrol uni a he node performs processing conrol packe. A ime ], he scheduling decision is no really made, bu virually; i.e., he virual scheduling decision does no reserve any nework resources bu marks he inerval o indicae ha he inerval may be used for a daa burs. Before he elecronically buffered conrol packe is sen o he downsream node, he scheduling decision is really made a ime (]'-T o ). During he period from ] o (]'-To), he channel assignmen is based on he order of daa burs arrival sequence. Since he conrol uni a node i can gaher more informaion from arriving conrol packes, a beer scheduling decision may be made and he nework resource can be used more efficienly. Since more informaion of he subsequen DBs is available o he node, beer resource allocaion can be achieved. Many sophisicaed scheduling schemes of he DBs in FDLs are possible wih he proposed scheme. n his paper, however, he DBs in he FDLs are scheduled only according o heir acual arrival sequence o demonsrae he principle ofhe DRD scheme c c1 c1' i-!- i c2' i!c3!! c3'! i!!!!c4 i! iii 4 y ii 84 i (a)!,' ie (c) Figure 3. (a) The arrival and deparure sequence ofconrol packes and he corresponding daa burss, (b) channel assignmen according o he reservaion scheme in general OBS wih FDLs; he new incoming daaburs cannobe scheduled direcly, (c) he new incoming daa burs canno be scheduled afer delay T d Under he general reservaion scheme in OBS neworks wih FDLs, we noice ha, afer conrol packes successfully resolve conenion using FDLs, hey have o be elecronically buffered wih he same delay as he seleced FDLs. n fac, he decision of he nework resource allocaion is made immediaely afer processing he conrol packe. The process of elecronic buffering does no conribue o he nework performance. Hereby, we propose he delayed reservaion decision (DRD) scheme. The proposed scheme akes advanage of he process of elecronic buffering of conrol packes wih he aim o improve he resource uilizaion efficiency. As shown in Figure 2, under he proposed scheme, raher han making resource allocaion decision afer processing CP x a ime, he reservaion decision is made jus before sending ou he updaed conrol (b) (c) (d) Figure 4. (a) The arrival and deparure sequence ofconrol packes and he corresponding daa burss, (b) a 1, Bl, B2 and B3 are virually scheduled a he oupupor, (c) a (1 '-To), Bl, B2 andb3 are re-scheduled according o he bursarrival sequence, and he ulimae scheduling decision ofbl is made, (d) he ulimae scheduling decisions ofbl, B2 and B3 when C4 arrives. Figure 4 shows ha he proposed scheme can achieve beer resource allocaion han he general reservaion scheme as illusraed in Figure 3. Afer processing c1 a, e1 is elecronically buffered unil 5 and is corresponding daa burs B1 is virually scheduled on channel D1. e2 and c3 are

5 processed in he same way. Thus, afer 3, Bl, B2 and B3 are virually scheduled a he oupu por as shown in Figure 4(b). This virual scheduling follows he arrival sequence of conrol packes and has a similar channel assignmen as he general reservaion scheme. A ime (/] '-To), ahead of sending he updaed conrol packe cl' o he downsream node, he ulimae scheduling decision of B1 is made. Based on he virual scheduling, Bl, B2 and B3 are re-scheduled according o he burs arrival sequence. Therefore, Bl is finally scheduled on channel Dl, B3 is virually scheduled on channel Dl oo and B2 is virually scheduled on channel D2 as shown in Figure 4(c). The ulimae scheduling decision ofb2 and B3 follows a similar process. Consequenly, Bl and B3 are scheduled on Dl, and B2 is scheduled on D2. When c4 arrives a he node, i can successfully schedule is daa burs B4 on channel D2 wihou burs conenion resoluions, as shown in Figure 4(d). There