Maher Ali

Cost-effective implementation of multicasting in wavelength-routed networks

Multicasting in the optical domain has been recently shown to provide substantial savings in terms of the network-wide average packet hop distance and the total number of transceivers in the network. Current proposed multicasting architectures [e.g., splitter-and-delivery (SaD)] employ power splitting mechanisms which have the side effect of high fabrication cost due to the large number of splitters and the need for optical amplifiers. We propose a low-cost novel architecture called tap-and-continue (TaC) for realizing multicasting. This architecture provides a natural evolution from current unicast cross-connects and is based on tapping devices. We prove that any multicasting session can be feasibly realized in networks employing only TaC cross-connects, and the problem of finding the optimal multiple-destination minimum cost trail in such networks is NP-complete. Therefore, we develop a 4-approximation algorithm for multiple-destination routing. Simulation results demonstrate that the TaC cross-connect provides a realistic, cost-effective approach for implementing multicasting with negligible blocking degradation especially in multifiber networks.

Power-efficient design of multicast wavelength-routed networks

In this paper, we introduce the power-efficient design space for multicast wavelength-routed networks. The power-efficient design space is based on the impact of power on the overall design of wavelength-routed networks. Two cross-connect architectures on this design concept are investigated. One is an existing architecture called splitter-and-delivery (SaD). The other is a new architecture called multicast-only splitter-and-delivery (MOSaD). The MOSaD architecture uses power splitters for multicast connections only, allowing unicast connections to pass without enduring unnecessary power losses. Our cross-connect design provides a strictly nonblocking service for unicast connections while eliminating unnecessary power loss of the SaD cross-connect. Experimental results demonstrate that the MOSaD architecture provides substantial savings in cost and reduction in signal power loss with minimal effects on the blocking performance of the network.

Routing and wavelength assignment with power considerations in optical networks

Abstract Previous studies have solved many variations of the routing and wavelength assignment (RWA) problem in optical networks under the assumption of perfect conditions regarding the power of a signal. In this paper, we investigate the RWA problem while allowing for degradation of routed signals by optical components. The problem is formulated as a mixed-integer non-linear program. We propose a two-phase approach. In the first phase, we solve the pure RWA problem using fixed routes for every connection. In the second phase, power assignment is accomplished by either using a heuristic or using a genetic algorithm. This paper demonstrates that: (i) connections do interact affecting the power of each other, and (ii) global search meta-heuristics provide better solutions.

Allocation of Splitting Nodes in All-Optical Wavelength-Routed Networks

In this paper, we introduce the splitter placement problem in wavelength-routed networks (SP-WRN). Given a network topology, a set of multicast sessions, and a fixed number of multicast-capable cross-connects, the SP-WRN problem entails the placement of the multicast-capable cross-connects so that the blocking probability is minimized. The SP-WRN problem is NP-complete as it includes as a subproblem the routing and wavelength assignment problem which is NP-complete. To gain a deeper insight into the computational complexity of the SP-WRN problem, we define a graph-theoretic version of the splitter placement problem (SPG), and show that even SPG is NP-complete. We develop three heuristics for the SP-WRN problem with different degrees of trade-off between computation time and quality of solution. The first heuristic uses the CPLEX general solver to solve an integer-linear program (ILP) of the problem. The second heuristic is based on a greedy approach and is called most-saturated node first (MSNF). The third heuristic employs simulated annealing (SA) with route-coordination. Through numerical examples on a wide variety of network topologies we demonstrate that: (1) no more than 50% of the cross-connects need to be multicast-capable, (2) the proposed SA heuristic provides fast near-optimal solutions, and (3) it is not practical to use general solvers such as CPLEX for solving the SP-WRN problem.

Route optimization of multicast sessions in sparse light-splitting optical networks

In this paper, we investigate the multicast routing problem in sparse splitting networks (MR-SSN). The MR-SSN problem is to find a route from the source node of a session to all destinations of the session such that the total number of fibers used in establishing the session is minimized while the multicast capable nodes are evenly distributed throughout the network. We present a heuristic based on Tabu search that requires only one transmitter for the source node and one wavelength for a multicast session in this paper. We test our heuristic on a wide range of network topologies and random sessions and conclude that the difference between our solution and ILP optimal solution in terms of the number of fibers used for establishing a multicast session is within 10% nearly all the time and within 5% in about half of the time.

Routing algorithms for all-optical networks with power considerations: the unicast case

In this paper, we investigate the problem of routing connections in ail-optical networks while allowing for degradation of routed signals by different optical components. To overcome the complexity of the problem, we divide it into two parts. First, we solve the pure RWA problem using fixed routes for every connection. Second, power assignment is accomplished by either using the smallest-gain first (SGF) heuristic or using a genetic algorithm. Numerical examples on a wide variety of networks show that: (a) the number of connections established without considering the signal attenuation was most of the time greater than that achievable considering attenuation; and (b) the genetic solution quality was much better than that of SGF, especially when the conflict graph of the connections generated by the linear solver is denser.

Allocation of multicast nodes in wavelength-routed networks

We investigate the allocation of multicast nodes and formalize it as splitter placement in wavelength-routed networks (SP-WRN) problem. The SP-WRN problem entails the placement of multicast nodes so that the overall network blocking probability is minimized. To gain a deeper insight into the computational complexity of the SP-WRN problem, we define a graph-theoretic version of the splitter placement problem (SPG), and show that even SPG is NP-complete. We develop three heuristics for the SP-WRN problem with different degrees of trade-off between computation time an quality of solution. The first heuristic uses CPLEX, the second heuristic is based on a greedy approach, and the third heuristic employs simulated annealing. Numerical examples demonstrate that: (i) no more than 50% of the cross-connects need to be multi-case capable, and (ii) the iterative simulated annealing heuristic provides fast near-optimal solutions.

Routing and wavelength assignment (RWA) with power considerations in all-optical wavelength-routed networks

Routing and wavelength assignment (RWA) is an important problem that arises in wavelength division multiplexed (WDM) optical networks. Previous studies have solved many variations of this problem under the assumption of perfect conditions regarding the power of a signal. We investigate this problem while allowing for degradation of routed signals by components such as taps, multiplexers, and fiber links. We assume that optical amplifiers are preplaced. We investigate the problem of routing the maximum number of connections while maintaining proper power levels. The problem is formulated as a mixed-integer nonlinear program and two-phase hybrid solution approaches employing two different heuristics are developed.

Assignment of multicast switches in optical networks

This paper addresses the optimal placement of multicast nodes in all-optical networks. An approximate analytical model for the blocking probability in multicast networks is developed. The blocking model is used to guide two heuristics for the placement problem. The two heuristics are: (1) the famous warehouse allocation heuristic (ADD) and (2) simulated annealing.

Preplanned recovery with redundant multicast trees in optical networks

In this paper, we investigate the problem of Preplanned Recovery with Redundant Multicast Trees (PRRMT) in optical networks. The redundant trees ensure the source node remains connected to all destination nodes for a multicast session request under single edge failures. Our objective is to minimize the total number of links used for both trees. We formulate PRRMT as an integer linear program (ILP), and also develop a heuristic algorithm. The ILP approach and heuristic algorithm are experimentally evaluated on 14-node NSFNET and 21-node Italian network. Experimental results show that: (1) ILP approach leads to optimal solutions but requires prohibitively long time, (2) Our heuristic algorithm yields optimal or near-optimal results in very short time, and (3) The edge-disjoint trees can protect the transmission for an edge failure.