756 IEEE COMMUNICATIONS LETTERS, VOL. 9, NO. 8, AUGUST 2005 Dynamically Survivable WDM Network Design with p-Cycle-Based PWCE Zhenrong Zhang, Wen-De Zhong, Senior Member, IEEE, and Sanjay Kumar Bose, Senior Member, IEEE Abstract— The concept of a protected working capacity en- velope (PWCE) is very attractive for designing a dynamically survivable WDM network. We consider a p-cycle-based PWCE approach for designing survivable WDM networks where the traffic demand is dynamically changing. We develop an integer linear programming formulation to determine the protected working capacity envelope and use simulations to evaluate the blocking performance for different network models. Index Terms— ILP, p-cycle, optical networks, PWCE. I. I NTRODUCTION T HE method of preconfigured protection cycle, known as p-cycles [1], [2], can provide ring-like fast protection speed and mesh-like high efficiency of spare capacity for sur- vivable WDM optical network. Integer Linear Programming (ILP) methods have been used earlier to configure p-cycles for WDM networks where the traffic demand is known a priori, i.e. static as in [1], [2]. In order to solve the p-cycle design problem in a simpler fashion, heuristic algorithms such as the ER-based unity-p-cycle design algorithm in [3] or capacitated iterative design algorithm (CIDA) in [4] have been proposed recently. The heuristic algorithm of [3], [4] can tackle both static and dynamic traffic demands as in [5]. The concept of protected working capacity envelope (PWCE), introduced by Grover in [6], [7], is attractive because it does not need protection resources to be dynamically config- ured thereby simplifying network management and reducing processing overheads. For a given network topology with specified edge capacities, it first assigns some spare capacity to each edge to create an “envelope” of protected working capacities. As a result, any dynamically arriving lightpath demand routed within this PWCE is automatically protected. This eliminates the process of finding protection paths for dynamically arriving traffic demands. Once the PWCE is determined for a given network, the issue remaining is to simply route dynamically arriving traffic demands within the already determined PWCE in the network. In this paper, by incorporating the advantages offered by p-cycles, we consider a p-cycle-based PWCE approach [8]–[11] to design dynami- cally survivable WDM networks and present a modified ILP formulation for solving this design problem. Our simulations Manuscript received January 10, 2005. The associate editor coordinating the review of this letter and approving it for publication was Prof. Michael Devetsikiotis. Z. R. Zhang was with the School of EEE, Nanyang Technological Univer- sity, Singapore, and is currently with Guangxi University, China. W. D. Zhong and S. K. Bose are with the School of EEE, Nanyang Technological University, Singapore (e-mail: ewdzhong@ntu.edu.sg). Digital Object Identifier 10.1109/LCOMM.2005.08024. show that the p-cycle-based PWCE is considerably more effec- tive than other existing protection methods for such networks. The significance in this work is in quickly reporting a first independent confirmation of the predictions on blocking made in [6] and corroboration of the main findings on performance published in [9]. II. p- CYCLE- BASED PWCE For a p-cycle-based PWCE, spare capacities are pre- allocated to each edge to configure p-cycles so that an “enve- lope” of protected working capacities can be created with pre- determined p-cycles. A dynamically arriving traffic demand may be routed in this pre-determined protected working ca- pacity envelope using any efficient routing algorithm that one prefers to use. A dynamically survivable WDM network is then designed with the following two steps: 1) Allocate spare capacities to configure p-cycles; PWCE can then be created with those pre-configured p-cycles. 2) Route dynamic traffic demand in the pre-created PWCE using any efficient routing algorithm (e.g. shortest path). Fig. 1. (a) one p-cycle with 4 spare capacities, (b) the corresponding protected working capacity envelope. Fig. 1 shows a simple example where a p-cycle (1-2-3-4- 5-1) is configured with 4 spare units on each on-cycle edge so as to provide a protected working capacity envelope. This p-cycle protects 4 working units on an on-cycle edge and 8 working units on any straddling edge. As shown in Fig. 1(b), this creates a PWCE with 4 protected working units on each of edges 1-2, 2-3, 3-4, 4-5 and 8 working units on each of edges 2-5, 3-5 and 1-3. III. ILP FORMULATION FOR p- CYCLE- BASED PWCE DESIGN As in [2], we model a WDM network as a directed graph G(V,E) with V as the set of nodes and E as the set of directed edges. A directed edge transmits data in one direction. Any edge in the network may consist of two counter directional edges whose capacity may not be equal. We define a unit 1089-7798/05$20.00 c  2005 IEEE