P-Cycle Based Protection Schemes for Multi-Domain Networks

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1 P-Cycle Based Protection Schemes for Multi-Domain Networks Arnold Farkas, Janos Szigeti, Tibor Cinklert High-Speed Networks Laboratory Department of Telecommunications and Media Informatics Budapest University of Technology and Economics Magyar tudosok krt. 2., H Budapest, Hungary Tel: +(36) Fax: +(36) Abstract- In multi-domain networks, the domains are typically run by different operators, which do not want to share all the internal information with competitors. Furthermore, broadcasting all the internal informations would unnecessarily overload the network with state messages. We decompose the multi-domain resilience problem into two sub-problems, namely the higher level inter-domain protection, and the lower level intra-domain protection. Building p-cycles at the higher level is accomplished by certain tasks at the lower level including straddling link connection, capacity allocation, and path selection. We show that p-cycles offer high availability and acceptable resource usage without the knowledge of paths that makes p- cycles very good candidate for protecting inter-domain traffic. Index Terms-p-cycle, multi-domain networks, resilience, restoration, protection. I. INTRODUCTION In optical transport networks, there is a huge amount of traffic that belongs to different service classes and has typically different availability requirements as well. For enhancing availability of the services, several failure handling methods exist, that vary in their spare resource requirements, re-routing time, and some other properties [1]. The resilience mechanism which uses pre-calculated backup paths and pre-assigned resources, that effects shorter re-routing time, is defined as protection. The mechanism that calculates and sets up backup paths based on topology information update after a failure has taken place is defined as restoration. Resilience has high resource efficiency, however, it is typically slower that protection and often it can not guarantee full restorability of the affected traffic. The resilience methods can be grouped by their scope: a method can be path-, segment- (sub-network) or link- (span)oriented, and the protections can be sorted further according to their accessibility: if only one traffic has the right to use the resources reserved for protection, then that This work has been supported by the EC within the FP6 IP NOBEL ( framework ttibor Cinkler has been supported by the OTKA Postdoctoral Research Grant D42211 and by the Janos Bolyai Postdoctoral Research Fund protection is dedicated, otherwise, if multiple traffic instances may alternatively use the resources, is referred to as shared. The existing traditional failure handling methods, such as 1+1 automatic protection switching (1+1 APS), shared backup path protection (SBPP), BLSR ring, p-cycles, mesh path restoration and mesh link restoration [2] are not feasible in multi-domain networks since they require detailed information about the inner topology of the domains, which is inaccessible. Furthermore, for the shared protection schemes not only the topology and the capacities are required, but the route of the working and of the protection paths for all demands as well. A. p-cycles The p-cycle combines the advantages of the ring and of the mesh: it realizes ring-like recovery speed while retaining the capacity efficiency of the mesh-based methods. The p-cycle is a cyclic, pre-calculated, pre-assigned, closed path with a certain amount of allocated spare capacity [3]. It provides protection for any link that has both end nodes on the cycle as either an on-cycle link or a straddling link. Similarly to the BLSR rings, if an on-cycle link fails, then it is replaced by a protection path along the remaining cycle. In contrast to the BLSR rings, however, the p-cycle is also able to protect straddling links that results in higher capacity efficiency than for the ring of the same size [4]. In the case of a straddling link failure the p-cycle can simultaneously protect two units of working capacity on the straddling link by providing two alternative backup paths around the p-cycle. P-cycle is a shared link protection method and therefore it has higher capacity efficiency than dedicated link protection: a p-cycle of one capacity unit can protect one unit of working capacity of any each on-cycle link if it fails or two units on capacity of any straddling link that fails. Generally, the capacity efficiency of p-cycle based schemes is much higher than that of 1+1 APS, but does not match that of SBPP [5]. Since it is a link protection method, it has a shorter fault detection and re-routing time than path oriented methods. The backup path is pre-calculated and pre-assigned, and therefore it is faster than restorations that must signal and configure switches along the paths at the time of failure occurence /05/$ IEEE 223

