Design of Hybrid Optical Networks With Waveband and Electrical TDM Switching

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1 Design of Hybrid Optical Networks With Waveband and Electrical Switching Shun Yao Department of Electrical and Computer Engineering, University of California, Davis, CA currently with Park, Vaughan & lemming, LLP Canhui (Sam) Ou and iswanath Mukherjee Department of Computer Science, University of California, Davis, CA Corespondence author: Shun Yao, Tel: , ax: Abstract We propose a hybrid optical-electrical architecture which integrates all-optical waveband ing and electrical time-division-multiplexed () ing. y grouping pass-through traffic into wavebands and ing them alloptically, the hybrid architecture can significantly reduce the size of electrical and number of associated transponders. As a result, it provides tens of terabits per second throughput with a small footprint. The proposed architecture is capable of ing, wavelength conversion, and multicasting. In this paper, we investigate the network-optimization problem with static traffic to minimize the overall network cost. A mathematical formulation of the optimization problem and its solutions are presented. ast heuristic approaches for nearoptimal solutions are also proposed and evaluated. I. INTRODUCTION As link capacity of optical networks continues to increase, there is a growing need for ultra-high-capacity ing. Expensive electrical-optical-electrical () transponders required at each port of the current es will render their cost prohibitively high in future optical networks. Micro electro-mechanical systems (MEMS) based all-optical wavelength ing is a promising technology to replace ing. Since MEMS es are insensitive to wavelength, they are capable of ing, all-optically, not only wavelengths but also wavebands (groups of wavelengths). Recently, the concept of using both waveband-level and wavelength-level ing in a hierarchical manner has received growing attention. The authors in [1] presented a twolayer optical crossconnect (OXC) architecture and demonstrated a two-stage multiplexing/demultiplexing scheme. References [2] and [3] provided qualitative discussions on similar ideas. The works in [4] showed how to reduce the OXC size by using waveband ing and fiber ing. The authors in [5] proposed a destination-based lightpath-grouping heuristic algorithm to take advantage of waveband ing. Reference [6] developed a mixed integer linear programming (MILP)- based approach to design a generic two-layer network. The work in [7] presented heuristic approaches for designing twolayer (waveband and wavelength) networks. Recent works in [8] and [9] presented network-design algorithms for threelayer (fiber, waveband, and wavelength) all-optical ing. Input To local lower speed tributaries DEMUX patch panel All-optical waveband electrical Output MUX Output Waveband Drop Wavelength Add W1 W2 W3 W4 rom local lower speed tributaries Waveband MUX/DEMUX Wavelength MUX/DEMUX Wavelength Drop Input Waveband Add ig. 1. Architecture of proposed optical incorporating all-optical waveband ing and ing. Although all-optical ing removes the bottleneck, its fundamental limitation persists: the lack of ing, arbitrary wavelength conversion, and multicast capabilities. We propose a hybrid architecture (ig. 1) incorporating all-optical waveband ing and electrical ing. This architecture offers the flexibility of sub-wavelength ing and the scalability of all-optical waveband ing. In this study we present the network-design algorithms, which is to minimize the network cost by properly planning waveband paths and routing client connections over the waveband paths. The paper is organized as follows: Section II describes the hybrid architecture, Section III formulates MILPbased network optimization, Section IV presents heuristicsbased approach, and Section V concludes the paper. GLOECOM /03/$ IEEE

