Spare Capacity Planning Using Survivable Alternate Routing for Long-Haul WDM Networks

Transcription

1 Spare Capacity Planning Using Survivable lternate Routing for Long-Haul WDM Networks in Zhou an Hussein T. Mouftah Department of lectrical an Computer ngineering Queen s University, Kingston, Ontario, K7L 3N6 Canaa {zhoubi,mouftah}@ee.queensu.ca bstract Spare capacity planning is to fin the minimum amount of spare capacity to be allocate throughout a network so that the network can survive from network component failures. In this paper, the spare capacity planning problem is investigate for long haul wavelength ivision multiplexing (WDM) networks. three-step metho is evelope for solving the problem. First, heuristic approaches are use to select both caniate working routes an protection routes in orer to achieve approximate optimal performance while maintaining a computational feasibility. Secon, traffic requests are istribute on the obtaine caniate working an protection routes optimally using genetic algorithms (G). Finally wavelengths are assigne to working lightpaths an share protection lightpaths. The major avantage of the new approach is the ability to incorporate nonlinear constraints an nonlinear cost functions into the G, which are introuce by sharing protection links between share risk link groups (SRLG). Moreover, by consiering SRLG constraints in the spare capacity planning phase, wavelengths can be allocate to each share protection route before failures happen, so that shorter restoration latency can be achieve. Numerical results illustrate that the propose approach is more cost-effective than the single-path protection metho. 1. Introuction ackgroun Wavelength Division Multiplexing (WDM), which allows a single fiber to carry multiple signals simultaneously, will soon become the core technology to cope with the rapily increasing eman for banwith in the next generation Internet. n averse result of exploiting this avance technology is the increase network vulnerability in the sense that a single network failure can significantly reuce the capability to eliver services in large-scale information systems. Therefore, network survivability has been a crucial concern in highbanwith optical network. Survivable network architectures are base on either eicate resources, or share resources. With eicate protection, such as 1+1 an 1:1 protection, network service is restore with the eicate network resources. In the occurrence of a failure, the traffic is switche to the pre-configure protection lightpaths or links. Deicate protection has very low recovery latency an can guarantee full restoration against single network failures. However, it will result in inefficient use of network resources. In contrast, with share protection, network protection resource can be use for several ifferent failure scenarios. Major research efforts on share protection schemes focus on reucing the resource overhea by means of techniques such as sharing resources across connections or between isjoint lightpaths for the same connection. Unlike the eicate scheme, a connection can be establishe over the share protection resources only after a network failure is etecte. Share protection approaches can be classifie into link protection an path protection. In link restoration, all the connections that traverse the faile link are reroute aroun that link. The source an estination noes of the connections traversing the faile link are oblivious to the link failure. In path restoration, when a link fails, the source noe an the estination noe of each connection that traverses the faile link are informe of the failure. The source an the estination noes of each connection inepenently reroute the traffic to backup routes on an en-to-en basis. Link protections are expecte to have shorter recovery latency than path protections. However, it has been reporte that share path protection schemes are more economical than link protections in the sense that they require much lower reunant network resources. Moreover, path protections are more feasible than link protections with available technologies. Link protection schemes epen on fault localization, while no fault localization is necessary for path protection. Path rerouting performe at the ege of the network may allow some or all of the recovery functions to be move into the en-system. Thus, it simplifies network esign, an allows applications to make use of application specific information such as tolerance for latency in making rerouting ecisions. ccoringly, this paper focuses on esigning cost-efficient path protection algorithms. Proceeings of the Seventh International Symposium on Computers an Communications (ISCC ) / $ I