2 Applying p-cycles in a network does not restrict the working paths in any way, allowing them to choose the best, i.e. the most reliable routes through the network. The actual working efficiency of a p-cycle can be estimated by Xp,i = E: Xp,i E (p)= ViEL,where whr (1) I 2 if i is a straddling link within cycle p with unprotected working capacity of at least 2 units, I if i is a straddling link within cycle p with unprotected capacity of 1 unit or an on-cycle link with unprotected capacity of at least 1 units, 0 otherwise. L is denoting the set of links in the network, and C is the resource cost of the p-cycle. B. Multi-Domain Network Model In multi-domain networks, the domains are typically operated by competitor providers, who are reluctant to share information with their competitors. Furthermore, broadcasting every minor change in the network state would unnecessarily overload the network with state messages, causing problem in scalability. Applying a network model with hierarchical constitution is an obvious solution for scalability and since different informations are available at the different levels, the hiding of the topology also succeeds. The model we use consists of two levels. At the higher level only the border nodes and inter-domain links are known. The domains are connected, therefore, at the higher level, virtual links between the border nodes form the inner topology. At the lower level, the internal informations of the separated domains are known. An example for such a multi-domain network is shown in Figure 1. II. DEFINING p-cycles FOR THE HIGHER LEVEL The knowledge of the whole topology, available capacities on each link and traffic demands is required for applying conventional shared protection schemes which are not accessible at the higher level. At the lower level only internal informations are available. Therefore a possible solution for multi-domain shared link protection scheme is to handle the failures at the higher level (for the inter-domain links) and at the lower level (for the intra-domain links) separately. At the lower level traditional protection schemes can be applied as intra-domain protection, but for inter-domain link protection a novel method is required. Our goal is to define higher level p-cycles (without the knowledge of the internal structures of the domains) that are capable of protecting inter-domain links similarly to conventional p-cycles. There are three steps of forming a higher level p-cycle: Fig. 1. Multi-domain network model Assign inter-domain links as on-cycle or straddling interdomain links. Define the required logical internal connections for the higher level p-cycle. Realize the required logical internal connections. The first two steps are tasks of the higher level, the last one is the task of the lower level. A. Assigning Inter-Domain Links The first task is to handle domains as simple nodes at the higher level and compose p-cycles in the transformed network in a conventional way. This means defining on the one hand inter-domain links that will be on-cycle links within the higher level p-cycle an on the other hand inter-domain links that the p- cycle can protect as a straddling links (with the proper internal connections). At the lower level, a domain is notified which border nodes are connected to on-cycle inter-domain links within the higher level p-cycle (on-cycle border nodes - cbns), and which border nodes are connected to straddling inter-domain links (straddling border nodes - sbns). Within an affected domain, a higher level p-cycle can have no more than two cbns since only simple cycles can be composed in a conventional way. The limit for the number of sbns is the number of nodes in the domain. Of course, it can occur that within a domain the two cbns of the higher level p- cycle are identical and/or an sbn is a cbn as well, furthermore, there is no sbn. B. Define Required Logical Internal Connections Since the internal structure of the domains is hidden from the higher level, that level can only assign the required logical connections over affected domains to the considered higher level p-cycle. 224

3 Those border nodes that are either on-cycle cbn or straddling sbn must be interconnected suitably in order that the higher level p-cycle can handle failures analogously to a conventional p-cycle. As an example, consider the case when a straddling interdomain link fails. Then those two domains are responsible for protection switching where its two end nodes (sbns) are. For this purpose, two backup path within both affected domains must exist. These connect sbn to the two cbns. The other case is when an on-cycle inter-domain link fails. Then a backup path must be assigned between the two cbns. Figure 2(a) shows an example for an on-cycle inter-domain link failure, and an example is depicted in Figure 2(b) for a straddling inter-domain link