2 II. NETWORK ARCHITECTURE A. Hybrid Switch Architecture igure 1 illustrates the hybrid architecture. The entire node consists of a fiber patch panel, an all-optical waveband, an electrical, traffic aggregation equipment, and waveband and wavelength multiplexers/demultiplexers. The fiber patch panel configures the fiber topology, which stays static once configured. (We assume the fiber topology to be fixed in this study.) The waveband uses an entire waveband as its ing granularity. In case a wavelength or a sub-wavelength circuit needs to be ed to a different waveband and/or fiber, the parent waveband is ed to the electrical. The is capable of arbitrary time-slot interchange, arbitrary wavelength conversion, and multicast. The ports of the operate at a wavelength s data rate. oth waveband and have add and drop ports for local traffic. A waveband add/drop port operates on one waveband, while a add/drop port operates on one wavelength. The traffic aggregation/deaggregation equipment performs multiplexing/demultiplexing for local lowerspeed connections. All connections travel across the fiber topology in one or more waveband paths. A waveband path is an un-disrupted waveband connection that originates and terminates at a waveband add/drop port or a. A variation of the hybrid optical is to insert a wavelength-ing layer between waveband and. The wavelength can potentially help reduce ports on the. However, the additional layer will introduce more ports for interconnecting the layers, which adds to the overall cost. A comparative study of twolayer and three-layer architecture is part of our on-going work. In this study we mainly focus on the two-layer architecture. The waveband, since no wavelength conversion is involved, can be constructed in a waveband-layered approach, as shown in the zoom-in part of ig. 1. This is similar to a wavelength-dilated in wavelength-routed networks. y doing so, we can significantly reduce the port-count requirement of individual all-optical device, since the number of input/output fiber pairs (which is equal to the nodal degree) is usually less than 10. Hence, it is possible to construct the entire all-optical waveband with inexpensive low-portcount MEMS devices (e.g., 2-dimension mirror arrays). We present a comparison between the hybrid and the full- in ig. 2. The node has degree of 6 ( =6). We assume that 25% of traffic is added or dropped. In the hybrid (ig. 2(a)), A and D denote the number of waveband add and drop ports. W and W denote the number of ports connecting the waveband to/from the, respectively. WA and WD denote the number of wavelength add/drop ports, respectively. Since there is 25% add/drop traffic, we ensure that at least 25% of all the capacity from input fibers can be dropped and added. Selection of W and W depends on how much - capability one desires the node to have. or the all-optical waveband, we assume that it is constructed with four 8 8 MEMS A W All-optical waveband full W D full- WD WA (a) (b) ig. 2. Comparison of hybrid hierarchical and full-. es in a waveband-layered approach (ig. 1). or the, the fabric consists of chips constructed in a non-blocking Clos network, and OE and EO transponders are necessary. As an example, we choose a set of these parameters as follows: =4, W = W =4, and A = D =4. The values of WA and WD are chosen such that two wavebandful of wavelengths can be added and dropped. In the node architecture using only electrical (full-) shown in ig. 2(b), all incoming fibers are demultiplexed directly to wavelengths. The state-of-the-art chip is assumed to be a OC-48 (port rate) chip with STS-1 -ing granularity. or wavelengths operating beyond OC-48, extra ports are required, e.g., a OC-48 chip can be used as a OC- 192 chip. We assume that the requires 4N transponders where N is the number of input/output ports. Table I shows the comparison results at OC-768 data rate. The hybrid demonstrates 73%-93% reduction of electrical chips and transponders, while its added cost being 8 8 MEMS es and waveband multiplexers/demultiplexers. The hybrid can achieve a throughput of 7.68 Tbps in a 32-wavelength network with only 79 chips. The trade-off is in the reduced capability in the hybrid architecture. Nevertheless, the reduction of network flexibility can be alleviated by nearoptimal network design, as shown in later sections.. Establishment of Waveband Paths Since a client connection always travels inside a waveband path, the goal of network design is to minimize the network cost by properly planning waveband paths and routing client connections over the waveband paths. This problem resembles the virtual topology design and traffic grooming problem in wavelength-routed networks. However, the hierarchical and hybrid nature of the, the variable number of wavebands per fiber and wavelengths per waveband, and the unique ways in which waveband paths are shared among connections all add new dimensions to the problem. WA WD GLOECOM /03/$ IEEE