2 1. 2. Relate work Many stuies have been to proviing cost-effective protection schemes for WDM networks [1-13]. Protection esign aims to protecting network services from a set of failure scenarios, such as a single span cut, or a noe loss. Protection esign methos can be classifie into two categories: inepenent esign an coorinate esign [6]. Inepenent esign consiers failure scenarios inepenently, in contrast, coorinate esign consiers planning all reconfigurations simultaneously for several failure scenarios, in a coorinate manner. In [6], lanyali, an yanoglu evaluate two iterative methos, inepenent an coorinate esigns, through simulation for three restoration schemes: full reconfiguration, path-base, an link-base. The numerical results showe that coorinate esign with full reconfiguration achieves the most economical solutions in terms of fiber requirements. They also showe that coorinate planning of several failure scenarios provies more efficient esigns than those obtaine by consiering the failures inepenently. The iterative methos nee much shorter computational time than centralize optimization metho using CPLX. However, the solutions obtaine from the iterative methos are not approximately optimal. ecause increasing the number of iterations cannot result in improve solutions in any case. Some other work consiers all possible scenarios together in coorinate esigning without using iterative approaches [1-5, 9,13]. Typically, the path protection problem is to optimally etermine two isjoint alternate paths, a working path an a protection path, for each source-estination pair, so as to minimize require network capacity an cost. However, these algorithms unnecessarily restrict that all traffic for each noe pair follows the same working path an protection path after failures. In contrast, an optimal esign scheme for survivable WDM transport networks is propose in [3], which focuse on optimally istribute traffic among several caniate working path an protection path pairs. However, the algorithm requires that a single restoration path is preetermine for each working path on a one-toone basis. In this paper, we name this approach as singleprotection path metho. Most of the previous work on spare capacity allocation of mesh WDM networks moele the static protection esign as an integer linear programming (ILP) problem. The esign objective is to minimize the total spare capacity require for the restoration in specific failure scenarios. Unfortunately, the resulte ILP formulation is NP-har [13]. To obtain the optimal solution for even a small size network, such as a few tens of noes, is very time consuming using available mathematical tools. Relaxation methos are propose to approximate the IP solution. Lagarangian relaxation, which ecomposes the original complex problem into several easier subproblems, was use by Doshi et al [9]. Simulate annealing an Tabu searching base methos were propose in [1,13] an [2] respectively Our approaches In this work, our objective is to solve the spare capacity planning problem efficiently for long-haul optical networks, where traffic matrix is known for network planner. Centralize optimization is evelope to esign cost-effective path protection for all traffic requests. Then the erive protection results are loae to each source noe of traffic requests, so that upon a network failure, a istribute restoration algorithm can be activate to reroute the traffic to backup lightpaths. survivable alternate routing algorithm is evelope in this paper, which improves the performance of previous work using the following schemes: In orer to reuce the computational complexity of the spare capacity planning problem, a set of caniate working routes an corresponing caniate protection routes for each wavelength request are pre-compute an use to compose the search space for the esign variables. In this paper, we present a survivable routing algorithm, which can calculate a set of caniate working routes an several isjoint caniate protection routes for each working route. To istinguish our metho, we name it as multi-path protection. Multi-path protection can istribute traffic more evenly over networks, thus sharability of protection paths can be improve. The traeoff of multi-path protection is that more complex restoration signaling is necessary after occurrence of failures. Multiple signaling proceures must be simultaneously performe for all of these restoration paths. However, the increase of the complexity is not significant. To aress the share path protection problem, the Share Risk Link Group (SRLG) concept is consiere as one of the most important constraints concerning the constraint-base route computation of optical channel routes [15]. share risk link group is efine as a group of links that share a component which failure causes the failure of all links of the group. We efine sharable protection link group (SPLG) as collections of protection routes which corresponing primary routes o not belong to any SRLG. SRLG constraints can be efine as follows: protection paths can share links if an only if they belong to a common SPLG. Knowing which fibers are in share risk groups is very ifficult, because logical isjoint paths may not be physically iverse [14]. In the existing transport networks, ientification of SRLGs is not conucte effectively. However, recently, management Proceeings of the Seventh International Symposium on Computers an Communications (ISCC ) / $ I

