Survivability in waveband switching optical networks: Challenges and new ideas

Computer Communications 31 (2008) 2039–2046

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Survivability in waveband switching optical networks: Challenges and new ideas q Xingwei Wang a, Lei Guo a,b,*, Xuetao Wei c, Weigang Hou a, Fei Yang a, Lan Pang a a b c

College of Information Science and Engineering, Northeastern University, Shenyang 110004, China Key Lab of Broadband Optical Fiber Transmission and Communication Networks, University of Electronic Science and Technology of China, Chengdu 610054, China Department of Computer Science and Engineering, University of California, Riverside, CA 92521, USA

a r t i c l e

i n f o

Article history: Received 14 November 2007 Received in revised form 7 March 2008 Accepted 9 March 2008 Available online 20 March 2008 Keywords: Optical networks Wavelength-division-multiplexing (WDM) Waveband switching (WBS) Survivability

a b s t r a c t With the developmental trend of ultra-high speed and ultra-large capacity required in optical networks, the number of wavelengths in each fiber greatly keeps increasing and the consequence is that the size (i.e., number of switching ports) and cost of conventional Optical Cross-Connects (OXCs) are excessively enhanced. Therefore, the technique called Waveband Switching (WBS) has been proposed to reduce the number of switching ports for saving the costs. At the same time, the survivability for optical networks is a very important issue since each wavelength channel has the transmission rate over several gigabits per second and the failures may lead to a lot of traffic blocked. Combining the WBS technique and survivability in optical networks, in this paper we comprehensively review the existing survivable schemes in WBS optical networks which are quite different from conventional survivable schemes in Wavelength-Routed (WR) optical networks. Then, we analyze the shortages of the current researches and prospect the challenges for survivability in WBS optical networks. Finally, we propose new ideas for designing survivable schemes to well guide the future work of researchers in WBS optical networks.  2008 Elsevier B.V. All rights reserved.

1. Introduction 1.1. Waveband switching In current years, Wavelength-Division-Multiplexing (WDM) optical networks that are equipped with Optical Cross-Connects (OXCs) and employ Wavelength-Routed (WR) selection has become the most feasible systemic solution for wide area backbone networks. With the development and maturation of WDM technique, the number of wavelengths in each fiber keeps increasing such that the size (i.e., number of switching ports) and cost of OXCs greatly are enhanced and the corresponding control and management become more and more complicated. Therefore, we can say that, although the WDM has become more mature, the fibers have become more popular, and OXCs and optical switching techniques have obtained great advancement, the

q

This work was supported in part by the National Natural Science Foundation of China (Nos. 60673159, 70671020), the National High-Tech Research and Development Plan of China (No. 2006AA01Z214), the Key Project of Chinese Ministry of Education (No. 108040), the Specialized Research Fund for the Doctoral Program of Higher Education (Nos. 20070145096, 20070145017, 20060145012), the Program for New Century Excellent Talents in University, and the Open Foundation of Key Laboratory of Broadband Optical Fiber Transmission and Communication Networks, Ministry of Education (UESTC), China. * Corresponding author. Address: College of Information Science and Engineering, Northeastern University, P.O. Box 134, Wen Hua Road, Shenyang, Liao Niang 110004, China. Tel.: +86 24 83687575; fax: +86 24 23906321. E-mail addresses: [email protected], [email protected] (L. Guo).

0140-3664/$ - see front matter  2008 Elsevier B.V. All rights reserved. doi:10.1016/j.comcom.2008.03.011