failure. (a) Logical internal connections / / (b) Most Reliable (c) Least Cost (d) Ring Based Fig. 2. (a) On-cycle inter-domain link failure (b) Straddling inter-domain link failure / Failure handling with higher level p-cycle Figure 3(a) shows the required internal connections of a domain (gray shape) that has five border nodes. The upper three are sbns, while the bottom two are cbns. The external dashed lines represent straddling inter-domain links of the higher level p-cycle. C. Realizing Logical Internal Connections To configure the real internal connections is the objective of the lower level, since the required internal informations are accessible only at this level. At the lower level, within the domains there are seven paths that correspond to the seven connections (logical links) between the border nodes of Figure 3(a). These paths are not necessarily disjoint. Assuming that only a single straddling inter-domain link fails at time we can share resources allocated for protecting straddling links. This reduces the cost significantly. Note, that a unit of capacity on the p-cycle can protect Fig. 3. Internal connections two units of capacity of straddling links. Therefore the set of links must be defined, where one unit of spare capacity is not enough, since they are the extensions of the straddling inter-domain links. We have developed different strategies, namely the Most Reliable, the Least Cost and the Ring Based one for realizing the logical connections. 1) Most Reliable: Within a domain the two cbns should be interconnected with the most reliable path. Each sbn should also be interconnected with the cbns via the most reliable paths. On the common segments of the two cbn - sbn paths two capacity units must be allocated. If the two cbns are identical, i.e., the higher level p-cycle does not traverse, just "touches" the domain, then two units of capacity must be allocated on each link in the internal connection, since the whole structure corresponds to the inner extensions of the straddling inter-domain links. This strategy guarantees that, in the case of a failure, the used backup path is the most reliable one, i.e. it provides the best availability. Figure 3(b) shows an example of realizing logical internal connections with the Most Reliable strategy. Within the domain, the thicker links represent the links, where one unit of capacity should be allocated for the higher level p-cycle, while the dashed links represent the part of the internal connection where two units of capacity are needed. 2) Least Cost: The first step is to determine which sbn is the closes to one of the cbns, i.e., which sbn requires the least resources to connect to one of the cbns. That sbn should be connected to the closest cbn with the path that requires the least resources, since that segment cannot be shortened further (i.e., cannot be made cheaper). The next step is to connect the closest sbn among the remaining sbns to the connected structure via the shortest path, thereby that sbn is connected with the minimal resource requirement. This step must be 225

4 repeated until no sbn remains. Finally, connect in the same way the remaining cbn, if any. In this structure, two units of capacity must be allocated in each used link except on the links of the shortest route between the two cbns where one unit is enough. With this greedy strategy the spare-capacity requirement of the the internal connection is minimized. An example for realizing the logical internal connection with the Least Cost strategy is shown in Figure 3(c). 3) Ring Based: To realize the logical internal connections with the Ring Based strategy the shortest ring is used that contains all the cbns and sbns in the domain. Only one unit of capacity must be assigned on each link. In the case when a connected straddling inter-domain link fails, the traffic flows between the two cbns and the affected sbn on the two alternative paths around the ring. In other case of failure the traffic flows between the two cbns on the shortest path along the ring. An example for a Ring-Based realization is depicted in Figure 3(d). The two previous strategies can be applied in any connected graph including trees where a Ring-Based internal connection cannot be established, therefore we do not suggest to use this method. To realize the same logical internal connection in the same domain by the three different strategies require different amount of spare capacity: Most Reliable needs the most (14), Least Cost requires the least (9) and Ring Based is in the middle with the requirement of 12 units of spare capacity. The actual working efficiency of a higher level p-cycle can also be estimated by Equation (1). III. REFERENCE METHOD To analyze the performance of the proposed protection schemes we have applied different kinds of protection strategies. As the reference protection method we have applied the conventional dedicated (end-to-end) path protection, since it is a common scheme. We have compared its performance to the proposed multi-domain oriented methods to analyze the advantages and disadvantages of dividing the network into domains. The generated traffics are assigned with dedicated path protection, where the backup path is disjoint from the working path, and the failure handling is not separated to lower and higher level mechanisms. Full information of topology is required for this protection scheme, which is generally inaccessible in a multi-domain network. IV. PROPOSED METHODS In a multi-domain network, the internal structure of the domains is concealed at the higher level, and therefore the link protection mechanisms should be differentiated to interdomain and intra-domain failure handling. Applying protection schemes at the higher level requires spare capacities for their internal connections at the lower level, but can handle only the failures of the inter-domain links. Furthermore, at the lower-level additional spare capacities are required for intra-domain protections. Therefore the multidomain oriented schemes require more spare capacity to protect the same traffics than the conventional methods do. Previous works have shown that p-cycles have high capacity-efficiency [5] compared to other protection schemes and offer high availability that makes them very good candidate for protecting inter-domain traffic. A. Enumerating p-cycles In order to assign and allocate p-cycles a suitable set of candidate p-cycles must be enumerated. Since the Grow and Expand algorithms [6] do not consider that replacing a link to a disjoint route may restrict the replacement of another link in the p-cycle, therefore the output of the algorithm depends on the sequence of the on-cycle links, which results in a smaller set of p-cycles. Figure 4 shows an example where, starting from the same link of the same primary p-cycle, the result of the Grow algorithm depends on the on-cycle link sequence (clockwise or counterclockwise). Fig. 4. Sequence dependency of the Grow algorithm This fact is not a problem in the case of a dense network topology containing a lot of links and nodes, since the difference in the spare resource requirement to the optimal set of capacitated p-cycles is less significant [6], but in the case of less denser topology the difference is not negligible. For this reason, we suggest the ExhaustiveGrow algorithm based on Grow for less denser networks. Like its predecessor, this algorithm also requires a primary set of p-cycles that can be generated by the Straddling Link Algorithm (SLA) [8], for instance. Unlike the Grow algorithm this considers all the possible sequences of the on-cycle links which effects a larger set of p-cycles. The algorithm can be represented in pseudocode as follows: ExhaustiveGrow( Cycleset) For each cycle p in Cycleset Mark all links and nodes on cycle p For each link i in cycle p { Dijkstra(i, unmarked links/nodes) If Dijkstra() returns a route r Let cycle p' = p Add route r to cycle p' Remove link i from cycle p' Add cycle p' to Cycleset } } } I Dijkstra() Find the shortest route r between the 226

5 } end nodes of i using the unmarked links and nodes only Return route r We have used the aforementioned ExhaustiveGrow algorithm to assign the on-cycle inter-domain links of higher level p- cycles, and have realized the internal connections with Least Cost and Most Reliable strategy either. To enumerate the set of candidate internal p-cycles at the lower level we have used the ExhaustiveGrow or Grow algorithm depending on the size and the density of the domain. B. Multi-Domain Oriented Schemes Since the p-cycle allocation and assignment methods based on ILP [7] [8] [9] are difficult and time-consuming, we applied the CIDA algorithm [6] to allocate and assign capacitated p- cycles at both levels. The applied protection schemes: CIDAO - we have applied only higher level p-cycles, there was no intra-domain protection. CIDA - the higher level CIDAO protection scheme is combined with p-cycles at the lower level. CIDED - at the higher level we have used CIDAO protection scheme, and at the lower level the intra-domain protections were dedicated segment protections. The realizations of logical internal connections for CIDAO, CIDA and CIDED strategies can be either Most Reliable (M) or Least Cost (L) strategy, depending on whether the higher availability or the minimized allocated resource is primary. V. EVALUATING CRITERIA The performance of the applied protection scheme can be evaluated by many properties such as availability, reliability, traffic loss, traffic delay etc., we have chosen the full capacity requirement, the availability and the relative thrift that considers both of them. A. Capacity Requirement Since in the real networks the available capacity is not infinite, however, applying a protection scheme enhances the availability of the