3 an istribution of SRLG information has receive attention from both inustry [18] an Internet ngineering Task Force [19]. y applying the SRLG constraint to the constraintbase path computation, one can select a route taking into account resource an logical structure isjointness that implies a lower probability of simultaneous lightpath failure. However, to the best of our knowlege, general SRLG constraints were selom consiere explicitly in previous work in globally optimizing pre-compute path protection esigns for static traffic moels. Without consiering SRLG constraints, the reserve restoration capacity is either over-reserve or uner-reserve for protection. The typical example of the former case is 1:1 protection, which results in costly esign because of capacity reunancy. In [5], protection paths for a single eman can share reserve banwith uner the conition that their working paths are mutually isjoint. However, the metho can be improve by applying more general SRLG constraints. In the latter case, protection paths can be multiplexe by other working paths or protection paths without SRLG constrains to reuce network resource reunancy. However these methos result in partial restoration after failures happen. nother effect of neglecting SRLG constraints at protection esign stage is that wavelength allocation for backup paths has to be one ynamically after network failures occur [9]. For longhaul WDM networks, the propagation elay of control signaling is long, which contributes significantly the restoration latency. In our approach with the consieration of SRLG in protection esign, wavelengths are allocate to each backup path at esign stage. Thus, it results in short restoration latency. The remainer of this paper is organize as follows: In Section II, fixe survivable alternate route selection an wavelength routing algorithm is presente. The survivable alternate routing with consieration of SRLG constraints is formulate as a non-linear integer programming problem. n approximate solution using genetic algorithms is introuce. Section III presents simulations an numerical results of the propose routing algorithms. Section IV summarizes an conclues the paper. 2. Survivable lternate Routing We propose a survivable alternate routing (SR) heuristic to simplify the searching of working paths an protection paths. We efine a working route set (WRS) between a source noe an a estination noe as a set of pre-compute caniate routes, through which wavelength requests between the noe pair is route. protection route set (PRS) for a working route rw is a set of caniate isjoint routes of rw, through which wavelength requests on rw are reroute accoring to pre-reserve banwith on each backup route if the working route rw is broken. Working routes an protection routes can be either inactive or active. route is active when banwith (wavelengths) is reserve along the route for a traffic request. D F Working path Protection path Figure 1. WRS an PRS Figure 1. shows two working routes, --C-H an - D--H from noe to noe H. For each working route, two backup routes is assigne, e.g. -D--H an -F-G- H are backup routes for --C-H; --C-H an -F-G-H backup -D--H. The SR can be realize as follows: First, for each source an estination pair a set of working routes is precompute using k-shortest path routing algorithm [22]. The maximum size of WRSs is Kw. The Kw shortest routes o not have to be mutually isjoint. Secon, for each working path, up to Kp isjoint backup paths are searche using breath first protection path searching algorithm. In practice, the size of the route sets, Kw an Kp, is below ten, typically 2 to 6. Then a lightpath layout algorithm is execute to establish working lightpaths over the WRSs, an to eploy protection lightpath over the corresponing PRSs. The optimization objective of lightpath layout algorithm is to minimize the total number of wavelengths use on each optical link. The consiere networks are flat all-optical mesh networks. ach noe in the networks enotes an optical channel-switching router, which is strictly bufferless. In the networks, each link inclues two uniirectional fibers, one for each irection, an each fiber can be multiplexe by numbers of wavelengths. anwith requests are in terms of numbers of lightpaths (wavelengths). Figure 2 shows an example result of spare capacity planning by using survivable alternate routing. Traffic requests, working lightpaths, reserve backup lightpaths an the wavelengths allocate for them are shown in Table 1. This example also shows the avantage of multi-path protection. With multi-path protection, banwith can be reserve for protecting working path - on both backup paths, -C-D- an -C--F-D-, so that the traffic can be balance better than that in single-path protection case C G H Proceeings of the Seventh International Symposium on Computers an Communications (ISCC ) / $ I

