M.H. MacGregor

Optimal capacity placement for path restoration in STM or ATM mesh-survivable networks

The total transmission capacity required by a transport network to satisfy demand and protect it from failures contributes significantly to its cost, especially in long-haul networks. Previously, the spare capacity of a network with a given set of working span sizes has been optimized to facilitate span restoration. Path restorable networks can, however, be even more efficient by defining the restoration problem from an end to end rerouting viewpoint. We provide a method for capacity optimization of path restorable networks which is applicable to both synchronous transfer mode (STM) and asynchronous transfer mode (ATM) virtual path (VP)-based restoration. Lower bounds on spare capacity requirements in span and path restorable networks are first compared, followed by an integer program formulation based on flow constraints which solves the spare and/or working capacity placement problem in either span or path restorable networks. The benefits of path and span restoration, and of jointly optimizing working path routing and spare capacity placement, are then analyzed.

Optimal capacity placement for path restoration in mesh survivable networks

The total capacity required by a transport network to satisfy demand and protect it from failures contributes significantly to its cost. Previously the spare capacity of a network with a given set of working span sizes has been optimized to facilitate span restoration. Path restorable networks can, however, be even more efficient by defining the restoration problem from an end to end re-routing viewpoint. We now provide a method for capacity optimization of path restorable networks and quantify the capacity benefits of path over span restoration. The further benefits of jointly optimizing working path routing and spare capacity placement in path restorable networks is also quantified by extending the capacity optimization method presented.

Optimized k -shortest-paths algorithm for facility restoration

The problem of finding shortest paths arises in many contexts; testing restoration algorithms and developing design packages for large telecommunications networks are two cases where the simple task of finding sets of restoration paths can consume up to 95 per cent of the execution time of an application program. This paper presents experimental studies of several well‐known shortest‐paths algorithms adapted to the task of finding the k‐successively‐shortest link‐disjoint replacement paths for restoration in a telecommunications network with n nodes. The implementations range in complexity from O(kn2) when based on Dijkstras original method, through several improvements to an efficient implementation of O(kn[v+longn]) complexity, and finally to an O(kn) implementation for the special case of edge‐sparse graphs with small integer edge weights. Here v is the maximum degree of a node in the network. Several alternatives were tested during the course of these studies, particularly with a view to minimizing the number of heap updates, These alternatives are possible because we are searching for several paths between a given pair of nodes, rather than just one path between one or more pairs of nodes. Two fairly straightforward changes yield a decrease in execution time, whereas a more complex heap management strategy consumes as much time in the added code as it releases from the main routine. Experimental results confirm the theoretical complexity of q k n log n) and demonstrate a speed‐up of nearly an order of magnitude over the simpler O(kn2) implementation in the largest networks tested. The optimized implementation is recommended for planning and operational applications of k‐shortest paths rerouting for telecommunications network restoration and restorable network design. If hop counts or small integer link weights can be used to measure distances, then the qkn) implementation is recommended, as typical telecommunications networks are edge‐sparse.

Two strategies for spare capacity placement in mesh restorable networks

Because the problem of optimal spare capacity placement in a mesh-restorable network is NP-hard, the authors consider two heuristic strategies to solve the spare capacity placement problem in a near-optimal way within reasonable time constraints. The spare link placement algorithm (SLPA) is based on the principle of iterative link addition to produce the greatest incremental change in network restorability. SLPA is shown theoretically and experimentally to have polynomial time complexity. The iterated cutsets heuristic (ICH) formulates spare capacity placement as a linear programming problem subject to constraints based on a subset of cutsets of the network. Iteration and heuristic rules are used to develop the constraint set required by ICH. Theoretical time complexities are assessed for both SLPA and ICH. Comparative experimental tests on 36 trial networks are discussed. ICH network designs require slightly less redundant capacity. On average, SLPA placed 5% more spare capacity than ICH.<<ETX>>

Optimized design of ring-based survivable networks

We describe an original method for the automated synthesis of transport network designs based on multiple SONET self-healing rings. A heuristic procedure is based on insights about the trade-off between capacity efficiency and traffic capture efficiency of ring sets to find a minimum-cost composite design. Test cases show designs that range from nearly optimal capacity efficiency to nearly optimal traffic capture, depending on the relative costs of transmission capacity and costs for ring-to-ring transition interfaces. The proposed approach may be used directly to produce complete multi-ring designs, or in an automated “ring proposer” context to be integrated with existing ring-planning tools. Results from a real-world long-haul network design involving a total of 141 self-healing rings are presented.

The self-traffic-engineering network

Characterizes several techniques for the distributed, dynamic management of telecommunications network transmission facilities. Several methods are presented for changing trunk-group sizes in order to respond to extraordinary network loads. These methods are given the generic title of self-traffic-engineering (STE), because the network effectively reengineers itself to adapt to unforeseen circumstances. Self-traffic-engineering is a new network management application for telecommunication facilities networks. One of these methods is based on a new interpretation of network reliability, which is embodied in a metric called connectability. The paper reports the results of a simulation study in which self-traffic-engineering is used to enable the Telecom Canada network to adapt to a focused overload.

SACK TCP resilience

The selective-acknowledgement transmission control protocol (SACK TCP) is the most up-to-date version of TCP commonly used in the Internet. Understanding the resilience behaviour of SACK TCP is very important because congestion episodes and failure events are relatively frequent. In this paper, the descriptive power of Petri nets is leveraged to present the reactions that take place inside SACK TCP when network failures or congestion occur. An algebraic model is also presented that can be used to calculate the critical number of lost segments, nC, above which SACK TCP resilience is significantly degraded.

Optimal multicast in DWDM networks: minimizing wavelength conversion

The problem of assigning wavelengths and routing multicast sessions in DWDM networks has given rise to a host of heuristic and approximate techniques. In this paper we demonstrate that it is feasible to find optimal solutions for many instances of this problem using integer linear programming. The technique presented is also useful as an optimal standard to which heuristics can be compared in order to determine the quality of solutions they generate. The goal of this paper is to document an integer programming tableau for routing multicast sessions in DWDM networks, while accounting for a variety of limits on the abilities of the nodes in the network to drop, split and convert wavelengths. Examples of applying the tableau to problems of moderate size are included.

Blocking analysis and optimized design of a class of circuit-switched multicast networks

We present a blocking analysis and optimized design method for a wide-area multicast videoconferencing service which interconnects subscriber locations via circuit-switched DS-1 multicast control units (MCUs). Optimization leads to a configuration of MCUs in which the end-to-end blocking of random multicast traffic meets a target specification with a minimum total number of MCUs. The analytical model is validated by a detailed network emulation. The analysis and simulator are intended for use in a planning system to support operation of the planned multicast services.