Dynamic Routing and Spectrum Assignment Algorithms in Flexible Optical Networks: An Overview

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1 2012 7th International ICST Conference on Communications and Networking in China (CHINACOM) Dynamic Routing and Spectrum Assignment Algorithms in Flexible Optical Networks: An Overview Nan Hua*, Yang Liu, Xin Wan, Xiaoping Zheng State Key Laboratory on Integrated Optoelectronics, Tsinghua National Laboratory for Information Science and Technology, Department of Electronic Engineering, Tsinghua University, Beijing , P.R China * huan03@mails.tsinghua.edu.cn Zhigang Liu Institute of China Electronic Equipment System Engineering Company, Beijing, , P. R. China Abstract-In this paper, we examine the current research on the dynamic routing and spectrum assignment (RSA) problem in flexible optical networks. The existing RSA algorithms are classified into three main categories: fixed routing (FR), fixed alternative routing (FAR) and adaptive routing (AR). We select the representative algorithms within each category, and briefly introduce their concept, advantage and drawback. Comparison among these algorithms is made in respect of blocking performance, computational complexity and so on. Keywords - flexible optical networks; routing and spectrum assignment (RSA); orthogonal frequency division multiplexing (OFDM); spectrum segments I. INTRODUCTION Traditional wavelength division multiplexing (WDM) is an enabling technology for high-speed optical networks to meet the exponential growing demands of the Internet traffic [1, 2]. However, since the optical WDM networks strictly follow the rigid ITU-T grids, they suffer from coarse bandwidth granularity and significant granularity mismatch, thus causing inefficient resource utilization. Optical orthogonal frequency division multiplexing (OOFDM) has been proposed as a promising solution to this problem due to its high spectrum utilization and flexibility [3-5]. Using this technology, novel flexible optical network architecture, i.e. spectrum-sliced elastic optical path network (SLICE), has been proposed in [6] and [7]. As one of the key enabling technologies in flexible optical networks, the dynamic routing and spectrum assignment (RSA), which computes a path for a connection request and assigns an available spectrum segment with contiguous frequency slots, has recently attracted a lot of attention. In flexible optical networks, the conventional wavelength-continuity constraints (WCC) that exists in the traditional WDM networks are transformed to more rigorous spectrum-continuity constraints (SCC). Two rules should be satisfied in SCC. The first is the spectrum continuity rule. This rule guarantees that the spectrum segment which is assigned to a certain lightpath consists of contiguous frequency slots, and is the same over all the links comprising this lightpath. The second one is the spectrum nonoverlap rule. This rule ensures that the two signals transported on the same link are assigned to different spectrum segments without overlaps. The SCC make the RSA problem more complicated than the well-known routing and wavelength assignment (RWA) problem in WDM networks. In order to solve the dynamic RSA problem, a number of different algorithms have already been proposed in the literature. These algorithms can be classified into three main categories: fixed shortest-path routing (FR), fixed alternative routing (FAR) and adaptive routing (AR), which are all inherited from the traditional WDM networks. Literature [8-12] focuses on the FR-based RSA solution. In order to obtain lower blocking probability, FAR-based RSA algorithms are proposed in [7, 13-17]. Among these FR- and FAR-based algorithms, the first-fit (FF) spectrum assignment scheme is most used [7-10, 12, 13, 17]. To increase spectrum resource utilization, the most-used (MU) spectrum assignment scheme is used in [14] and [16], which is widely employed in the traditional WDM networks. In [15], a maximize common large segment (MCLS) scheme is proposed. This scheme achieves higher resource utilization than FF and MU. In order to reduce blocking during the signaling process, a collisionaware weight based fit (CWF) scheme is proposed in [11]. Adaptive routing can significantly reduce the blocking probability in dynamic flexible optical networks. Related work can be found in [18-22]. In [18], a least congested conditioned to modulation format (LC MF) algorithm is proposed. This algorithm computes a path at the source node based on available aggregated bandwidth information flooded by an OSPF-TE extension. For spectrum assignment, the FF scheme is used during the signaling process. In [19] and [22], the authors propose a modified Dijkstra shortest path (MSP) algorithm, which perform complete RSA computation (both routing and spectrum assignment) at the source node. The MSP algorithm relies on OSPF-TE extensions to disseminate the spectrum state of each link. As a shortest-path-based routing algorithm, MSP is not able to obtain optimal routes in flexible optical networks. In order to obtain optimal routes, two traversal-based algorithms, i.e. the spectrum-constraint path vector searching (SCPVS) algorithm and the entire path searching (EPS) algorithm, are proposed in [19, 22] and [20], /12/$ IEEE