4 (in this example, no solution can be foun to satisfy all traffic requirements with single-path protection). C λ 2 D F Working path ackup path Figure 2. lternate survivable routing Table 1. n example of alternate survivable routing. WP, λ w : working path an wavelength ID reserve for traffic along the working path, P, λ p : backup path an wavelength ID reserve along the protection path. S-D PIR TRFFIC WP, λ W P, λ P 3 -, 1 -, 2 -, 3 -C-D-, 1 -C-D-, 2 -C--F-D-,3 C D 1 C-D, 3 C--F-D, 3 F 2 -F, 1 -F, 2 -C-D-F, 1 -C-D-F, reath first protection path searching algorithm We evelope an algorithm to search a protection route set for each primary path. We assume that, after the failure of the primary path, it is not necessary to provie the same quality of service in terms of propagation elay on the protection path. Since one of the main objectives of esigning share path protection is to reuce the banwith reunancy by improving the sharing of protection banwith among ifferent SRLGs. ccoringly, we select protection paths base on their iversity other than their cost (elay). The more iverse the protection paths in a protection route set are, the more common backup links can be share by the working paths belonging to ifferent SRLGs. For this reason, instea of using the k-shortest path algorithm to generate PRS, a breath first iverse routing algorithm is evelope. To search protection paths for a working path, a tree is generate from the original network topology graph by eleting the intermeiate noes an links in the working path. Let the source noe be the root noe of the tree. breath first searching metho is use to search the protection paths from root noe to estination noe. Let k enote the traverse times of backup link () by backup paths in the kth protection route set of traffic request, thus, once a new backup path is foun for kth k working path of, increases its counts. To make the obtaine backup paths as iverse as possible, we confine between one to a certain pre-efine value K l which k is less than K p. If k >K l, then the newly foun protection path will not be inclue in the backup route set, an the searching proceure continues until K p protection paths are foun or the last leaf noe is reache. Figure 3. shows the tree generate for working path -- C-H. Protection paths, -D--H, -F-G-H, -D--G-H, -D-F-G-H, -F-D--H, -D-F-G--H, an -F-D--G- H are obtaine sequentially by using broa first searching algorithm. If K l =2 an K p =4, only the -D--H, -F-G- H, an -D--G-H are eligible solutions D F D G G H G H H H G H H Figure 3. reath first backup path searching tree Survivable alternate routing We efine the following notation. N = the set of noes L = the set of links C = the cost for link (), which can be consiere as propagation elay (in time units) over the link λ s = the banwith requirement from noe s to noe in term of number of wavelengths as the basic unit W = the maximum number of wavelength per fiber X = a set of working paths that use link (i, j) Y = a set of backup paths that use link (i,j) Z s = the set of working paths between source noe s an estination noe = the total amount of banwith reserve by active paths that use link (i, j) = the total amount of banwith reserve by backup paths that use link () R w = the set of all working paths, which are numbere from 1 to K p N(N-1) R wk = the kth working path in R w P uv = the uth protection path in PRS of R wv H F Proceeings of the Seventh International Symposium on Computers an Communications (ISCC ) / $ I

5 c k = the amount of banwith is route through the working path R wk uv = the amount of banwith on the backup route P uv Without loss of generality, we assume that paths that share a common link belong to a SRLG. Therefore, X also enotes a SRLG, in which all paths share link (). Then, = c k v P uv an k X uv = c = u Max uv ( mn), ( mn) ( ) u X mn, v Y Constraints: ( ) ( c k k Z s + ) W = λ s Optimality Criterion: Cost Optimization O a : Minimizing ( ) Loa balancing O b : Minimizing ( ) C + ( ) Max + ( ) ecause in most cases, there is no optimal solution which can satisfy all the criteria at the same time. Some compromises have to be mae. Let O an O max* b max* a enote the estimate maximum value of O a, an O b respectively. The objective function can be formulate as follows: O Min + O a b ( wa w max* b max* Oa Ob where w a an w b are the weights for optimality criterion a) an b), respectively. For the above moel, the optimization problem is to fin, between a source an a estination, a set of optimal routes that give the largest amount of resiual banwith along the routes while satisfying the specifie constraints on routing cost. Since the banwith allocate in WDM networks is in number of wavelengths, it is obvious that the formulate problem is an non-linear integer programming problem with multiple objectives. It can be easily proven that it is an NP-har problem. In practice, approximate algorithms are neee to solve such a problem for a large WDM network. ecause integer programming methos involve long computing perio an high computation intensity to get an optimal istribution of traffic over the route sets, we ) evelop a survivable routing algorithm base on Genetic lgorithms (G) to efficiently search approximately optimal solutions. Compute WPSs an PPSs Generate initiate traffic partitions valuate solutions base on: Mutation cost optimization loa balancing Crossover Selection Meet optimization no criteria? yes llocate wavelengths Figure 4. Survivable alternate routing using G G is a stochastic search metho that mimics the metaphor of natural biological evolution. G operates on a population of potential solutions applying the principle of survival of the fittest to prouce better an better approximations to a solution. t each generation, a new set of approximations is create by the process of selecting iniviuals accoring to their level of fitness in the problem omain an breeing them together using genetic operators, i.e. crossover an mutation, borrowe from natural genetics. This process leas to the evolution of