Fast Approximate Dimensioning and Performance Analysis of Mesh Optical Networks
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Design of Reliable Communication Networks (DRCN) 2003, Banff, Alberta, Canada, October 19-22, 2003 Fast Approximate Dimensioning and Performance Analysis of Mesh Optical Networks Jean-François Labourdette*, Senior Member, IEEE, Eric Bouillet*, Member, IEEE, Ramu Rama- murthy#, Ahmet A. Akyamaç*, Member, IEEE *Tellium, 2 Crescent Place, Oceanport, NJ 07757, USA #Ciena Corp., 5965 Silver Creek Valley Road, San Jose, CA 95138, USA E-mail: [email protected], [email protected], [email protected], [email protected] Abstract – This paper presents a collection of approximation tivity between client equipments such as routers, also shown formulas that allow a network planner to quickly estimate the in Figure 1. size of a mesh optical network with limited inputs. In particular, it provides a set of equations that relate number of sites, average There exist several schemes for providing protection and fiber connectivity, demand load and capacity for various mesh restoration of traffic in networks. They range from protecting protection architectures. These results can be used to easily and single links or spans to protecting traffic end-to-end at the path quickly estimate the amount of traffic that can be carried over a level. In addition, the protection capacity can be assigned in given network, or, conversely, given the traffic to be supported, advance to pre-computed back-up routes or those routes can to assess the characteristics of the topology required (in terms of be computed in real-time after the failure. Different schemes number of nodes, connectivity). Finally, this analysis can be used achieve different trade-offs between restoration speed and to estimate the restoration performance that can be expected capacity efficiency [1]. without requiring any extensive simulation studies. Index Terms – Optical networks, mesh networking, restoration, performance analysis. I. INTRODUCTION While investigating, designing, or even negotiating a data- Network port transport network it is always valuable to quickly anticipate a realistic gross estimate of its dimensions and cost. Very often DWDM system the available information and/or time are insufficient to pro- Lightpath ceed with a full-scale study of the network. The task is further Digital hindered by increasingly complex protection architectures. For IP-router Fiber ports instance, in shared mesh restoration, additional capacity is reserved to secure for every demand an alternate route that serves as backup in case of failure occurrence along its pri- Add/drop Optical Switch Optical line ports mary route. Since not all demands will be affected by a single fabric failure, the reserved capacity can be shared among multiple demands. The amount of sharing and average time to re- Figure 1: Optical mesh network establish services after any failures are difficult to estimate. In end-to-end or path protection, the ingress and egress The objective of this paper is to provide the framework and nodes of the failed optical connection attempt to restore the the formulas to estimate fundamental network characteristics signal on a predefined backup path, which is SRLG1-disjoint, and performance within an acceptable range of reality without or diverse, from the primary path [2,3,4,5]. Path diversity having to resort to advanced network planning and modeling guarantees that primary and backup lightpaths will not simul- tools. These tools will then be used in a second phase when taneously succumb to a single failure. Unlike local span pro- more detailed designs are required. The model and formulas tection, back-up paths are provisioned with the working paths presented are also very useful in understanding some funda- and thus the restoration does not involve further real-time path mental behavior of networks as they capture and highlight the computations. Another aspect of path protection is that the key relationships between different network characteristics restoration processing is distributed among ingress and egress (size, node degree, switch size, utilization…) and traffic de- nodes of all the lightpaths involved in the failure, compared to mand characteristics, and network performance (capacity, res- local span protection where a comparable amount of process- toration times,…). We apply our model and techniques to op- ing is executed by a smaller set of nodes, those adjacent to the tical mesh networks shown in Figure 1 made of optical switches connected to each other over inter-office DWDM systems. The optical switches provide lightpath-based connec- 1 The concept of Shared Risk Link Group (SRLG) is used to model the failure risk associated with links riding the same fiber or conduit [1,3]. 