expensive costs, high complexities, and unknown reliabilities of large-scale switching equipments have also baffled the comprehensive applications. Under this situation, the technique called Waveband Switching (WBS) has been proposed to reduce the size of OXCs for saving the costs. The main idea of WBS is to perform waveband grouping method to bind several lightpaths of wavelength level into one waveband which can be switched by only one port such that the switches in conventional WR networks can be reduced and further the cost of OXCs can be saved. In order to support the waveband switching meanwhile provide efficiency for conventional wavelength switching, the authors in [1] proposed the Multi-Granular OXCs (MG-OXCs) structure which includes multi-layer and single-layer models. The main difference between the multi-layer MG-OXCs and single-layer MG-OXCs is that the single-layer MGOXCs removes the modules among different layers, i.e., fiber to band (FTB) module, band to wavelength (BTW) module, band to fiber (BTF) module, and wavelength to band (WTB) module, so that the realization, configuration and control for single-layer MG-OXCs can be simpler. Although many researchers have investigated the problems in WR optical networks and the WR technique is also the foundation in design for WBS optical networks, the difference of motivations between WBS and WR is quite large. Generally, the motivation for designing WR optical networks is to reduce and optimize the number of consumed wavelength-links or wavelength-hops while the motivation for designing WBS optical networks is to reduce and optimize the number of consumed switching ports in

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MG-OXCs. Current research in [1] has indicated that, the method for minimizing the number of consumed wavelength-links or wavelength-hops cannot efficiently reduce the number of consumed switching ports in MG-OXCs, and even a simple WBS algorithm also cannot be obtained by slightly extending the conventional Routing and Wavelength Assignment (RWA) algorithm in WR optical networks. Therefore, the current opinion is that an efficient WBS algorithm can make a performance trade-offs between the number of consumed wavelength-links or wavelength-hops and the number of consumed switching ports, and generally the number of consumed switching ports can be significantly reduced by slightly increasing the number of consumed wavelength-links or wavelength-hops. Therefore, due to the difference of motivations between WBS and WR, the techniques that can be suitable for WR optical networks cannot be directly suitable for the design of WSB networks, such that the researches on new and dedicated techniques for WBS, such as WBS routing algorithms, wavelength or waveband assignment methods, waveband grouping schemes, are quite necessary. Some papers have investigated the design for new techniques in WBS optical networks. In [1–7], the authors proposed several typical waveband grouping schemes including Same Source Grouping (SSG) scheme in which the lightpaths of wavelength level with the same source node can be grouped to the common waveband, Same Destination Grouping (SDG) scheme in which the lightpaths with the same destination node can be grouped to the common waveband, and Same Sub-Path Grouping (SSPG) in which the partial lightpaths with the same segmented source node and segmented destination node can be grouped to the common waveband, where the SSPG scheme has the better performance of saving the number of switching ports. In [8–11], the authors performed the performances test for WBS switching with considering the full wavelength conversion capacity, limited wavelength conversion capacity and interior wavelength conversion capacity in the same waveband, respectively. In [12–20], the authors proposed several WBS routing algorithms with heuristic methods. In [21], the authors presented the technique of waveband merging to improve the switching performance. In [22], the authors studied the technique of discontinuous waveband in which the wavelengths with discontinuous location in the same fiber can be grouped to the common waveband. In [2,16], the authors described the Integer Linear Programming (ILP) formulation for WBS routing and wavelength assignment to search the optimal or near-optimal result. In [23,24], the authors investigated the placement of optical nodes with sparse MG-OXCs that can guide the future researches on segment routing or multi-domains routing in WBS optical networks. In [25,26], the authors addressed the problem of optimizing the size of wavebands, i.e., optimal number of wavelengths in each waveband. In [27], the authors proposed the WBS multicast routing algorithm based on the suggested auxiliary graph. In [28], the authors studied the impairment-aware waveband switching routing algorithm. 1.2. Survivability in optical networks In optical networks, the failures may lead to a lot of traffic blocked since each wavelength channel has the transmission rate over several gigabits per second. Therefore, survivability in optical networks is a very important issue and has been studied for many years. The main idea of survivability is to allocate redundant backup resources for active working resources, where the backup resources should be failure-disjoint with the working resources. In normal cases, the traffic can be carried by working resources. If the failures lead the working resources to be unavailable, the traffic can be switched and carried by backup resources since the backup resources are failure-disjoint with the working resources; namely, they will not be unavailable simultaneously. In optical networks,