traffics, it is also important how many allocated resources are needed to provide it, therefore the first property is the extra-capacity requirement, i.e. the redundancy. B. Availability Since the failure probability of a real optical link is of the order of 10 6, a long simulation time is required for an appropriate simulation, therefore we have estimated the availabilities. The evaluation of the availability needs extensive calculations being in the range of 0(22N), so we have applied a worst-case based estimation which is suitably fast, and has an acceptable accuracy. To estimate the availability of a segment of traffic t in domain d with shared link protections, we have used the following formula: As(t,d) =JJpi ±+ (1 ler ler where r is the set of links in the affected segment of the route, pl denotes the probability of link 1 is operational, B is denoting the set of links in the backup path for link 1, and S is the set of links that the protection is also assigned for. In the case of applying dedicated segment protection the formula for estimating the availability of the inner segment can be expressed as: As(t,d) HPI + (1 HP) fpi ler ler ieb The availability of the whole traffic can be estimated with two different formulae depending on applying dedicated path protection or combined inter-domain and intra-domain protection schemes. When using dedicated path protection the formula for estimating the availability of a traffic is the following: A(t)Ipi +R(1]p B)pi, 1ER 1ER ie:b where R is denoting the set of links in the whole route and B is the set of links in the backup path for the route. In the cases of combined inter-domain and intra-domain resilience mechanisms the availability can be estimated by the following formula: A(t) Pi As(t, i) +,(1 Ppi) pi, (5) IEI ied IEI ierubus\{i} where I denotes the set of inter-domain links in the whole route, D is the set of domains used by the traffic. This estimation is worst-case based, because it allows only one failure in each internal segment or one in the external segment of the path, and in case of shared link protection all the other links that can be protected with the same protection must be operational, otherwise the traffic fails. In fact a traffic can still operate in the case of a multiple failure if they are handled with independent protections and/or the failed link of the traffic has the priority to use the shared protection. Although it is a worst-case based estimation formula, it gives a low-bound with acceptable accuracy since the probability of a traffic is operational in the case of a multiple failure is very low, and therefore it causes a little difference in availability when considering all possible states. C. Relative Thrift The relative thrift of protection method i can be defined as: 1 Ai 1 Ao (6) ri + 1' where Ai is the average availability that the protection offers, Ao is the average availability of the unprotected traffics, and ri is the redundancy of the applied protection scheme. Pi) IH. Pi, (2) ierubus\{l} (3) (4) 227

6 VI. NUMERICAL RESULTS We have performed tests in two multi-domain networks. The first is a hypothetical, randomly generated network where all the links have enough free capacity to apply any kind of resilience mechanism without any restrictions. That network is named TNet that is depicted in Figure 5(a). And we have performed several tests in the model of the European core network - El-Net [10], which is shown in Figure 5(b). 50% of the links of the route of traffics is protected for single link failure in the TNet is shown in Table I. The multi-domain oriented methods handle the failures of the two levels separately, therefore allocated resources are required at the lower level for the intra-domain protections, and for the internal connections of the inter-domain protections, that results in higher capacity requirement than the conventional methods. TABLE I CAPACITY REQUIREMENTS OF TNet Prot. scheme Cap. req. Redundancy >0% >50% 100% Unprotected % 0% 0% 0% I % 98.2% 98.2% 98.2% CIDAO-L % 84.3% 3.1% 0.3% CIDAO-M % 84.3% 3.1% 0.3% CIDA-L % 100% 100% 100% CIDA-M % 100% 100% 100% CIDED-L % 100% 100% 100% CIDED-M % 100% 100% 100% (a) TNet Fig. 5. (b) El-Net Tested networks The probability of link failure in the networks can be calculated as: q = l qo, (7) where 1 is denoting the length of the link in km, and qo denotes the link failure coefficient (LFC) with a default empirical value of [ 1 ]. If the weight of a link is set as: C = logi q = logp (8) then the shortest route is the most reliable one as well. Since q C (0; 1), its logarithm is definitely negative, therefore the weight must be positive, and Dijkstra's algorithm can be applied to find the most reliable routes, and it is unnecessary to use the Belmann-Ford algorithm as opposed to [11]. By altering the value of LFC, we have examined the alteration of the availability, which is equivalent to analyze the