are proposals for delaying he resource reservaion o improve he sysem performance bu he focus is on he oupu channel scheduling. Virual Fixed Offse ime (VFO) [11] is one such proposal. n VFO, each DB is assumed o be delayed by a fixed amoun of ime; herefore he node can schedule DBs according o heir acual arrival imes. However, o implemen VFO, all inpu pors a each node mus have an FDL module wih a fixed delay. Since each DB needs o be buffered wih he same delay, burs conenion may no be effecively resolved. For he example shown in Figure 4, if he VFO reservaion scheme is deployed, all four daa burs are buffered wih he same delay by he FDL a he inpu pors. Since B4 is unnecessarily buffered and FDLs are no used o provide burs conenion resoluion, daa burs B4 is dropped. n he proposed delayed reservaion scheme, FDLs are no only used o resolve burs conenion bu also o ake advanage of he process of elecronic buffering associaed wih he FDL delay o achieve more efficien resource allocaion. n he following secion, we compare he performance ofhe VFO approach and he delayed reservaion decision scheme by simulaions.. PERFORMANCE EVALUATON We run simulaions o invesigae he nework performance under he proposed DRD scheme in he NSFNET opology, as shown in Figure 5. Following he common simulaion assumpions [11], he inpu nework raffic is in he form of daa burss and follows he Poisson disribuion. The daa burs lengh is an exponenial disribuion. The average conrol packe processing ime is assumed o be one enh of he average daa burs ransmission ime. We assume ha each link has one dedicaed wavelengh as he conrol channel and en wavelenghs for carrying daa burss. Each node in he opology may be an edge node or a core node, and full wavelengh conversion capabiliy is available a each node. When a new daa burs arrives a an edge node, i randomly chooses a desinaion from he res of he nodes in he nework and he shores pah rouing algorihm is used. All nodes are assumed o receive he same offered load in he nework. Figure 6 compares he performance of he proposed DRD scheme wih ha of he exising OBS neworks wih opical buffers. The performance in loss probabiliy of he general OBS wih FDLs is ploed in doed line; and ha of DRD scheme is ploed in solid line. The lines wih dos and crosses indicae ha each node has an FDL wih a fixed delay of 0.1 ime unis, hose wih circles and aserisks indicae ha each node has an FDL wih a fixed delay of 1 ime uni, and hose wih squares and plus signs indicae ha each node has an FDL module whose granulariy is one and he maximum delay is 10 ime unis. We can observe ha DRD has beer performance. Alhough he DRD scheme requires he same hardware complexiy, is delayed reservaion decision can uilize resources more efficienly and herefore more DBs can be served. 0.45r---, , ,--,--,-----, , FDL,B=O.1 - DRD,B = FDL, 8 = DRD, 8 = 1 (J. FDL,B= DRD, 8 = 10 e 0.25 =-..;l Figure offered load The loss probabiliy ofhe general OBS wih FDLs and he DRD scheme Figure 5. The NSFNET opology, 1991 (he original map is available a fp.uu.ne/ine/maps/nsfne/). Figure 7 gives he performance comparison in loss probabiliy of he delayed reservaion decision scheme and he VFO scheme [11]. The VFO scheme, ploed in doed lines, buffers each incoming daa burs wih a fixed delay, and hus he FDL share per link archiecure mus be deployed. The DRD scheme, deploying he FDL share per node archiecure, is ploed in solid lines. The lines wih squares and crosses indicae ha each node has an FDL wih a fixed delay of one ime of he conrol packe processing ime, i.e., 0.1 ime unis. Those wih circles and aserisks indicae ha each node has an FDL wih a fixed delay of en imes of he conrol packe

