A NOVEL APPROACH TO JOINT WAVELENGTH AND WAVEBAND ROUTING IN HIERARCHICAL OPTICAL NETWORKS

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1 A NOVEL APPROACH TO JONT WAVELENGTH AND WAVEBAND ROUTNG N HERARCHCAL OPTCAL NETWORKS Saurav Gorai, Arabind Dash, and Ranjan Gangopadhyay ndian nstitute of Technology Kharagpur Kharagpur , ndia sauravgorai@yahoo.com only2dash@yahoo.com ranjan@ece.iitkgp.emet.in Piero Castoldi Scuola Superiore Sant 'Anna Center ofexcellence for Communication Networks Engineering Pisa, taly castoldi@sssup.it Abstract The work presents a new approach to the design and planning of WDM networks based on joint wavelength and waveband routing (JwwzsRi. The approach follows the concept of dividing the entire network into suitable subnets with one express node per subnet. Both aggregation of calls into wavebands and segregation of wavebands into individual wavelengths are perjormed by the express nodes. An algorithm is presented for routing and wavelength/waveband assignment of calls in the network. Performance results indicating various trade-offs in regard to waveband granularity. subnet divisions and trafic pattern are presented. Keywords: Waveband; subnet; lightpath grouping; multigranularity optical crossconnect. 1. NTRODUCTON The increasing number of wavelengths per link in wavelength division multiplexing (WDM) networks makes the routing process through optical crossconnect (OXC) more difficult to be implemented and managed. One simple solution to obviate these difficulties is to introduce the multi-granularity switch architecture [], which includes the fiber crosscnnnect (FXC), band crossconnect (BXC) and wavelength crossconnect (WXC) layers. This multi-granularity optical crossconnect switch (MG-OXC) is therefore capable of routingkwitching of wavelengths, wavebands and fibers and reduces the cost and complexity involved in the processing of individual wavelengths. Previous research on joint wavelength and waveband routing has been carried out with the main aim of minimizing the number of switching ports in a crossconnect node for a given traffic demand. Such studies [2, 31 become useful in the design of optical switches and have also justified the use of hierarchical routing strategies in WDM networks. However, in a WDM network not all nodes need to use MG-OXC; in fact some nodes can use ordmary OXCs and the rest can use MG-OXCs. From the viewpoint of WDM network design and planning there are several important issues which have not been adequately addressed in the current literature such as the required number of MG-OXCs, number of wavelengths, number of wavebands and their granularity in meeting a specified performance/cost goal. There are of crime some recent publications [l, 41 which have provided useful algorithmic approach for both wavelengwwaveband routing and assignment for static traffic [2] as well as dynamic traffic [4] scenarios. The present work intends to address several tradeoffs possible with regard to number of subnets (i.e. number and placement of MG-OXCs in the network), number of wavebands and waveband granularity, traffic patterns, etc, with JWWBR to meet the target network blocking and to compare its performance with the same network when it is purely wavelength-routed. n DU work we have capitalized the semilightpath algorithm [S for the purpose of wavelengthiwaveband routing in both the wavelength layer and waveband layers. The simulation strategy adopted here is based on dividing the entire WDM network into several subnets in which each subnet is supposed to have an express node where traffic aggregation takes place for the waveband formation. We differentiate between inter-subnet and intra-subnet traffic. We call the inter- CCECE CCGE 2004, Niagara Falls, May/mai /04/$ EEE