resources mainly include such as wavelength channels, switching ports, optical transceivers, wavelength converters, and so on. Since resources are generally finite, the design for survivability in optical networks often need consider the resource optimization. Survivable schemes in optical networks mainly include protection scheme and restoration scheme, in which the protection scheme can be simpler configured and also have faster recovery time so that most previous researches in [29–32] are favorite for employing protection scheme. Protection scheme can be further classified into three categories, i.e., path-based protection, linkbased protection, and segment-based protection. Generally, pathbased protection can perform better resource utilization ratio than the link-based protection and segment-based protection, while link-based protection and segment-based protection can perform faster recovery time than the path-based protection. Most previous researches consider the path-based protection since it is easier to implement in the current phase than the link-based protection and segment-based protection. In path-based protection, each demand will be assigned to a working path and a link-disjoint backup path such that any single-link failure can be tolerated. Path-based protection includes two categories, i.e., dedicated-path protection (DPP) and shared-path protection (SPP). In DPP, the backup paths cannot share the backup resources in any condition; but in SPP, any two backup paths can share the backup resources if their corresponding working paths are link-disjoint due to the single-link failure constraints. 1.3. Survivability in WBS networks With the number of wavelengths in each fiber keeps increasing, the number of switching ports and the cost of conventional OXCs are greatly enhanced. However, the increased switching ports and cost can be actually saved by the WBS technique and then the saved cost can be used for the construction of optical networks. Therefore, the problem of survivability in WBS optical networks has become the hot research in recent years. However, due to the difference of motivations between WBS and WR, the techniques that can be suitable for WR optical networks cannot be directly suitable for the design of WSB networks, so that the researches on new and dedicated techniques for WBS are too necessary. Comparing with the above mentioned survivable schemes which are all dedicated for WR networks in Section 1.2, only few referred researches [33–39] have elementarily addressed the survivability in WBS optical networks so that there are still many challenges and problems which need to be solved in future work. As the opinion in [33], the survivability in WBS optical networks has the quite attraction and challenge to researchers, and many issues are still unexplored at the current stage. Therefore, in Section 2 of this paper, we comprehensively review the existing survivable schemes in WBS optical networks and we analyze the shortages of current schemes. In Section 3, we present the possible challenges for survivability in WBS optical networks and propose new ideas for designing survivable schemes that can well guide the future work of researchers in WBS optical networks. In Section 4, we prospect the trend of new generation optical networks. In Section 5, we conclude this paper. 2. Related work Due to the late begin of studying the survivability in WBS optical networks, the existing works are few. In [33], the authors reviewed the survivable WBS schemes and proposed a heuristic algorithm based on layered-graph. The proposed algorithm that employs the fixed routing selection can try its best to assign available wavebands to demands if the required bandwidths are greater than one wavelength channel in the first stage; in the second stage

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can be realized by configuring one or more trees for networks. However, the proposed algorithm only can be suitable for searching the solutions under the situation of static demands. In [37], the authors proposed a shared-path protection algorithm in WBS networks, in which the backup wavelengths in backup wavebands with higher sharing degrees will have larger probabilities of being selected such that the number of switching ports can be saved. However, the proposed algorithm ignores the problem presented in [34]: improvement for sharing degree of backup wavelengths is contrary to reduction for the number of switching ports; that is, if the backup wavelengths have been shared by many backup paths, in protection switching process of different failure scenarios, many backup switching ports may be required by different backup paths. For example in Fig. 1, the circles denote the ports, solid lines denote the backup waveband and dashed lines denote the backup wavelength. We assume the backup waveband contains three backup wavelengths that can be shared by nine backup paths. We can see that: (1) in case 1, after the failures occur, the traffic can be switched to backup paths 1, 2 and 3 which are on the same backup waveband path from port A to port F, and here six ports are consumed; (2) in case 2, after the failures occur, the traffic can be switched to backup paths 4, 5 and 6 which are on different backup wavelength paths from port A to port G, to port H and to port J, respectively, and here six ports are consumed; (3) in case 3, after the failures occur, the traffic can be switched to backup paths 7, 8 and 9 which are on different backup wavelength paths from port A to port I, to port K and to port L, respectively, and here eight ports are consumed. Therefore, it is obvious that although the backup wavebands and wavelengths can be shared to improve the resources utilization ratio, the number of switching ports also may be significantly increased since different backup paths may require different switching ports. The same problem also has been ignored by [38]. Therefore, the

