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WAVELENGTH ROUTING IN OPTICAL NETWORKS

Abstract
Optical networks are high-capacity telecommunications networks based on optical technologies and component that provide routing, grooming, and restoration at the wavelength level as well as wavelength-based services. This paper deals with the twin concepts of wavelength routing and wavelength conversion in optical networks. The paper talks about the various categories of wavelength switches, routing algorithms, wavelength conversion and categories of wavelength conversion. Finally this paper deals with industry related issues like the gap between research and the industry, current and projected market for optical networking & DWDM equipment and future direction of research in this field.

1.Introduction
An optical network consists of wavelength routers and end nodes that are connected by links in pairs. The wavelength-routing switches or routing nodes are interconnected by optical fibers. Although each link can support many signals, it is required that the signals be of distinct wavelengths. Routers transmit signals on the same wavelength on which they are received. An All-Optical wavelength routed network is that wavelength-routed network that carries data across from one access station to another without any O/E (Optical/Electronic) conversions.

2. Categories of Wavelength Switches (or routers):
(i) Non-reconfigurable switch: These types of switches, for each input port and each wavelength, transmit onto a fixed set of output ports at the same wavelength. These cannot be changed once the switch is built. Networks that contain only such switches are called non-reconfigurable networks.

(ii) Wavelength-Independent Reconfigurable switch: These type of switches have input-output pattern that can be dynamically reconfigured. However, the input-output pattern is independent of the wavelength of the signal i.e. there are only fixed sets of output ports onto which an incoming signal can be transmitted.

(iii) Wavelength-Selective Reconfigurable Switch: These types of switches combine the features of the first two categories. Also known as generalized switch, they basically have both the properties of dynamic reconfiguration and the routing pattern being a function of the wavelength of the incoming signal.

Reconfigurable routers are of bounded degree, while nonreconfigurable routers may not be. That is, the complexity of non-reconfigurable networks can be ignored as it is not of a fixed degree. However, the complexity of reconfigurable networks is strongly dependent on its degree and it is bounded.

3. Efficient routing Algorithms

(i) Permutation routing problem:
Each end node in a permutation problem is the origin of atmost one session and also the destination of atmost one session at any given time. A new concept called the widesense nonblocking criterion has been introduced. This criterion effectively insures that at any instant of time, the session present in a network constitute a permutation problem and that no session is every blocked. A routing scheme is oblivious if it always uses the same wavelength to satisfy a given connection request; it is partially oblivious if the wavelength must be chosen from a subset of available wavelengths. Bounds on the number of wavelengths needed for oblivious,

nonoblivious, and partially oblivious wide-sense nonblocking permutation routing for nonreconfigurable networks were calculated. For reconfigurable networks, bounds are given on the number of routers needed, with the number of wavelengths as a parameter. Researchers focussed on the permutation routing problem in a homogeneous WDMA network, i.e a network having both an input/output port and a switch. A lower bound as well as an upper bound on the number of wavelengths that are necessary for permutation routing as a function of the size and the degree of the network was calculated. Topologies considered were the hypercube, Debruijn and the multistage perfect shuffle.

(ii) Lower bound :
By simply counting the number of links in the network , it was concluded that the number of wavelengths must grow at least as fast as W (log N/log d) where N is the number of nodes in the network and d is the degree of the network. "A session requires h link-wavelengths if it is routed on an h hop path since it uses one wavelength channel on each of the h links. The upper bound is O((log N)3) and is independent of the degree of the network. Research work has been done in solving the problem of routing connections in a reconfigurable optical network using WDM. An upper bound on the carried traffic of connections is derived for any routing and wavelength assignment(RWA) algorithm in such a network. A fixed-routing algorithm achieves this bound asymptotically. The RWA problem was formulated as an Integer Linear program (ILP). This bound was found to be good for optical network using dynamic wavelength convertors. Two routing node architectures were presented. In the first structure it was found that as the number of edges increased the reuse factor increased. Also the reuse factor with wavelength convertors was higher than that without one for small values of wavelength systems. Also it is assumed implicitly that in networks without wavelength convertors , two connections can be assigned the same wavelength as long as they don’t share any link in the network.

An important aspect was to find the reuse factor for larger networks as a function of the number of nodes, edges and wavelengths via simulation. Based on the results, it was inferred by the researchers that it is possible to build

all-optical networks without wavelength convertors. However, only a modest number of connections per node with a reasonable number of wavelengths is supported. Using 32 wavelengths it is possible to provide 10 full-duplex connections to each node in a 128-node random network with average degree 4, and 5 full-duplex connections per node in a 1000-node random network with average degree