2 subnet traffic as the express traffic and it comprises all calls that have their sources and destinations in different subnets. The waveband layer handles this express traffic. The wavelength layer handles the intra-subnet traffic, which comprises calls with sources and destinations in the same subnet. 2. JONT WAVELENGTH AND WAVEBAND ROUTNG Traffic grooming at more than one level is a key solution for efficient switching and routing to meet the desired minimum overheads. Consequently, efficient algorithms will be required to provide strategies for routing and scheduling in hierarchical networks employing MG-OXCs. Although adequate work has been done on wavelength-routed networks and wavelength routing is still fundamental to joint wavelength and waveband routing, the work on joint wavelength and waveband routing (JWWBR) is quite different from simple wavelength routing in terms of the objectives and techniques involved. A most common objective in the design of wavelength-routed networks is to reduce the number of wavelengths used or the number of used wavelength hops. However, in hierarchical networks one can achieve a drastic reduction in the number of ports while not using the traditional optimal RWA algorithm. JWWBR for hierarchical networks incorporates several lightpathgrouping strategies to put as much express traffic as possible in the transparent waveband layer. By virtue of intelligent lightpath grouping strategies one can enable efficient routing of calls with the added benefit of reduced number of ports. With JWWBR once a call is placed into a waveband at the express node it does not have to pass through individual wavelength filters thus simplifying the multiplexer and demultiplexer design as well. Finally, all of these also result in reduced complexity of controlling the switch matrix and network provisioning. 3. WAVEBAND ROUTNG. n a typical wavelength-routed network, the OXC node performs the routing and wavelength assignment dynamically as the calls arrive at a node. n the case of optical networks having MG-OXC, the following functions need to be carried out: (i) routing of wavebands; (ii) lightpath grouping; (iii) wavelength assignment to each routed call within a waveband. Since the complexity of the design formulation for a large size network is significantly high, a heuristic design method is desirable for network dimensioning and performance evaluation. f there are m wavelengths or more for the traffic from the same source node to the same destination node, one can group these m wavelengths into one waveband to route this waveband as a single unit instead of routing the m wavelengths separately through the network. The nature of waveband formation has direct impact on the requirements of the crossconnect nodes. Several traffic routing scenarios may happen. which may lead to different waveband formation schemes. For example, one can group lightpaths with common intermediate links (i.e. traffic having different source or destination nodes but with a common sub-path to form bands or fibers on this subpath). The latter may be termed as intermediate grouping. This may include grouping of some traffic from the same source to different destination nodes, from different source nodes to the same destination node and grouping of traffic from different source nodes to different destination nodes (which are on longest common sub-paths in each case). 4. SMULATON STRATEGY For our simulation we have divided the network under consideration into several subnets. Traffic from a node to another node in a different subnet comes under express or inter-subnet traffic whereas traffic between two nodes in the same subnet is local or intrasubnet traffic. Waveband routing is done for the express traffic whereas wavelength routing is done for the local traffic. Each subnet has an express node at which formation and segregation of wavebands take place. For waveband formation we adopt the following lightpath grouping strategies: End-to-end grouping: Grouping the traffic between same sources and destinations One-end grouping: Grouping the traftic from the same source to different destinations and from different sources to the same destination Sub-path grouping: Grouping the traffic between different sources and different destinations. The following network parameters have been taken into account while carrying out the simulation: Total number of nodes (N) Node to node connectivity (given, each link is bidirectional) Number of wavelengths per link (K) Number of subnets (M) Wavelength crossconnects (Single T,& pair per wavelength, h-conversion allowed) Waveband crossconnects (Granularity G = 2,4,...)

3 Network Topology, Subaeh, Eiprei Nod- Drmmd Matrix, Number 01 Wwcleoghs and Waveband Grmdsrify, G Le1 W-Numberofrueh~allr. NO Sileclnext p.lraisubnee (l,jhi#j. P.~ mm.tnlng~.n~ using remiughtprtb muting. Fig. 1. Flowchart showing the basic steps of JWWBR n our algorithm, there are several nested loops that iteratively perform waveband formation and semilightpath placement. When we place a waveband 60m one express node to another, we actually set up a group of granularity number of lightpaths following the same sequence of links between the express nodes. As soon as we find that a waveband can he placed 60m one express node to another, we start placing one by one, the semilightpaths from the sources to the point of waveband formation and correspondingly, the semilightpaths fiom the point of segregation of wavebands to the destinations. This process is repeated till we are able to find as many calls as required to fill the waveband completely. We then move on to the formation of another waveband. When we find that no more wavebands are required between two subnets or no more wavebands can be placed between two subnets, we move on to the next pair of subnets. For ease of understanding a flowchart explaining the fundamental steps of JWWBR is given in Fig. 1. However, in order to ensure a fair distribution of wavebands among each pair of subnets, in our implementation, we have done waveband formation in levels. The first level corresponds to grouping of calls having their sources and destinations within zero hops of the express nodes. This level of waveband formation is carried out for each pair of subnets. n the second level, we do the grouping of