3 TALE I HYRIDITCHVS. ULL- ITCH (OC-768). s Tbps Hyb. full- Saving MEMS sw. chips 4 none sw. chips % txponders % MEMS sw. chips 4 none sw. chips % txponders % MEMS sw. chips 4 none sw. chips % txponders % MEMS es 4 none sw. chips % txponders % As shown in ig. 2(a), a waveband path can originate from a waveband add port (one of the A ports), or from an - to waveband- port (one of the W ports). Similarly, it can terminate at one of the D ports or the W ports. As a result, there are four types of waveband paths: -, -W, W-, and W-W (see ig. 3), where and W indicate the waveband and, respectively ( for band and W for wavelength ). A client connection can only be ed from one waveband path to another through a. Waveband add/drop ports can only appear at a connection s source or destination node. waveband waveband (a) - waveband path waveband waveband waveband waveband (b) -W waveband path waveband waveband (c) W- waveband path (d) W-W waveband path ig. 3. Different types of waveband paths. III. MILP-ASED NETWORK DESIGN A. MILP ormulation We use the following notations in the MILP formulation: m and n denote end-points of a physical link. i and j denote originating and terminating nodes for a waveband path. s and d denote source and destination of a client connection. b denotes index of a waveband. y denotes data rate of connection requests (y {1, 3, 12, 48, 192}). t denotes index of OC-y connection request between a source-destination pair (s, d). The given conditions are: N, number of nodes in the network;, number of wavebands in a fiber; W, number of wavelengths in a waveband; mn, number of fibers connecting from node m to n; C, capacity of one wavelength channel (e.g., C = 192); Λ, set of connection request matrices (Λ = {Λ y }, where y indicates the data rate of connection requests, Λ y,sd denotes the number of OC-y connection requests between node pair (s, d)); and α, the ratio of the cost of an port (including the associated transponders) to the cost of an all-optical waveband port. The variables are: 1) Node configuration variables: D b i (number of waveband drop ports on waveband b at node i), A b i (number of waveband add ports on waveband b at node i), Wi b (number of ports on the waveband that lead to on waveband b at node i), Wi b (number of ports on the waveband that accept traffic from on waveband b at node i), WA i (number of wavelength add ports at node i) and WD i (number of wavelength drop ports at node i). 2) Waveband-path virtual topology variables: Vij b, VWb ij, VWb ij, and VWWb ij (number of -, -W, W-, and W-W waveband paths from node i to node j on waveband b). 3) Variables for routing of the waveband paths over physical topology: Pmn, ij,b PWmn ij,b, PWmn, ij,b and PWWmn ij,b (number of -, -W, W-, and W-W waveband paths from node i to node j that traverses fiber link (m, n) on waveband b). 4) Variables for routing connections over waveband-path virtual topologies: ij,y, W ij,y, W ij,y, and W Wij,y (binary variables, whether the tth OC-y connection request from source s to destination d is routed through a -, -W, W-, or W-W waveband path from node i to node j). Our objective is to minimize the network cost: i b (Db i + Ab i + W i b + Wi b)+α i b (W Wi b + W Wi b)+α i (WA i + WD i ), where α is the ratio of the cost of an port (including associated transponders) to the cost of an all-optical waveband port. The optimization is subject to the following constraints: 1) On node configuration and waveband-path virtual topology variables: j Vb ij + j VWb ij Ab i i Vb ij + i VWb ij Db j j VWb ij + j VWWb ij Wb i i VWb ij + i VWWb ij W i b, i, j N, b. They ensure that the number of waveband paths originating/terminating at a node is bound by the ports on the waveband. 2) On routing of virtual topology over physical topology: m Pij,b m PWij,b mk m m PWij,b PWWij,b mk mk = n Pij,b kn = n PWij,b kn mk = n PWij,b kn = n PWWij,b kn, k i, j, i, j, k N, b. They ensure that, for an intermediate node k on waveband path (i, j) on waveband b, the number of incoming waveband paths is equal to the number of outgoing waveband paths. n Pij,b in n PWij,b in n PWij,b in n PWWij,b = Vb ij = VWb ij = VWb ij in = VWWb ij, i, j N, b. They ensure that, for the originating node i of waveband path (i, j) on waveband b, the total number of outgoing waveband paths destined for node j is equal to the total number of waveband paths between node pair (i, j) on waveband b. m Pij,b mj m PWij,b mj m PWij,b mj = Vb ij = VWb ij = VWb ij GLOECOM /03/$ IEEE