the algorithm can assign available wavelengths to residual demands that cannot be allocated the available wavebands in the first stage. In [34], the authors investigated the protection in WBS networks with considering the shared backup wavelengths and proposed both ILP formulations and heuristic algorithm subject to minimizing the number of switching ports. This research has shown that the improvement for sharing degree of backup wavelengths is contrary to the reduction for number of switching ports, so that the performance trade-offs between them should be considered in the future work. In [35], the authors focused the protection design in WBS optical networks with the Shared-Risk Link Groups (SRLG) constraints and proposed two heuristic algorithms, i.e., PBABL and MPABWL. In order to tolerate the single-SRLG failure, in PBABL each working waveband path will be assigned to a SRLG-disjoint backup waveband path, while in MPABWL each working waveband path will be assigned to a SRLG-disjoint backup waveband path and multiple SRLG-disjoint backup wavelength path. Simulation results have shown that MPABWL has better performances since it can more efficiently utilize the wavebands and wavelengths. However, the deficient issues are: (1) the backup wavebands or wavelengths are dedicated but not shared such that the resources utilization ratio may be low; (2) both PBABL and MPABWL perform the fixed routing selection in which the SRLG-disjoint paths for each node pair need to be pre-computed by an offline manner, such that the fixed paths cannot be adaptive and may not be suitable for the current dynamic situation; (3) the demand matrixes need to be given in advance, such that the proposed algorithm cannot properly handle dynamic demands. In [36], the authors extended the ILP formulations of sharedpath protection for conventional WR survivable networks to the new ILP formulations for WBS survivable networks and also proposed a modified spanning tree algorithm in which the protection

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Fig. 1. Backup switching ports assignment with shared backup wavelengths.

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trade-off schemes between improving sharing degree of backup wavelengths and reducing the number of switching ports need to be studied in the future work. In [39], the authors studied the protection technique of considering non-uniform wavebands and proposed the ILP formulations based on SSG and SSPG schemes. However, the authors only consider the dedicated backup wavelengths and static demands such that more efficient heuristic algorithms for shared backup wavelengths and dynamic demands are challenges for the future work. According to the above presented contents, the main characters of current researches on survivable WBS optical networks can be summarized as follows: (1) survivable routing algorithms, especially protection routing algorithms, have got more attention at the current stage; (2) due to the difference of optimal objectives between WBS and WR, even a simple WBS survivable algorithm cannot be obtained by slightly extending the conventional WR survivable algorithm, and WBS survivable algorithms with the unique characters have been proposed but they are still in beginning stage; (3) in the case of shared backup wavelengths, the trade-off between the sharing degree and the number of switching ports needs to be considered when designing the waveband grouping schemes for survivable algorithms; (4) current researches mostly focus on unicast WBS survivable routing, while researches on multicast WBS survivable routing are still vacant; (5) researches on WBS survivable routing for multi-layer and multi-domain optical networks are not yet reported; (6) the problems of both routing and network configuration have been mainly solved by some fixed routing methods for static demand matrixes, so that the efficient dynamic heuristic algorithms are missing. By concrete analysis, the main disadvantages of current researches on survivability in WBS optical networks can be summarized as follows: (1) for dynamic survivable routing algorithms, current researches mostly only consider the single constraint but ignore the multiple constraints; (2) for static programming in WBS networks, current researches mostly only consider the optimal configuration of switching ports but ignored the globe optimization for other factors, such as sparse MG-OXCs, wavelength converters, splitting capacity, and so on; (3) current researches mostly employ the fixed routing methods, fixed size of waveband and continuous wavelength assignment in waveband, such that the efficient dynamic routing algorithms and the techniques of adaptive assignment for wavelength or waveband are missing; (4) only [27] refers the multicast routing in WBS networks but not consider the survivability, that is to say, the research on multicast survivability in WBS optical networks is blank; (5) current researches only study the survivability in single-layer and singledomain WBS optical networks, while researches on survivability in multi-layer and multi-domain WBS optical networks are not yet reported; (6) survivability on differentiated services for WBS networks is still in progress. It is obvious that the researches on survivability for WBS networks are still in beginning stage, and many related issues are still under solving, such that we can have a lot of innovative chances and challenges. In the following section, we will present the challenges and propose new ideas for designing survivable schemes to well guide the future work of researchers in WBS optical networks.

