Wavelength-division-multiplexing (WDM) networks have emerged as a method of providing Terabits-per-second capacity for ever-increasing bandwidth demands.
Such a network is composed of optical cross-connects (OXCs) interconnected by fiber links, with each fiber supporting tens to hundreds of wavelength channels. End users in the network communicate with each other via one or several all-optical channels, i.e., lightpaths, with transmission rate ranging from one to tens of Gigabits per second.
Although increase in number of wavelength channels and fibers between node pairs may increase the available capacity, this may cause a scalability problem in maintenance and manufacturing of the optical cross-connects (OXCs). An effective way of handling this problem is to bundle a group of consecutive wavelength channels together and switch them as a single unit on a specific route to reduce the required resources of intermediate cross-connects along the route. The tunnel-like passage created by the bundled wavelength channels is defined as a waveband/fiber tunnel. Wavelengths in a tunnel must be switched together except at the two ends of the tunnel. Nodes that support such multigranularity switching, e.g. wavelength, waveband and fiber-switching, are termed hierarchical cross-connects or multigranularity optical cross-connects (MG-OXCs). This thesis examines the tunnel allocation and protection problems related to the networks that using the node architecture, MG-OXC.
1.1 Multi-Granularity Optical cross-Connects (MG-OXC)
The network is based on the node architecture [1] shown in Fig. 1. A MG-OXC mainly comprises fiber-, waveband-, and wavelength-switching boxes and waveband
and wavelength multiplexer/de-multiplexers. The fiber- and waveband-switching boxes on the left-hand side serve as selectors on the input fibers and wavebands while the fiber- and waveband-switching boxes on the right-hand side serve as OXCs that switch fibers and wavebands. In MG-OXC networks, a tunnel is defined as a group of consecutive wavelength channels that are bundled and switched together as a single unit, which could be either a fiber or waveband tunnel depending on the size of the grouped wavelengths. All of the channels in a waveband or fiber tunnel must be switched together. A tunnel is said to be allocated if link capacity along the route of the tunnel is dedicated to that tunnel. For an allocated tunnel to be used by lightpaths, a sufficient number of wavelength-switching ports at the ingress and the egress of the tunnel have to be further dedicated to that tunnel so that lightpaths can be grouped or de-grouped at both ends. The number of wavelength-switching ports dedicated to the tunnel at the two ends of the tunnel is equal to the number of the wavelengths that the tunnel carries. We say that a tunnel is brought up if wavelength-switching port at the both end are dedicated to the allocated tunnel. Wavelength-switching ports at the two ends of the tunnel can be freed when there is no lightpath traversing it. The advantage of using MG-OXC is the cost reduction in the size of switch fabric. Fig.2 illustrates an example of the saving.
Fig. 1. Architecture of an MG-OXC
Fig. 2. Advantage of MG-OXC
In this thesis, we make the following assumptions. We assume that each directional link between two nodes consists of F fibers in which F1, F2, and F3 fibers are assigned as fiber-, waveband-, and wavelength-switching fibers respectively (i.e.
F = F1 + F2 + F3). Accordingly, the number of ports of a node is dependent on its node degree. That is, for example, for a node with node degree , there are F
i Δi
3⋅ ⋅|W| wavelength-switching ports for that node, where W is the set of wavelengths in a fiber. We also assume that each node is equipped with sufficient wavelength conversion capability in the wavelength-switching layer. Therefore, a lightpath in the wavelength-switching layer can be converted into any other wavelength if necessary. However, waveband conversion is not assumed, and therefore waveband continuity still has to be maintained. We also assume that a tunnel can only traverse on the shortest path from its ingress node to its egress node.
Δi
1.2 Tunnel Allocation and Protection Problem
Although applying MG-OXC can reduce network costs, some problem also arise.
Tunnels complicate the routing and wavelength assignment (RWA) problem and should be allocated carefully to achieve higher network performance. Additionally, the protection problem in MG-OXC networks should also be examined, since it has
not been intensively studied. This work investigates problems related to MG-OXC networks, including the tunnel allocation problem and the protection problem. The remainder of this thesis is organized as follows.
In chapter 2, we consider the problem of routing and wavelength assignment (RWA) with tunnel allocation in MG-OXC networks. Given a set of static lightpath requests, our problem is to (a) allocate a set of fixed-length tunnels (b) find routes from the source nodes to their respective destination nodes, and (c) assign wavelengths to theses route. The objective here is to minimize the blocking probability. Furthermore, we extend our work for the dynamic RWA problem. Given a historical traffic matrix, the dynamic RWA problem is how to build a set of tunnels off-line to accommodate the future dynamic lightpath requests in such a way that the blocking probability can be minimized. In order to utilize the wavelength ports and fibers efficiently, each tunnel established should follow a tunnel length constraint which could be equal to the average network hop distance. Based on this criterion, a novel graph model is proposed [7] in which edges are added only for the node pairs whose hop distance follow the tunnel length constraint to form an auxiliary graph.
We further analyze the impact of tunnel length on blocking rate based on the hypothesis of fixed tunnel length constraint in chapter 3. This hypothesis occurred originally in [1], but the authors didn’t explain its motivation clearly. A blocking probability model used Erlang loss formula is provided to estimate the performance of tunnel allocation with different tunnel length constraint.
In chapter 4, an efficient fault-recovery protection scheme for the lightpaths was proposed. A segment-based protection scheme, called Tunnel Based Segment Protection (TSP) is proposed to recover the communications interrupted by a fiber cut.
Another scheme directly perceived through the senses, called Tunnel based Path
Protection (TPP) is also proposed for comparison. Finally, after the tunnels have been allocated by TSP or TPP, we issue another ILP formulation for the static RWA problem.
Chapter 5 concludes the results of our works and suggests some possible future works.