4 m PWWij,b mj = VWWb ij, i, j N, b. They ensure that, for the terminating node j of waveband path (i, j) on waveband b, the total number of incoming waveband paths originated from node i is equal to the total number of waveband paths between node pair (i, j) on waveband b. ij (Pij,b mn + PWmn ij,b + PW ij,b mn + PWW ij,b mn ) mn, m, n N, b. This constraint ensures that a waveband b on any fiber can be used for at most one waveband path. 3) Routing of connections over the virtual topology: W sk,y + i W Wik,y = W kd,y + j k s, d, s, d, k N, y {1, 3, 12, 48, 192}, t [1, Λ y,sd ]. This constraint ensures that incoming multi-hopping connections arriving in a -W or W-W waveband path always leave in a W- or W-W waveband path. sd,y + W sd,y + j + W (Wsj,y (W W W kj,y, + W Wsj,y )=1 + W Wid,y )=1, sd,y sd,y + i id,y s, d N, y {1, 3, 12, 48, 192}, t [1, Λ y,sd ]. These two constraints ensure that every connection request is successfully routed. (y ij,y ) W C b Vb ij (y W ij,y ) W C b VWb ij (y W ij,y ) W C b VWb ij (y W Wij,y ) W C b VWWb ij, i, j N. These constraints ensure that the total amount of load routed on all the waveband paths from node (i, j) does not exceed these wavebands total capacity. The conventional routing and wavelength assignment (RWA) problem is known to be NP-complete. In this problem, if it is assumed that only one type, namely -, of waveband path is allowed, and that all the connection requests are of an entire waveband s capacity, then the problem reduces to the standard RWA problem. Therefore, the optimization of a hybrid ed network is at least NP-complete.. Illustrative MILP Results and Discussion Due to the computational complexity, the optimization was performed on a six-node network operating with 12 or 16 wavelengths at OC-192. We choose the connection granularities to be OC-3, 12, 48, and 192. The connection requests are uniformly distributed. Total connection requests are equivalent to 800 Gbps, and are distribute evenly among the four granularities. igure 4 presents the cost and alloptical waveband cost with different configurations. We vary the cost ratio α to be 0.5, 1, and 1.5 to capture the impact of cost variation. It is observed that, when the waveband granularity is finer, the optical-ing cost increases and the -ing cost decreases. When the total number of wavelengths per fiber increases (from 12 to 16), the overall network cost decreases. This can be explained by the more frequent use of waveband ports when there are more wavelengths in the network. On the other hand, the network cost of the full- network increases linearly with the total number of wavelengths per fiber. This demonstrates network cost full Wf =12 full Wf =16 =3 W =4 =4 W =3 0 5 cost waveband cost α=0.5 α=1.5 =6 W = α=1.0 =4 W =4 ig. 4. MILP optimization results. the excellent scalability of the hybrid architecture. It is also observed that when α =0.5, the full network and the hybrid network has approximately similar overall cost. When α reaches 1 and 1.5, there is significant cost reduction in the hybrid network, since the portion of the overall cost is much smaller. IV. HEURISTIC APPROACH This section investigates heuristic approaches that address following issues: Sharing of Waveband Paths Among Connections. There are four types of sharing among connections: 1) connections with same source and same destination, 2) connections with only same source, 3) connections with only same destination, and 4) connections sharing only one or more physical links. They can be grouped into -, -W, W-, and W-W waveband paths, respectively (we refer to this as waveband sharability constraint). Setting Up New Waveband Paths. When a connection is routed, it faces a topology combining the actual physical topology and the virtual waveband path topology. We should use existing waveband paths as much