behaviour of a metropolitan, national, continental, and worldwide network with the same topology. We have generated randomly 1000 traffic demands of average bandwidth requirement of 5 units to both networks. Since the traffics are generated at random, the distribution of the capacity requirement on each link is not uniform. A. Capacity Requirement The full capacity requirement of the traffics and the applied protection schemes, and the rate where 100% and at least The capacity requirements of the El-Net is shown in Table II. Although applying traditional dedicated path protection would require less spare capacity, in the case of a realistic network such as this network, where a domain may have only one border node and/or has a tree-structure, the two disjoint path do not definitely exist, that makes dedicated path and segment protection infeasible. This explains why only 7.4% of the traffic demands could be protected with end-to-end protection. On the other hand, in a multi-domain network, where the internal structures are not visible at the higher level, the conventional link protection schemes are not usable. Therefore a multi-domain oriented protection scheme with higher capacity requirement must be applied. TABLE 11 CAPACITY REQUIREMENTS OF El-Net Prot. scheme Cap. req. Redundancy >0% >50% 100% Unprotected % 0% 0% % 7.4% 7.4% 7.4% CIDAO-L % 74.7% 0.9% 0% CIDAO-M % 74.7% 0.9% 0% CIDA-L % 99.4% 95.8% 21.6% CIDA-M % 99.4% 95.8% 22.5% CIDED-L % 95.3% 64.6% 20.1% CIDED-M % 95.3% 65.7% 20.9% B. Availability The relation between LFC and the estimated average availabilities of the traffics of the El-Net is depicted in Figure 6. The X axis indicates the altering LFC and on the Y axis is the average availability of the traffics. Since in the El- Net only a small portion of traffic could have been protected with dedicated path protection, the improvement of average availability is irrelevant. At smaller LFC values, the CIDA- M strategy resulted the highest average availability, only at greater LFC values was it better to apply CIDED-M. For 228

7 realistic optical links, the link failure coefficient is of the order of 10 6, therefore it can be concluded that applying CIDA-M resulted in the highest average availability in this network. ma Average availabilities figure the rate of traffics is shown on the Y axis where the availability was at least that is indicated on the X axis. It can be observed that using CIDA-M resulted in the highest availabilities, and therefore applying p-cycles at both levels were the most efficient. As it can be seen in this figure either, applying dedicated path protection has not improved the availabilities significantly. Tail behaviours CIDA-M-- CIDA-L---' CIDED-L --A-- CIDAO-M - CIDA0-L Unprotercted - - _ - 1 e-06 1 e-05 LFC E E- Fig. 6. Average estimated availabilities in the El-Net The estimated average availabilities of the traffics of the TNet are shown in Figure 7. Applying dedicated path protection requires less spare capacity than the other methods, at the same time, only the CIDED-type strategies resulted in higher average availabilities. In the latter case, the inter-domain protection is combined with dedicated segment protection at the lower level. Because of the inner topology, the internal segments are very short, sometimes contain only one link, therefore sometimes the applied dedicated segment protection is dedicated link protection, which offers higher availability than a shared link protection scheme does. The CIDA-M and CIDA-L strategies were less efficient, because the backup paths for the inter-domain links were almost as long as the backup paths of the dedicated path protection, and in case of multiple failures -multiple long backup paths are required to be operational. Average availabilities Fig. 8. Availability > X Tail behaviours in El-Net Figure 9 shows the tail behaviour of availabilities in TNet at the link failure coefficient of [ 1 ]. The curves of the reference and CIDAO strategies are staged because according to the size of the network, a lot of traffic have used the same link, and since there has been no protection, or only applied at the higher level, in the case of failure of one of the unprotected links a lot of traffic has been interrupted.the CIDED-M strategy resulted the highest availabilities, that can also be explained by the dedicated short segment or link protection Tail behaviours E I CIDED-M _--, CiDED-L --A-: 1 + ti-.-',' CIDA-M -U- CIDA-L -y CIDA0-M *-: CIDAO-L - ' Unprotected ,...,.. 1e-06 I e-05 LFC Fig. 7. Average estimated availabilities in TNet Tihe tail behaviour of the availabilities in El-Net is depicted in Figure 8 at the LFC value of [ ']. In this 200 O Availability > X Fig. 9. Tail behaviours in TNet C. Relative Thrift The relative thrift of the applied protections schemes in the El-Net is depicted in Figure 10. The Y axis indicates the 229