4 calls having their sources and destinations within one hop of the destination (This also includes calls which could not be placed in the first level due to insufficient number of calls to fill the wavebands completely.). t may he noted here that the first level is essentially end to end grouping whereas the second level involves a more complicated case of both one-end grouping and sub-path grouping. We go on increasing our levels till all calls are considered for waveband formation. n this manner, we maximize the number of wavebands throughout in the entire network. After the stage of waveband formation, those calls, which could not be placed in wavebands, are placed in the wavelength layer using the semilightpath algorithm. These calls include the intra-suhnet calls as well as the unplaced inter-subnet calls. Those calls, which still could not be placed, are considered to be blocked. equals the performance in Fig. 4 for a 75% intersubnet traffic case. We might guess that a completely random traffic throughout the network is equivalent to 75% inter-subnet traffic when the number of subnets is four.?. ). 0.1 P P m e M E G m Fig node EON divided into 4 subnets -- ~ 0.000l am 2 1 tu 180 2m 320 YO 160 3m 4w 428 U0 (60 U0 Nvmkdmllsmnmork Fig. 2. Wavelength routing in EON, 24 wavelengths fiber, random baffic 5. RESULTS For the purpose of simulation we consider the 27- node European optical network (EON). When EON is viewed as a purely wavelength routed network, the blocking performance (Fig. 2) of this network can he obtained by using the semilightpath algorithm [S. We also perform the simulation of EON when it is divided into four subnets as shown in Fig. 3. n the simulation the traflic demand matrix is vaned to correspond to different percentage of inter-suhnet calls and network blocking versus the number of calls in the network is evaluated as shown in Fig. 4. t is observed from Fig. 4 that for a given number of calls, network blocking increases drastically with increase in the percentage of inter-suhnet calls. This is expected because intersubnet calls have a longer average hop distance which consume more network resources in terms of wavelengths on different links. n Fig. 2, a random traffic is considered and we observe that it nearly U YO Nvmkofmlli innetwork Fig. 4. Wavelength routing in EON, 24 wavelengths fiber, 4 subnets under consideration DO 310 YO )a uo W Nvmk of mlls in nctwork Fig. 5. J WBR in EON, 4 subnets, 24 wavelengths fiber, 25% inter-subnet traffic

5 W 320 YO NO Number of calls io netwok Fig. 6. JWWBR in EON, 4 subnets, 24 wavelengths fiber, 75% inter-subnet traffic that wavelength resources are used optimally and partially filled wavebands do not exist. 6. CONCLUSONS The present work reports a useful strategy for the design and planning of WDM networks utilizing joint wavelength and waveband routing. n our strategy for JWWBR implementation the semilightpath algorithm has been used to perform both wavelength and waveband routing. The performance evaluation of a representative network model (EON) under different traffic situations, sub-netting structures and waveband granularities has been carried out with both wavelength routing and JWWBR. The performance results indicate various practical trade-offs possible among network parameters. This study has included only wavelength conversion and not waveband conversion, that can be considered in future work. Acknowledgements 160 lea iw no w 319 MO 360 Numtaofcall~bo~ok Fig. 7. JWWBR in EON, 7 subnets, 24 wavelengths fiber, random traffic The impact of joint wavelength and waveband routing with different granularities (G = 2,4, 6 and 8) for the wavebands on the network blocking probability has then been shown in Fig. 5 and Fig. 6 for 25% and 75% inter-subnet traffic respectively. n both cases the number of subnets is equal to four. An interesting result has emerged that the network blocking performance with JWWBR can be strikingly better compared to the results for a purely wavelength routed network for random traffic (Fig. 2), when the intersubnet traffic is considerably less in the former case. The performance results for EON with number of subnets equal to seven and with random traffic are shown in Fig. 7. Our next observation is that the blocking probability decreases with increase in granularity. This is the result of the strategy used that each waveband must be completely filled before waveband routing takes place i.e. when the number of calls equal to granularity is available to fill the band. Otherwise, a waveband is not formed. When the granularity is increased and there are not sufficient demands, many calls would prefer the path in the wavelength layer rather than the waveband layer so The authors are greatly indebted to Prof. G. Prati who extended his hand of support for the summer internship of Saurav Gorai and Arabind Dash at Centro di Eccellenza per l hgegneria delle Reti di Comunicazione, Scuola Superiore Sant Anna, Pisa, taly, where a major part of this work was pursued. The authors are also indebted to M. M. Rameshkumar for the constant encouragement, guidance, help and support during their research. References [] Pin-Han Ho, and H.T. Mouftah, Network Planning Algorithms for the Optical nternet Based on the Generalized MPLS Architecture, EEE Globecorn, vol. 4, pp ,November, [2] Y. Suemura, 1. Nishioka, Y. Maeno, S. Araki, R. zmailov, and S. Ganguly, Hierarchical Routing in Layered Ring and Mesh Optical Networks, EEE CC, vol. 5, pp ,28 April - 2 May, [3] Pin-Han Ho, and H.T. Moueah, Path selection with hnmel allocation in the optical nternet based on generalized MPLS architecture, EEE CC, vol. 5, pp ,28 April - 2 May, [4] Xiaojun Cao, Vishal Anand, and Chunming Qiao, A Waveband Switching Architecture and Algorithm for Dynamic Traffic, EEE Comm. Len., vol. 7, no. 8, pp , August, [S. Chlamtac, A. Farago, and T. Zhang, Lightpath (Wavelength) Routing in Large WDM Networks, EEE J- SAC,vol. 14,no. 5,pp , June,