6 processing ime. We observe ha, only when he nework load is high, will VFO scheme have lower burs loss probabiliy. VFO uses FDL o delay daa burss, and he scheduler can resor daa burss according o heir arrival imes in order o uilize bandwidh more efficienly. When he nework offered load is high, FDL buffers in boh VFO and DRD are filled wih daa burss. n VFO, he inerval o opimize schedulingis fixed a B. n DRD, he inerval is dependen on he delay required o resolve burs conenion, which is less han B in mos cases. Therefore, VFO has lower burs loss probabiliy when he nework is heavily loaded. While he nework is no highly loaded, for example when he load is less han 0.8 wih buffer of one ime uni delay, he DRD scheme has much lower loss probabiliy han he VFO scheme. This is due o he fac ha VFO buffers each incoming daa burs and he burs conenion may no be effecively resolved r , , , :s f 0.25 l:l.. f'-l os Figure VFO, B = T cp -- DRD,B=lTcp 0 VFO, B = lot cp --+- DRD,B=lOT cp The loss probabiliy ofhe general OBS wih FDLs and he DRD scheme. V. 0.5 CONCLUSONS Based on exising OBS neworks wih opical buffers, we propose he delayed reservaion decision scheme o enhance he nework performance. The proposed scheme has he same hardware complexiy a each node as he exising OBS neworks wih opical buffers, bu allows exra elecronic processing of conrol packes o uilize he nework resources more efficienly. Compared o oher delayed reservaion schemes such as he virual fixed offse ime, he proposed DRD scheme decreases he hardware complexiy, and improves he nework performance in loss probabiliy when he nework is no highly loaded. ACKNOWLEDGMENT This research is suppored in par by he Areas of Excellence Scheme esablished under he Universiy Grans Commiee of he Hong Kong Special Adminisraive Region, China (Projec No. AoE/E-O 1/99). Addiional suppor is provided by a gran from The Hong Kong Polyechnic Universiy (Projec Number A-PF98). REFERENCES [1] K. C. Lee and V. O. K. Li, "A wavelengh-converible opical nework," Journal oflighwave Technology, vol. 11, no. 5, pp , [2] X. C. Yuan, V. O. K. Li, C. Y. Li, and P. K. A. Wai, "A novel selfrouing address scheme for all-opical packe-swiched neworks wih arbirary opologies," Journal oflighwave Technology, vol. 21, no. 2, pp , [3] M. Yoo and C. Qiao, "Jus-Enough-Time (JET): A High Speed Proocol for Bursy Traffic in Opical Neworks," in EEE/LEOS Technologies for Globle nformaion nfrasrucure 1997, pp , [4] C. Qiao and M. Yoo, "Opical burs swiching (OBS) - a new paradigm for an opical nerne," Journal ofhigh Speed Neworks, vol. 8, no. 1, pp , [5] M. Yoo, M. Jeong, and C. Qiao, "A high-speed proocol for bursy raffic in opical neworks," in SPE Proceedings, All Opical Communicaion Sysems: Archiecure, Conrol and Nework ssues, pp. 3230,1997. [6] C. Y. Li, G. M. Li, P. K. A. Wai, and V. O. K. Li, "Novel resource reservaion schemes for opical burs swiching," in Proceedings ofeee CC 2005, 3ed, pp ,2005. [7] Y. Xiong, M. Vandenhoue, and H. C. Cankaya, "Conrol archiecure in opical burs-swiched WDM neworks," EEE Journal on Seleced Arears in Communicaions, vol. 18, no. 10, pp ,2000. [8] M. Yoo, C. Qiao, and S. Dixi, "The effec oflimied fiber delay lines on QoS performance of opical burs swiched WDM neworks," in Proceedings ofeee CC 2002, 2ed, pp , [9] M. Yoo, C. Qiao, and S. Dixi, "QoS performance of opical burs swiching in P-over-WDM neworks," EEE Journal on Seleced Arears in Communicaions, vol. 18, no. 10, pp , Oc [10] X. Lu and B. L. Mark, "Performance modeling ofopical-burs swiching wih fiber delay lines," EEE Transacions on Communicaions, vol. 52, no. 12, pp ,2004. [11] J. Li, C. Qiao, J. Xu, and D. Xu, "Maximizing hroughpu for opical burs swiching neworks," in Proceedings of EEE NFOCOM 2004, 3ed, pp ,2004.