2 respectively. However, since these two algorithms have factorial computational complexity, they are not suitable to be used in large real networks. To solve this problem, a spectrumscan routing (SSR) algorithm is proposed in [20]. This algorithm can obtain an optimal route in polynomial time. The authors later extend the SSR and propose a discrete spectrumscan routing (DSSR) algorithm in [21], which can be applied to both the slot-based and the gridless flexible networks. Literature [23-25] focuses on spectral defragmentation, which rearranges the routes and allocated spectrum of existing connections, to accommodate new connection requests. The spectral defragmentation algorithms try to move the spectral fragments of a link into a continuous block, thus obtaining consecutive unallocated spectrum slots. The remainder of this paper is organized as follows. Section II introduces the FR- and FAR-based RSA algorithms. Section III introduces the adaptive RSA algorithms. Section IV makes a comparison among different RSA algorithms. Finally conclusions are given in Section V. II. FIXED ROUTING AND FIXED ALTERNATIVE ROUTING A. Fixed shortest-path Routing (FR) FR is a widely-used solution for the dynamic RSA problem because of its simplicity. FR is a two-step approach, where the routing and the spectrum assignment are processed separately. For routing, each node maintains a static pre-calculated routing table which contains the shortest paths for each destination node. When a connection request arrives, the routing controller of the source node searches its routing table by the destination address, and selects the corresponding path. Spectrum assignment is based on the spectrum aggregation operation, which explores the selected path (in the routing step) and calculates the aggregated available spectrum along this path. Based on the aggregated spectrum, the spectrum assignment schemes, e.g. the FF, MU, MCLS and CWF mentioned in Section I, will be used to calculate a spectrum segment for the connection. Spectrum assignment can be performed either in the source node or during the signaling process. In the second case, the change of the link state (availability of each frequency slot) is not required to be disseminated throughout the network, thus reducing the control plane (CP) bandwidth consumption. An example of the FR-based RSA computational process is shown in Fig. 1. B. Fixed Alternative Routing (FAR) The blocking probability of the FR is high. It can be seen in Fig. 2 that if the available resource on the pre-calculated path (P 1 ) can not satisfy a connection request, this connection will not be established, without considering other paths. Figure 2. An example of a FAR-based RSA computaional process (from source S to destination D). FAR is proposed to improve the blocking performance. It is also a two-step approach. Different from FR, at least two candidate paths will be pre-calculated for each destination. When a connection request arrives, the routing controller searches the pre-calculated routing table, selects the candidate paths in sequence, and processes the spectrum assignment for each candidate, either sequentially in the source node, or in parallel during the signaling process. In Fig. 2, the connection can be established successfully on the second shortest path (P 2 ) with the FAR-based RSA algorithm. III. ADAPTIVE ROUTING Adaptive routing is a one-step approach that jointly considers routing and spectrum assignment at the source node. AR requires real-time update of the whole network state, which can be realized through extended link state advertisement (LSA) flooding. In this section, the concept of four representative ARbased algorithms, i.e. MSP, EPS, SSR and DSSR, will be introduced. A. Modified Dijkstra Shortest Path (MSP) Different from the classic Dijkstra algorithm, the MSP algorithm performs the spectrum aggregation operation each time a new neighbor is to be added to the candidate list. If the aggregated available spectrum can not satisfy the required bandwidth, this candidate will be discarded and will not be added to the candidate list. Fig. 3(a) shows an example of MSP that successfully find a route. However in some cases, MSP are not able to find a route although such a route exists. It can be seen in Fig. 3(b) that <S- 1-2-D> is an available path that satisfies the requested bandwidth. However this path will never be added to the routing table since it will be discarded at Node 2 due to larger hops. Figure 1. An example of the FR-based RSA computational process (from source S to destination D). The computational complexity of MSP is O(N 2 ), where N is the node number of the network. This complexity is at the same level as that of the classic Dijkstra algorithm. 252