0-7803-8118-1/03/$17.00 © 2003 IEEE 428 Design of Reliable Communication Networks (DRCN) 2003, Banff, Alberta, Canada, October 19-22, 2003 failure. In the following we will only consider the cases where capacity utilization and recovery time. In mesh restoration, the protection path is failure-independent and is thus the same node-diversity between primary and backup paths does not for all types of failures. By way of this restriction, the restora- guarantee full protection against node failures. Additional tion paths may be computed and assigned before failure occur- sharing restrictions are required to guarantee restoration in rence. There are two subtypes of path protection: (1) dedicated case of node failure (for those lightpaths that did not terminate mesh (1+1) protection, and (2) shared mesh restoration. or originate at the failed node). Protection against node failure Dedicated or 1+1 mesh protection is illustrated in Figure 2. generally requires more bandwidth. See [2,3] for further de- The network consists of four logical nodes (A to D) and two tails and experimental results. demands (AB and CD) accommodated across an eight node optical network (S to Z.) The provisioning algorithm of this The procedure to route a lightpath consists of two tasks: (1) architecture computes and establishes simultaneously the pri- route selection, and (2) channel selection. Route selection in- maries and their SRLG-disjoint protection paths. During nor- volves computation of the primary and backup paths from the mal operation mode, both paths carry the optical signal and the ingress port to the egress port across the mesh optical network. egress selects the best copy out of the two. In the example of Channel selection deals with selecting individual optical chan- Figure 2, all the optical channels on primary and secondary nels along the primary and backup routes. The problems of paths are active. In particular, the configuration reserves two selecting a route together with selecting channels on the route optical channels between nodes S and T for protection. This is are closely coupled and if an optimal solution is sought both the fastest restoration scheme since for every lightpath one problems should be solved simultaneously. In this paper, we device is responsible for all the necessary failure detection and assume that routing computation is done with access to the restoration functions. But it is also the most exigent in terms complete network information, and that a k-shortest path ap- of resource consumption. If protection against node failure is proach is used for both the primary and back-up paths. See also desired, then primary and backup paths must be node dis- [12-14] for comparison of routing efficiency when only partial joint in addition to SRLG-disjoint. information is available. UWPrimary for demand AB UWPrimary for demand AB AB AB V V Bridging (both lightpaths are active) Optical line reserved for shared Secondary for demand AB protection of demands AB and CD TS TS Secondary for demand CD Cross-connections are established Cross-connections are not established on secondary paths before failure Y Y C D C D XZPrimary for demand CD XZPrimary for demand CD Figure 2. Dedicated mesh (1+1) protection Figure 3. Shared mesh restoration: before failure As in dedicated protection, in shared mesh restoration pro- tection paths are predefined, except that the cross-connections along the paths are not created until a failure occurs (see UWPrimary for demand AB is not affected Figure 3 and Figure 4). During normal operation modes the AB spare optical channels reserved for protection are not used. We V refer to such channels as reserved (for restoration) channels. Optical line carries Since the capacity is only “soft reserved”, the same optical protection of demands CD channel can be shared to protect multiple lightpaths. There is a TS condition though that two backup lightpaths may share a re- served channel only if their respective primaries are SRLG- Cross-connections are established disjoint, so that a failure does not interrupt both primary paths. after failure occurs Link failure If that happened, there would be contention for the reserved Y channel and only one of the two lightpaths would be success- C D fully restored. Shared mesh restoration involves slightly more XZ processing to signal and establish the cross-connections along the restoration path. There is thus an evident trade-off between Figure 4. Shared mesh restoration: after failure 429 Design of Reliable Communication Networks (DRCN) 2003, Banff, Alberta, Canada, October 19-22, 2003 The outline of the paper is as follows. In Section II, we pro- c d z vide an analysis for approximating the path length and the b protection capacity in mesh restorable networks. In Section e h III, we derive dimensioning formulas that approximate the a number of lightpaths that can be carried in a maximally loaded h=3 f network of given size and connectivity. We validate these ap- f proximations and give some examples in Section IV. In Sec- g i tion V, we use the analysis to estimate the restoration per- formance in mesh restorable networks. We conclude the paper in Section VI. c d z II. APPROXIMATE PATH LENGTH & PROTECTION CA- b PACITY ANALYSIS h e h’=6 In what follows, we represent a WDM network as a graph.