as possible to reduce cost. Hence, the routing is computed over a collapsed topology with modified link costs favoring existing waveband-path links. Waveband Assignment. When the route of a waveband path is chosen, a waveband is assigned to the waveband path. This is similar to the wavelength-assignment problem in a wavelength-routed network. Here we adopt the first-fit waveband-assignment. When searching for available wavebands, the algorithm starts from the lowest index and choose the first available waveband. Since - waveband paths offer the most savings, node pairs with enough connections for an entire waveband are considered first for - paths. After establishing the fullyfilled - waveband paths, we now decide which node pair (with unrouted connections) to route next. or nodes that are far apart (in hop distance), a waveband path along their route bypasses more es and reduces more cost. However, if such a long waveband path is under-utilized, the unused capacity is less likely to be used by other connections. or nodes that are closely located to each other, a waveband path GLOECOM /03/$ IEEE

5 between them is more likely to be used by other connections. We evaluate the following four approaches for selecting the next source-destination pair to route: : select node pair with the shortest hop route; if tie, select node pair with the lightest load. : select node pair with the shortest hop route; if tie, select node pair with the highest load. : select node pair with the longest route; if tie, select node pair with the lightest load. : select node pair with the longest route; if tie, select node pair with the highest load. Next, abiding the waveband sharability constraint, one needs to decide what type of waveband path is to set up. When all four types of waveband paths are applicable, we choose the type of new waveband path according to two load thresholds: TH and TH W W. When the offered load is larger than TH, a - path is set up. When the load is between TH and TH W W, a -W or W- (randomly selected) path is set up. If the load is less than TH W W,a W-W path is set up. elow is the complete algorithm: 1. Routing initial full - paths. Search all node pairs; find those with enough connections to fill one or more entire waveband paths. Set up - waveband paths for these connections using adap-tive routing. Update network state. 2. Routing residual connections. Repeat follow-ing steps until all connections are routed. 2.1) or all node pairs with residual connections, choose the one to route next according to the selection scheme (,, or ). 2.2) Construct a collapsed topology based on physical topology and four waveband virtual topologies (corresponding to -, - W, W-, and W-W paths). The links in virtual topologies are assigned much lower costs than in the physical topology to maximize sharing with existing waveband paths. 2.3) Route residual connections of the selected node pair over the collapsed topology. If a new waveband path is necessary, select the type of waveband path according to the load thresholds TH and TH W W. 2.4) Update network state. 3. Compute each node s port configuration and the overall network cost. We first apply the heuristics to the same 6-node network used for MILP study to evaluate performance of the algorithm. A similar set of waveband/wavelength configurations are used, and the cost ratio α is set to 1. The waveband-path type selection thresholds TH and TH W W are chosen to be as low as possible while all the connections are successfully routed. Thus, the maximum -cost reduction is always attempted. igure 5 shows the comparison results. Overall, the heuristics performance is comparable with that of MILP. In one case ( with { =6,W =2}), the heuristic fails to route all the connections. or a more realistic scenario, we apply the heuristics to a carrier s network (ig. 6). The total amount of uniform traffic is 2 Tbps. We plot the network cost ratio of hybrid over full- against α, which is varied between 0.1 and 10. igure 7 shows the results of one scheme (), with different and W combinations. We have obtained similar network cost 3 network cost ratio: hybrid over =4, W =3 MILP =6, W =2 unsuccessful MILP cost all-optical cost =4, W =4 MILP ig. 5. Comparison of heuristics and MILP optimization (α =1) ig A carrier s network topology used in this study α ig =8 W =2 =6 W =3 =9 W =2 =4 W =5 =5 W =4 =6 W =4 =8 W =3 Network cost with different cost ratio GLOECOM /03/$ IEEE