3 S 2 D 1 no available spectrum request: can cover the slots of requested bandwidth. By shifting the SW slot-by-slot from the start to the end of the spectrum pool, SSR can scan all available spectrum segments (Fig.5). Each time the SW shifts to a new scanning point (SP), the network topology is pruned of occupied links, and then the classic Dijkstra algorithm can be used to calculate a candidate shortest route. Finally, SSR selects the shortest route among all the candidates as the optimal route. (a) no available spectrum S 2 D request: 1 longer path Figure 5. The spectrum-scan routing procedure. (b) Figure 3. Two examples of the MSP algorithm: (a) successful routing computation; (b) failed routing computation. B. Entire Path Searching (EPS) In order to achieve optimal routing (find a shortest available path if it exists) for dynamic flexible optical networks, the traversal-based algorithms such as EPS are proposed. Fig. 4 shows an example of the EPS procedure. The only difference between EPS and MSP is, the former will never discard a longer available path before a shortest path to the destination node is found. D. Discrete Spectrum-Scan Routing (DSSR) DSSR is the extension of SSR, which can be applied to the gridless flexible networks. The DSSR algorithm is based on spectrum discretization, which discretizes a continuous spectrum with the end frequencies of the existing spectrum segments. The DSSR procedure in a three-node ring network is described in Fig.6. In this procedure, the end frequencies of the spectrum segments assigned to all established connections (C1, C2, and C3) are regarded as the SPs, which mark the possible start of a new spectrum segment. Then DSSR uses the same scanning procedure as that in SSR to find the optimal route. Figure 4. An example of the EPS procedure. (a) It can be seen that EPS considers every possible path to the destination, and consequently has a factorial computational complexity - O(N!). Because of the high complexity, such kind of algorithms is not suitable to be used in large real networks. C. Spectrum-Scan Routing (SSR) The SSR algorithm is proposed to reduce the computational complexity for optimal routing in dynamic flexible optical networks. SSR requires the spectrum pool to follow discrete frequency slots. It introduces a spectrum window (SW) which (b) Figure 6. The DSSR procedure for a new connection request (C4): (a) spectrum discretization; (b) topology and established connections. 253

4 TABLE I. COMPARISON OF DYNAMIC RSA SCHEME FR/FAR LC MF MSP SCPVS/EPS SSR DSSR Optimal Path Computation Computational Complexity Resource Utilization Adapted to gridless networks Blocking Performance CP Bandwidth Consumption - the best - the worst The number of times that DSSR implements the Dijkstra algorithm (complexity - O(N 2 )) depends on the number of established connections, which will not increase faster than polynomial with the network scale. Therefore, the total computational complexity of DSSR is also polynomial. IV. ALGORITHM COMPARISON An overall comparison among the FR, FAR, LC MF, MSP, SCPVS, EPS, SSR and DSSR algorithms are shown in Tab. 1. As the optimal RSA algorithms, the SCPVS, EPS, SSR and DSSR outperform others in respect of the blocking performance and the resource utilization. However among these four algorithms, since SCPVS and EPS have factorial computational complexity, they are not suitable to be applied to real large networks. Comparatively, SSR and DSSR have much lower computational complexity of polynomial and become the best among all the algorithms. FR and FAR have both low blocking performance and inefficient resource utilization. However, they are still practical solution for the dynamic RSA problem because of their low computational complexity and low CP bandwidth consumption. V. CONCLUSIONS In this paper, we examine the current research on the dynamic RSA problem in flexible optical networks. The existing RSA algorithms can be classified into three main categories: FR, FAR and AR. We select the representative algorithms in each category, briefly introduce their concept, and compare their performance such as the blocking performance and computational complexity. The DSSR algorithm