3. Challenges and new ideas 3.1. Survivability in unicast WBS networks Same with other routing algorithms based on graphs, the key problem here is to construct the proper auxiliary graph that should be suitable for running the shortest-path algorithm to find the survivable routes for unicast demands in WBS networks. In reviewing

current studies of WBS survivable algorithms, we have noted that most of them only consider the single constraint of number of switching ports, which may not be practical for real situations. In future work, when designing the WBS survivable algorithms we need to consider multiple constraints which include the number of switching ports, wavelength and waveband continuity, waveband multiplexing or de-multiplexing, transmission delay, recovery time, etc. We can employ the loopless k-shortest path algorithm [40] and Waveband/Wavelength Layered Graph (WWLG) to solve the waveband and wavelength continuity constraints, in which WWLG can be generated by modifying the conventional Wavelength Layered Graph (WLG) [41]. For example in Fig. 2, we assume the given network has four nodes and five fibers, each fiber has four wavelengths, and two wavelengths can be grouped to one waveband. Then, WLG can be constructed with four different wavelength plans which correspond to k1, k2, k3, k4, respectively. If k1 can be converted to k3, there will exist the dashed m links between nodes N m k1 and N k3 8m ¼ 1; 2; 3; 4. Based on WLG, we can construct WWLG in which there are two additive waveband plans, i.e., B1 and B2. If assume (k1, k2) belong to B1 and (k3, k4) belong to B2, there will exist the dashed links between nodes N m By and Nm kx where y = dx/2e "x = 1,2,3,4. With the proposed WWLG, we can construct the full connected auxiliary graph by assigning the proper costs to different links for addressing multiple constraints of the number of switching ports, wavelength and waveband continuity, waveband multiplexing or de-multiplexing, transmission delay, recovery time, etc. Therefore, the problem for WBS survivable routing under multiple constraints can be changed to the problem for constructing and modifying of peculiar auxiliary graph. Based on constructed WWLG, some challenges and new ideas can be proposed and researched. (A) Since the fiber links may traverse the common physical resources, such as conduits, cables, etc, they may have the correlated failure probability, which can be defined as SRLG, such that the SRLG-disjoint survivable design in optical networks has become an important constraint. For computing the pair of working path and link-disjoint backup path, the authors in [42] proposed the Suurballe’s algorithm which can fast obtain the solution by running only one time. Under dynamic situations, however, computation for the pair of WBS working path and SRLG-disjoint backup path may be more complicated. Currently, only [35] reported the WBS survivable routing with SRLG constraints, but the proposed algorithm only supports the static demand matrixes. We can suggest the future work could consider the extended k-shortest path algorithm [30] with integrating the SSG or SDG scheme to design the dynamic survivable routing algorithm. Another idea for fast sub-path protection algorithm with trap avoidance [31] also can be combined with SSPG scheme to design the dynamic survivable routing algorithm in WBS networks. (B) Under the background of various services in current networks, survivability which is an important factor for evaluating the Quality of Service (QoS) of networks also should be differentiated; that is, survivability should provide different service levels for different requirements or failures. Although researches on survivability for supporting different service levels have been well studied in WR networks, the same researches in WBS networks have not been reported so that challenges and innovative space for designing differentiated survivable schemes in WBS networks are wide. First, we can consider the survivable schemes with different service levels for different users; that is, the services can be categorized to six levels from high to low which are shown in Fig. 3, i.e., Diamond which is to provide the best-effort protection for multiple failures, Platinum which is to provide the complete protection for dual failures, Goal which is to provide complete protection for single failure, Argentine which is to provide best-effort protection for

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Fig. 2. Auxiliary graph for layered wavelength and waveband plans.

