Finding Protection Cycles in DWDM Networks 2002 IEEE ICC on Volume 5, 28 April-2 May Page(s): Reporter: Jyun-Yong Du

Outline Introduction P-cycle protection problem Classical ring finding methods A new protection cycle finding algorithm Performance analyses Conclusions

Introduction(1/3) Optical networks based on single physical ring are favored for their fast restoration (usually within 50ms). The down side of single-ring-based networks is that they are bandwidth inefficient, since half of the total bandwidth has to be reserved for protection purpose. Also self-healing ring technique is usually used in metro networks, it is impractical to cover all the switches in a long-haul network with a single ring.

Introduction(2/3) On the other hand, mesh networks have a much higher link diversity which implies a lower bandwidth redundancy. It is generally hard to find a protection path upon link failure and finish all the required optical cross-connects within 50ms before protocols in upper layers timeout. As a compromised yet reasonable solution in practice, multiple logical rings can be chosen to cover the whole physical mesh networks; each of them behaves like a single physical self-healing ring by itself.

Introduction(3/3) The approach of protecting mesh network with logical rings can be further categorized as follows: 1) Pure ring cover (intra-ring and inter-ring) 2) Protection cycle (P-cycle) cover This paper is aimed at introducing mesh optical network logical ring cover problem in general, then proposing a new and efficient ring/cycle-finding algorithm in detail.

P-cycle protection problem(1/2) The problem of protecting mesh networks by multiple ring/p-cycles, under the constraint of a limited number of wavelengths on each physical fiber link, is an NP- complete problem. Hence, the authors propose to divide the whole problem into 2 independent sub-problems to reduce the complexity.  Sub-problem #1: Ring finding.  Sub-problem #2: Bandwidth planning.

P-cycle protection problem(2/2) Sub-problem #2 is usually based on linear programming (LP). There are various LP formulations for this sub-problem, based on their respective cost models. Each bandwidth planning formulation addresses it’s own specific objective function. This paper mainly focuses on sub-problem #1, i.e., to find a scalable, state-of-art ring/cycle finding algorithm.

Classical ring finding methods(1/2) There are various ring finding algorithms, some of them are classical methods that can select all the rings exhaustively from a network (graph). The classical algorithms belong to one of the following four classes: 1) Circuit vector space [6,8,9] 2) Backtracking algorithms [6,7,10] 3) Powers of adjacency matrix [6] 4) Edge-digraph [6]

Classical ring finding methods(2/2) In addition to classical methods, which are exhaustive, there is another category of ring-finding methods based on heuristics, such as: 1) Ring node routing [12] 2) Incapacitated ring cover [13] 3) Node disjoint routing [11] The heuristics can run faster than classical methods and yield good node/link coverage in most cases.

A new protection cycle finding algorithm(1/10) It has been proved theoretically [15,16,17] that in network protection, p-cycle topology is more efficient than any other topology patterns, such as trees and line segments, in the sense that p-cycle provides more protection paths per unit spare capacity consumed. As the authors have found from classical ring-finding algorithms introduced before, the number of rings/cycles in a graph/network increases exponentially, as the number of network nodes and links increases.

A new protection cycle finding algorithm(2/10) Here, the authors propose a scalable algorithm based on the idea of cycle straddling link. Such a new idea is based on the following observation: a p-cycle could be looked on as the combination of two node-disjoint paths between the two end nodes of a cycle-straddling link.

A new protection cycle finding algorithm(3/10) Given a general mesh network G=G(V,E) Before finding a set of p-cycle covering the network links, we can always first prune off those nodes of nodal degree 1 and the links adjacent to them, because no cycle could cover such nodes. Upon looking at the remaining links for p-cycle finding purposes, we can identify the following cases:

A new protection cycle finding algorithm(4/10) 1) Case of node degrees>=3 for both end nodes: there are 3 sub-cases

A new protection cycle finding algorithm(5/10) 2) Case of the node degree of at least one end node=2: there are 2 sub-cases

A new protection cycle finding algorithm(6/10)

A new protection cycle finding algorithm(7/10)

A new protection cycle finding algorithm(8/10)

A new protection cycle finding algorithm(9/10)

A new protection cycle finding algorithm(10/10)

Performance analyses

Conclusions To solve the scalability problem in sub-problem #1 (ring finding), the authors proposed the straddling link ring finding algorithm The algorithm has the feature that the number of p- cycles found does not exceed the number of links in the network. At the same time the resultant p-cycle set covers most network links which has been confirmed by the simulation results.