outperforms the others in respect of the blocking performance and the resource utilization. Its computational complexity is also acceptable. Although FR and FAR have low blocking performance, they are still practical solution for the dynamic RSA problem because of their low computational complexity and low CP bandwidth consumption. ACKNOWLEDGMENT This work was supported in part by projects under National 863 Program grant No. 2012AA011301, National 973 Program grant No. 2010CB328203, 2010CB328205, and NSFC under grant No REFERENCES [1] ITU-T G.694.1, Spectral grids for WDM applications: DWDM frequency grid, [2] C. A. Brackett, "Dense wavelength division multiplexing networks: principles and applications, " IEEE Journal on Selected Areas in Communications, 8(6): , [3] Q. Yang, Y. Ma, and W. Shieh, 107 Gb/s Coherent Optical OFDM Reception Using Orthogonal Band Multiplexing, IEEE/OSA OFC/NFOEC 2008, PDP7, [4] W. Shieh, X. Yi, Y. Ma, and Q. Yang, "Coherent optical OFDM: has its time come?," J. Opt. Netw. 7, , [5] L. Mehedy, M. Bakaul, and A. Nirmalathas, "Single-Channel Directly Detected Optical-OFDM Towards Higher Spectral Efficiency and Simplicity in 100 Gb/s Ethernet and Beyond," J. Opt. Commun. Netw. 3, , [6] M. Jinno, H. Takara, B. Kozicki, Y. Tsukishima, Y. Sone, and S. Matsuoka, Spectrum-efficient and scalable elastic optical path network: architecture, benefits, and enabling technologies, IEEE Communications Magazine, vol. 47, no. 11, pp , [7] M. Jinno, B. Kozicki, H. Takara, A. Watanabe, Y. Sone, T. Tanaka, and A. Hirano, Distance-adaptive spectrum resource allocation in spectrum-sliced elastic optical path network, IEEE Communications Magazine, vol. 48, no. 8, pp , [8] G. Shen, Q. Yang, From Coarse Grid to Mini-Grid to Gridless: How Much can Gridless Help Contentionless?, IEEE/OSA OFC/NFOEC 2011, OTuI3, [9] X. Wang, Q. Zhang, I. Kim, P. Palacharla, M. Sekiya, Blocking Performance in Dynamic Flexible Grid Optical Networks What is the Ideal Spectrum Granularity?, ECOC 2011, Mo.2.K.6, [10] S. Thiagarajan, M. Frankel, D. Boertjes, Spectrum Efficient Super- Channels in Dynamic Flexible Grid Networks A Blocking Analysis, IEEE/OSA OFC/NFOEC 2011, OTuI6, [11] X. Wang, Y. Zhao, X. Yu, J. Zhao, Collision-aware Spectrum Assignment Scheme for Flexible Bandwidth Optical Networks, ICCSEE 2012, [12] R. Muñoz, R. Casellas, R. Martínez, Dynamic Distributed Spectrum Allocation in GMPLS controlled Elastic Optical Networks, ECOC 2011, Tu.5.K.4, [13] T. Takagi, H. Hasegawa, K. Sato, et al, Dynamic Routing and Frequency Slot Assignment for Elastic Optical Path Networks that Adopt Distance Adaptive Modulation, IEEE/OSA OFC/NFOEC 2011, OTuI7, [14] K. Christodoulopoulos, I. Tomkos, E. Varvarigos, Dynamic Bandwidth Allocation in Flexible OFDM-based Networks, IEEE/OSA OFC/NFOEC 2011, OTuI5,

5 [15] Y. Sone, A. Hirano, et al, Routing and Spectrum Assignment Algorithm Maximizes Spectrum Utilization in Optical Networks, ECOC 2011, Mo.1.K.3, 2011 [16] R. Casellas, R. Muñoz, J. Fàbrega, et al, Experimental assessment of a combined PCE-RMA and distributed spectrum allocation mechanism for GMPLS Elastic CO-OFDM Optical Networks, IEEE/OSA OFC/NFOEC 2012, OM3G.1, [17] Qiushi Jin, Lei Wang, Xin Wan, Xiaoping Zheng, Bingkun Zhou, Study of dynamic routing and spectrum assignment schemes in bandwidth flexible optical networks, IEEE/OSA/SPIE ACP 2011, , [18] N. Sambo, F. Cugini, G. Bottari, P. Iovanna, P. Castoldi, Distributed Setup in Optical Networks with Flexible Grid, ECOC 2011, We.10.P1.100, [19] X. Wan, L. Wang, N. Hua, H. Zhang, X. Zheng, Dynamic Routing and Spectrum Assignment in Flexible Optical Path Networks, IEEE/OSA OFC/NFOEC 2011, JWA55, [20] Y. Liu, N. Hua, X. Wan and X. Zheng, A Spectrum-Scan Routing Scheme in Flexible Optical Networks, IEEE/OSA/SPIE ACP 2011, , [21] Y. Liu, N. Hua, X. Zheng, H. Zhang, B. Zhou, Discrete Spectrum-Scan Routing Based on Spectrum Discretization in Flexible Optical Networks, IEEE/OSA OFC/NFOEC 2012, JTh2A.49, [22] X. Wan, N. Hua, H. Zhang, X. Zheng Study on dynamic routing and spectrum assignment in bitrate flexible optical networks, accepted by Photonic Network Communications, [23] T. Takagi, H. Hasegawa, K. Sato, Y. Sone, A. Hirano, M. Jinno, Disruption Minimized Spectrum Defragmentation in Elastic Optical Path Networks that Adopt Distance Adaptive Modulation, ECOC 2011, Mo.2.K.3, [24] K. Wen, Y. Yin, D. Geisler, S. Chang, S. Yoo, Dynamic On-demand Lightpath Provisioning Using Spectral Defragmentation in Flexible Bandwidth Networks, ECOC 2011, Mo.2.K.4, [25] X. Yu, J. Zhang, Y. Zhao, T. Peng, Y. Bai, et al, Spectrum Compactness based Defragmentation in Flexible Bandwidth Optical Networks, IEEE/OSA OFC/NFOEC 2012, JTh2A.35,

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