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VIP, International E-Business....

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Bank operation, Domestic E-Busines...

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City area network operation...

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Campus, Company operation...

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E-mail....

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Fig. 3. Different levels for different survivable services.

single failure, Cuprum which is to provide nothing protection, and Iron in which the resources can be preempted by other services with high levels. We can note that current WBS routing algorithms and WBS survivable algorithms can provide the services to the levels below Goal, but nothing WBS survivable algorithms can address the services for Diamond and Platinum levels. Since previous researches have been down to solve the multiple failures [32] and dual failures [42] in WR networks, we can extend the ideas to design the survivable algorithms in WBS networks; that is, for multiple failures we can employ the idea of multi-sub-paths protection with SSPG scheme, for dual failures we can employ the idea of three link-disjoint paths with SSG or SDG scheme, so that the services for Diamond and Platinum levels can be solved. Since the holding times for real demands in practical networks are generally different, the survivable schemes based on differentiated holding times in WBS networks need to be studied. An idea is to classify the demands to three types, i.e., Real-time demands whose holding times are generally very short ranging from millisecond to second, Ordinary demands whose holding times are little long ranging from minute to hour, and semi-permanent demands whose holding times are long ranging from day to month. The chal-

lenge here is to accurately model the different types and employ the proper waveband grouping schemes and survivable strategies for different demands, that will be in progress in future studies. A character in WBS networks is that the affections to traffic from the failures of different fiber links are different. The reason for this is that the technique of waveband grouping may lead much traffic to converge on some fiber links, and then the failures of these fiber links may affect a lot of traffic. Therefore, the differentiated protections for crucial links which carry much traffic are required. An idea here is to employ the classification for crucial links, and different class can perform different protection algorithms with different waveband grouping schemes. Another idea here is to use load-balancing routing which can reduce the number of crucial links. 3.2. Survivability in multicast WBS networks Same with the survivability in unicast WBS networks, the key problem here is to construct the proper WWLG that should be suitable for running the minimal-tree algorithm to find the survivable trees for multicast demands. When constructing WWLG, we not

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only need consider these constraints which include the number of switching ports, wavelength and waveband continuity, waveband multiplexing or de-multiplexing, transmission delay and recovery time, but only need consider the unique constraints which include the splitting capacity, cross-sharing for backup resources of different backup trees and self-sharing between working and backup resources in the same demand [43]. Finally, we can construct the full connected auxiliary graph for survivable routing in multicast WBS networks, and therefore the problem for WBS survivable routing under multiple constraints can be changed to the problem for constructing and modifying of peculiar auxiliary graph. Based on constructed WWLG, the challenges and new ideas for multicast survivable routing can be proposed and researched. (A) Current researches only solved the problem of waveband grouping for unicast demands while the waveband grouping schemes for multicast demands have not been reported. Compared with previous unicast waveband grouping schemes, the challenge here is to consider the splitting capacity constraints in optical nodes. The ideas are gradually clear for designing four dedicated schemes to multicast waveband grouping, i.e., Same Root and Same Leaf Grouping (SRSLG), Same Root Grouping (SRG), Same Leaf Grouping (SLG), and Same Sub-Tree Grouping (SSTG), which are illustrated in Fig. 4, respectively. Based on the new multicast waveband grouping schemes, the future work can explore the heuristic survivable tree algorithms for dedicated or shared backup wave-

lengths and wavebands, and evaluate the respective performances of the four schemes to guide the following researches. (B) Since the resources, such as wavelength channels, switching ports, optical transceivers, wavelength converters, and so on, are generally finite, the design for survivability in WBS networks needs to solve the resources optimization. The problem of optimization generally includes dynamic optimization and static optimization, in which static optimization can be performed based on dynamic algorithms. Therefore, in the first step we can focus on designing dynamic optimal algorithms, and in the second step we need to propose some proper strategies to solve the static optimal problem by repeatedly running and adjusting dynamic algorithms. In dynamic algorithms, there are two policies to find the solutions: (1) the first policy is to decompose the problem of survivability for WBS multicast to two sub-problems, i.e., the first sub-problem is the multicast tree generation (including working tree and backup tree) for survivable routing and the second sub-protection is wavelength and waveband assignment for multicast tree, and to perform heuristic algorithms to solve the two sub-protection, respectively; (2) the second policy is to joint the multicast routing and wavelength/waveband assignment to one problem which can be directly solved on constructed auxiliary graph WWLG. Based on dynamic algorithms, the future work can focus on researching the resources optimization at the static demand cases.