6 network cost ratio: hybrid over =6 W =4 =9 W = α ig. 8. Comparison of four node-pair selection schemes. results from all four schemes. However, failed to route all connections with { =8,W =2} and { =6,W = 3}, and failed with { =8,W =2}, { =6,W = 3} and { =4,W =5}. Due to limited space, only is shown here. We observe that, with α between 0.5 and 1, the hybrid network s cost is comparable to or less than the network s cost. As α continues to increase (shaded area), the savings from hybrid network becomes more prominent. When α is equal to 2, all the schemes can achieve 40%-64% cost reduction. We remark that, as the line speed increases beyond OC-768, an port s cost will likely surpass that of an alloptical port, therefore the shaded quadrant is likely to represent the -cost savings in hybrid optical networks. It is also observed that hybrid networks perform better when there are more wavelengths in the system. Under such conditions, a full- network has a large amount of its -ing capability under-utilized. In a hybrid network, the cost is reduced at the price of less efficient usage of the wavelengths. On the other hand, with same number of wavelengths per fiber, finer waveband granularities perform better with α greater than 1. or example, { = 9,W = 2} offers lower network cost than { = 6,W = 3}. Since the network contains more waveband paths, more sharing can occur. However, finer waveband granularity means more wavebands; thus, more alloptical waveband ports are needed. As a result, when α is less than 1, finer waveband granularity induces higher overall network cost. igure 8 compares the four node-pair selection schemes. yields the lowest network cost. or the topology studied, satisfying first long connections can further reduce the cost. However, it requires more wavelengths to successfully route all the connections. In terms of successful routing, and perform better than and (corresponding results not shown here due to space limitation). These results imply that selecting shorter connections to route first increases sharing between connections, and improves the routing of longer connections at a later time. V. CONCLUSION We proposed a hybrid architecture with all-optical waveband ing and electrical ing. This architecture is shown to have excellent scalability. An MILP formulation was presented for the network-design problem. We also proposed heuristic algorithms that produce near-optimal results. The results indicate that, when an all-optical port s price approaches the price of an port and its transponders, significant network cost reduction can be achieved with the hybrid architecture. REERENCES [1] K. Harada, K. Shimizu, T. Kudou, and T. Ozeki, Hierarchical optical path cross-connect systems for large scale WDM networks, Proc. OC 99, vol. 2, pp , [2] O. Gerstel, R. Ramaswami, and W. K. Wang, Making use of a twostage multiplexing scheme in a WDM network, Proc. OC 00, vol. 3, pp , [3] O. Gerstel, R. Ramaswami, and S. oster, Merits of hybrid optical networking, Proc. OC 02, pp , [4] R. Lingampalli and P. Vengalam, Effect of wavelength and waveband grooming on all-optical networks with single layer photonic ing, Proc. OC 02, pp , [5] M. Lee, J. Yu, Y. Kim, C. Kang, and J. Park, Design of hierarchical crossconnect WDM networks employing a two-stage multiplexing scheme of waveband and wavelength, IEEE Journal on Selected Areas in Communications, vol. 20, no. 1, pp , Jan [6] G. Huiban, S. Perennes, and M. Syska, Traffic grooming in WDM networks with multi-layer es, Proc. IEEE International Conference on Communications (ICC 02), vol. 5, pp , [7] Y. Suemura, I. Nishioka, Y. Maeno, S. Araki, R. Izmailov, and S. Ganguly, Hierarchical routing in layered ring and mesh optical networks, Proc. IEEE International Conference on Communications (ICC 02), vol. 5, pp , [8] X. Cao, V. Anand, Y. Xiong, and C. Qiao, Wavelength band ing in multi-granular all-optical networks, SPIE, Optical Networking and Communications Conference (Opticomm) 2002, oston Massachusetts, vol. 4874, pp [9], Performance evaluation of wavelength band ing in multifiber all-optical networks, Proc. IEEE INOCOM 03. GLOECOM /03/$ IEEE