populations of iniviuals that are better suite to their environment than the iniviuals that they were create from, just as in natural aaptation. The programming iagram for traffic partitioning using G is shown in Figure 4. Optimization Criteria in G can be an estimate acceptable value of the objective function, an/or a number of generations in G. λ 1 µ 1 S 01 S S (N-1)(N-3) S (N-1)(N-2) λ 2 µ 2 λ K w 1 µ K p 1 µ λ K w Figure 5. G coing of survivable alternate routing The coing of solusion space is illustrate in Figure 5., where S enotes the traffic eployment for noe pair (i,j), ( i, j) λ enotes the traffic loa allocate on the kth working k, ) route of noe pair (i,j), an µ ( i j enotes the banwith l reserve on the lth protection route of the corresponing working route of noe pair (i,j). The length of the chromosome is N (N-1) K w K p. In practice, only the noe pairs which have traffic requests are consiere, thus a compact chromosome coing can be use. K p Proceeings of the Seventh International Symposium on Computers an Communications (ISCC ) / $ I

6 2. 3. Wavelength assignment Given a set of primary paths an backup paths, as well as the wavelength requests on each path, wavelengths can be assigne for each primary path an backup path so that no two primary paths traversing over a fiber link use the same wavelength, an two or more backup paths can share the same wavelength over a fiber link if an only if they belong to ifferent SRLGs. For large-scale networks with arbitrary topology, assigning wavelengths optimally to various lightpaths has been shown to be NP-har in [21]. We use G to solve the wavelength assignment problem. The optimization aims at minimizing the total number of require wavelengths to satisfy the traffic requests an protection requirements. 3. Simulations an numerical results To evaluate the survivable alternate routing algorithm, the numerical results reporte in this section are base on simulations that are conucte using a WDM network planning tool. The tool is evelope using C++ an runs in a Microsoft Winows environment. few objectoriente class libraries have been evelope incluing topology iscovery algorithm, k-shortest path routing, breath first iverse routing, genetic algorithms, wavelength assignment algorithms, an so on. The simulations are conucte on the network topologies shown in Figure 6. ach noe has a fully configurable optical switch, which can connect any wavelength in an ingress fiber to any egress fiber using the same wavelength. Network Network Figure 6. Network topologies use in the simulations Non-uniforme traffic matrixes are generate ranomly. First, traffic patterns are generate for each simulation. traffic pattern specifies the approximate volume of traffic between each noe pair. Three kins of patterns are efine: high volume, meium volume, an low volume. ach kin of pattern is mappe a pre-efine banwith range. Then the traffic matrix is generate, where banwith requirement for each noe pair is generate ranomly accoring to the preefine pattern. The main evaluation metric consiere in this work is the network reunancy ratio (C r ), which is efine as the ratio of the total spare capacity over the total working capacity: C r = ( ) ( ) Network reunancy highly epens on the network topology as well as the spare capacity allocation algorithms use. Using eicate path protection schemes, generally C r is over 100%. Without explicitly consiering SRLG constraints, the share path protection scheme propose in [6] obtaine much lower C r, from 74% to 87% for a thirty-two noe network. However the algorithm epen on fault localization. We compare the multi-path protection scheme with a single-path scheme by using ifferent traffic matrix over the two simulation topologies, the ten noe network an the twenty-three noe network. The numerical results are given in Table 2., which show that the multi-path protection scheme can obtain about a 20% reuction of network reunancy compare with the single-path protection metho. In turn, this reuces the cost of the survivable WDM network in terms of total number of wavelength require for certain traffic requests. Table 2. Comparison of Single-path protection an multi-path protection. T: number of active noe pairs; λ w : total wavelength require with no protection; λ p : total wavelength require with full protection. NTWORK T λ W SINGL-PTH MULTI-PTH PROTCTION PROTCTION λ p C r λ p C r % 10 51% % 14 49% % 19 46% % 14 58% % 17 54% % 19 55% 4. Conclusions This work aims at proviing cost-effective esign of survivable WDM long-haul networks with static traffic. survivable alternate routing metho is propose in this paper. Heuristic approaches are implemente to select both working routes an protection routes in orer to achieve approximate optimal performance while maintaining a computational feasibility. xplicit SRLG constraints are applie in the new metho. Therefore, more cost-effective network esigns an shorter restoration time can be achieve. The numerical results show that the multi-path protection metho outperforms single path protection approaches in terms of the resulting network reunancy. Proceeings of the Seventh International Symposium on Computers an Communications (ISCC ) / $ I

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