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Fig. 4. Waveband grouping schemes for multicast WBS networks.

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Current researches in [23–26] have indicated that the distribution of MG-OXCs and the size of wavebands (including uniform and non-uniform wavebands) will significantly affect the demand blocking probabilities so that how to configure the optical nodes with MG-OXCs capacities and how to determine the size of wavebands are challenges. In additional, other optimal problems which correspond to various network characters also should be studied, such as, optimizing the placement of optical nodes with wavelength or waveband conversion capacities, optimizing the configuration of continuous and non-continuous wavelengths in wavebands, optimizing the placement of electrical nodes with grooming capacities, optimizing the placement of splitters, and so on. The challenge here is to model these required optimal objectives according to the characters of traffic blocking probabilities and follow to propose efficient heuristic algorithms to optimize this model. Another innovative point for WBS multicast survivability is to study the mixed grouping scheme which has not been mentioned by previous researches. In the mixed grouping scheme, the working wavelengths and backup wavelengths can be grouped to the same waveband; that is, the same waveband not only can carry working traffic on part of wavelengths but also can reserve part of wavelengths for backup traffic, so that the number of switching ports can be further reduced and the waveband utilization ratio also can be improved. The challenge here is to solve the mixed grouping scheme under dedicated and shared backup wavelengths or wavebands. Previous research that can be referred is only [44] in which the authors proposed the mixed shared protection scheme in WR networks. Based on the idea [44] and current waveband grouping schemes, the mixed grouping scheme can be solved in future work. 3.3. Survivability in multi-layer and multi-domain WBS networks Since the technique of General Multi-Protocol Label Switching (GMPLS) [45] proposed in recent years can provide the seamless convergence between IP networks and optical networks, the sur-

vivability based on GMPLS control plan for IP/MPLS over WR optical networks shown in Fig. 5 has been investigated comprehensively. By extending the conventional MPLS, there are unique labels to support fibers, wavebands, wavelengths and time-slots in GMPLS so that the multi-layer survivability for IP/ MPLS over WBS optical networks can be efficiently supported by GMPLS control plan. The breaking point at here is to design the survivable virtual topology for tolerating the single failure or multiple failures. The main idea is to construct the pretty multi-layered virtual graph, which includes IP/MLSP layer, optical wavelength layer and optical waveband layer, by assigning proper link-costs with considering the factors, such as waveband grouping schemes, number of switching ports constraints, waveband and wavelength conversion capacities, and so on. Based on multi-layered virtual graph, single-layer survivable schemes or multi-layer survivable schemes can be evaluated, and multi-layer harmonious schemes also can be designed. Finally, we can well solve the survivability for multi-granular optical networks. Same with Internet, current optical networks have been actually separated to multiple domains that are called Automated Systems (ASs) and each AS has its own network provider and management policy, so that survivability for multi-domain WR networks is the hot research all through these years. However, survivability for multi-domain WBS networks has not been reported by current works so that it will have wide space for innovation. A feasible idea in [21] may guide the future work in WBS survivable routing in multi-domain networks, where the idea is to separate the networks to several ASs and each AS has one or more gateway MG-OXCs nodes which can deal with the waveband grouping in its own domain. We can integrate the idea [23] with existing multi-domain survivable algorithms such as the local segment shared protection algorithm [46] to solve the survivability in WBS multi-domain networks based on GMPLS. The challenge here is to solve the configuration of gateway nodes and MG-OXCs, routing computation in intra-domains and inter-domains, waveband grouping in intra-domains and inter-domains and control signals of inter-domains.