8 relative thrift while the LFC, that is indicated on the X axis, is altering. The CIDA-type strategies have the highest relative thrift in this network. Applying dedicated path protection resulted so low thrift that the thrift of the unprotected case was even higher. The CIDAO methods have the two lowest thrift, since both of them protect only against the failures of the inter-domain links, but require allocated resources inside the domains.. r- a: 2 Relative thrift ofthe protection schemes -.~~~-_-_- - CIDA-M - CIDA-L - CIDED-L - -v CIDED-M - CIDED-L - - Unprotected 1 +1 CIDAO-L ~~~CIDAO-M _- 1 e-06 LFC 1 e-05 Fig. 10. Relative thrifts in El-Net The relative thrift of the applied protections schemes in TNet is shown in Figure 11. It can be observed that the thrift of the CIDED methods are the two highest, since both methods offer outstanding availability. At lower coefficients the CIDA schemes have higher thrift than dedicated path protection has. The realistic LFC is of the order of 10 6, therefore the CIDA methods have higher thrift than dedicated end-to-end protection. r-.a,1 100' 10I 1 Relative thrift of the protection schemes e-06 e-05 LFC Fig. 11. Relative thrifts in TNet CIDED-M _.- CIDED-L CIDA-M 1 +1-,- -a. Unprotected CIDAO-M CIDAO-L **^ *. ~~~~~~~CIDA-L _ It can be seen in both figures that a scheme using higher level p-cycles with Most Reliable realization has the highest thrift, however, in every other cases, a scheme with Least Cost realization has higher thrift than the corresponding scheme of Most Reliable strategy. e CONCLUSION The most significant advantage of using higher level p- cycles is that it can be applied without the knowledge of the internal structure of the domains in contrast to the traditional shared link protection schemes, which are infeasible without knowing intemnal informations. A higher level p-cycle retains the benefits of a conventional p-cycle by offering high availability with acceptable sparecapacity requirement. One disadvantage of the higher level p-cycle is, that it is a link-protection method, and therefore it is not able to handle node failures in contrast to the path protections, which offer backup path if an inner node fails, thereby the service remains operational after a very short interruption. The disadvantage of the proposed multi-domain oriented protection schemes is, that they require more allocated resources than the traditional methods, since at the lower level spare capacity is needed for the intra-domain protection, and for the inner segments of the inter-domain protections. REFERENCES [1] D. Zhou, S. Subramaniam, "Survivability in optical networks," IEEE Network, vol. 14, no. 6, Nov./Dec. 2000, pp [2] D. A. Schupke, M. Jager, R. Huilsermann, "Comparison of resilience mechanisms for dynamic servcies in intelligent optical networks," Proc. Fourth International Workshop on the Design of Reliable Communication Networks (DRCN 2003), Banff, Alberta, Canada, Oct. 2003, pp [3] F. J. Blouin, A. Sack, W. D. Grover, H. Nasrallah, "Benefits of p- cycles in a mixed protection and restoration approach," Proc. Fourth International Workshop on the Design of Reliable Communication Networks (DRCN 2003), Banff, Alberta, Canada, Oct. 2003, pp [4] W. D. Grover, D. Stamatelakis, "Bridging the ring-mesh dichotomy with p-cycles," Proc. IEEE/VDE Workshop on Design of Reliable Communication Networks (DRCN 2000), Munich, Germany, April 2000, pp [5] J. Doucette, W. D. Grover, "Comparison of mesh protection and restoration schemes and the dependency on graph connectivity," Proc. Third International Workshop on the Design ofreliable Communication Networks (DRCN 2001), Budapest, Hungary, 7-10 Oct. 2001, pp [6] J. Doucette, D. He, W. D. Grover, 0. Yang, "Algorithmic approaches for efficient enumeration of candidate p-cycles and capacitated p-cycle network design," Proc. of Fourth International Workshop on the Design of Reliable Communication Networks (DRCN 2003), Banff, Alberta, Canada, Oct. 2003, pp [7] D. A. Schupke, C. G. Gruber, A. Autenrieth, "Optimal configuration of p-cycles in WDM networks," Proc. IEEE International Conference on Communications (ICC 2002), New York City, NY, 28 April - 2 May, 2002, vol. 5, pp [8] W. D. Grover, J. Doucette, "Advances in optical network design with p-cycles: Joint optimalization and pre-selection of candidate p-cycles," Proc. of IEEE/LEOS Summer Topicals 2002, Mont Tremblant, PQ, July 2002, pp [9] J. Kang, M. J. Reed, "Bandwith protection in MPLS networks using p- cycle structure," Proc. Fourth International Workshop on the Design of Reliable Communication Networks (DRCN 2003), Banff, Alberta, Canada, Oct. 2003, pp [10] Didna Mesk6 [Online], "European core network - El-Net," [11] A. Banati, I. Koncz, "Novel algorithms for graph reliability in packet switched networking," Proc. 5th International Workshop on Rare Event Simulation and Combinatorial Optimization (RESIM COP 2004), Budapest, Hungary, September