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Fig. 5. Multi-layer networks based on GMPLS.

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4. Prospects Challenges and new ideas of survivability for unicast, multicast, multi-layer and multi-domain WBS optical networks which have been proposed in Section 3 are not isolated, indeed they are associated cheek by jowl. For example, unicast is a special case of multicast, thus the problem of multicast survivability can be solved, the problem of unicast survivability also can be solved; the multi-layered auxiliary graph not only can be used to solve the multi-constrained survivable routing but also can be used to construct the virtual topology of multi-layer networks; survivable algorithms in single-domain networks are the basic of survivable algorithms in multi-domain networks. Therefore, we can prospect that the future way on survivability for new generation optical networks will trend to tolerate the failures on converging IP with optical networks for any-cast, multi-granularity, multi-layer and multi-domain based on GMPLS. 5. Conclusions This paper has comprehensively reviewed the existing survivable schemes in WBS optical networks and analyzed the shortages of current studies. Based on challenge for designing survivability in WBS optical networks, this paper has proposed new ideas, including multi-constrained survivable routing schemes, WBS auxiliary graph, differentiated services, multicasting waveband grouping schemes, and multi-layer and multi-domain WBS survivable policies which can well guide the future work of researchers. Acknowledgement The authors thank reviewers for valuable comments. References [1] X. Cao, V. Anand, C. Qiao, Framework for waveband switching in multigranular optical networks part I-multigranular cross-connect architectures, J. Opt. Netw. 5 (2006) 1043–1055. [2] M. Lee, J. Yu, Y. Kim, C. Kang, J. Park, Design of hierarchical crossconnect WDM networks employing a two-stage multiplexing scheme of waveband and wavelength, IEEE J. Select. Areas Commun. 20 (2002) 166–171. [3] X. Cao, V. Anand, C. Qiao, Waveband switching in optical networks, IEEE Commun. Mag. 41 (2003) 105–112. [4] Y. Zhou, Q. Zeng, Z. Zhang, Waveband switching in optical networks, Semiconduct. Optoelectron. 25 (2004) 57–61. [5] M. Li, W. Yao, B. Ramamurthy, Same-destination-intermediate grouping vs. end-to-end grouping for waveband switching in WDM mesh networks, Proc. ICC 3 (2005) 1807–1812. [6] X. Cao, V. Anand, C. Qiao, Waveband switching for dynamic traffic demands in multigranular optical networks, IEEE/ACM Trans. Netw. 15 (2007) 957–968. [7] A. Todimala, B. Ramamurthy, Algorithms for intermediate waveband switching in optical WDM mesh networks, Proc. HSNW 1 (2007) 21–25. [8] X. Cao, C. Qiao, V. Anand, J. Li, Wavelength assignment in waveband switching networks with wavelength conversion, Proc. GLOBECOM 3 (2004) 1943–1947. [9] X. Cao, V. Anand, J. Li, C. Xin, Waveband switching networks with limited wavelength conversion, IEEE Commun. Lett. 9 (2005) 646–648. [10] S. Yao, B. Mukhejee, Design of hybrid waveband-switched networks with OEO traffic grooming, Proc. OFC 1 (2003) 357–358. [11] S. Yao, C. Ou, B. Mukhejee, Design of hybrid optical networks with waveband and electrical TDM switching, Proc. GLOBECOM 5 (2003) 2803–2808. [12] L. Wan, W. Lv, P. Zhang, W. Gu, Waveband tunnel allocation algorithm in multigranular WDM network, Study Opt. Commun. 32 (2006) 33–35. [13] H. Song, Y. Xu, W. Jin, W. Gu, A dynamic routing and wavelength assignment algorithm in waveband switching optical network, J. Beijing Univ. Posts Telecommun. 27 (2004) 45–49. [14] M. Li, W. Yao, B. Ramamurthy, A novel cost-efficient on-line intermediate waveband-switching scheme in WDM mesh networks, Proc. GLOBECOM 4 (2005) 2013–2019.

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