Wavelength-Division Multiplexing (WDM)

Wavelength-division multiplexing (WDM)_wavelength-division multiplexing (WDM) is an approach that can exploit the huge opto-electronic bandwidth mismatch by requiring that each end-user's equipment operate only at electronic rate, but multiple WDM channels from different end-users may be multiplexed on the same fiber. Under WDM, the optical transmission spectrum (see Fig. 1.1) is carved up into a number of

  
Figure 1.1: The low-attenuation regions of an optical fiber.

non-overlapping wavelength (or frequency) bands, with each wavelength supporting a single communication channel operating at whatever rate one desires, e.g., peak electronic speed. Thus, by allowing multiple WDM channels to coexist on a single fiber, one can tap into the huge fiber bandwidth, with the corresponding challenges being the design and development of appropriate network architectures, protocols, and algorithms. Also, WDM devices are easier to implement since, generally, all components in a WDM device need to operate only at electronic speed; as a result, several WDM devices are available in the marketplace today, and more are emerging.

Research and development on optical WDM networks have matured considerably over the past few years, and they seem to have suddenly taken on an explosive form, as evidenced by recent publications [JLT96, JSAC96, JLT93, JSAC90, JHSN95] on this topic as well as overwhelming attendance and enthusiasm at the WDM workshops during recent conferences: Optical Fiber Communications (OFC '97) conference and IEEE International Conference on Communications (ICC '96). A number of experimental prototypes have been and are currently being deployed and tested mainly by telecommunication providers in the U.S., Europe, and Japan. It is anticipated that the next generation of the Internet will employ WDM-based optical backbones.

Current development activities indicate that this sort of WDM network will be deployed mainly as a backbone network for large regions, e.g., for nationwide or global coverage. End-users -- to whom the architecture and operation of the backbone will be transparent except for significantly improved response times -- will attach to the network through a wavelength-sensitive switching/routing node (details of which will become clearer later in the book). An end-user in this context need not necessarily be a terminal equipment, but the aggregate activity from a collection of terminals -- including those that may possibly be feeding in from other regional and/or local subnetworks -- so that the end-user's aggregate activity on any of its transmitters is close to the peak electronic transmission rate.

Let us examine below a sample networking problem on such a WDM network.

A Sample WDM Networking Problem

End-users in a fiber-based WDM backbone network may communicate with one another via all-optical (WDM) channels_all-optical channel, which are referred to as lightpaths. A ^lightpath may span multiple fiber links, e.g., to provide a ``circuit-switched''_circuit switching interconnection between two nodes which may have a heavy traffic flow between them and which may be located ``far'' from each other in the physical fiber network topology. Each intermediate node in the lightpath essentially provides an all-optical bypass facility to support the lightpath.

In an N-node network, if each node is equipped with N-1 transceivers [transmitters (lasers) and receivers (filters)] and if there are enough wavelengths on all fiber links, then every node pair could be connected by an all-optical lightpath, and there is no networking problem to solve. However, it should be noted that the network size (N) should be scalable, transceivers are expensive so that each node may be equipped with only a few of them, and technological constraints dictate that the number of WDM channels that can be supported in a fiber be limited to W (whose value is a few tens today, but is expected to improve with time and technological breakthroughs). Thus, only a limited number of lightpaths may be set up on the network.

Under such a network setting, a challenging networking problem is that, given a set of lightpaths that need to be established on the network, and given a constraint on the number of wavelengths, determine the routes over which these lightpaths should be set up and also determine the wavelengths that should be assigned to these lightpaths so that the maximum number of lightpaths may be established. While shortest-path routes may be most preferable, note that this choice may have to be sometimes sacrificed, in order to allow more lightpaths to be set up. Thus, one may allow several alternate routes for lightpaths to be established. Lightpaths that cannot be set up due to constraints on routes and wavelengths are said to be blocked, so the corresponding network optimization problem is to minimize this blocking probability.

In this regard, note that, normally, a lightpath operates on the same wavelength across all fiber links that it traverses, in which case the lightpath is said to satisfy the ^wavelength-continuity constraint. Thus, two lightpaths that share a common fiber link should not be assigned the same wavelength. However, if a switching/routing node is also equipped with a ^wavelength converter facility, then the wavelength-continuity constraints disappear, and a lightpath may switch between different wavelengths on its route from its origin to its termination.

This particular problem, referred to as the Routing and Wavelength Assignment (RWA)_routing and wavelength assignment (RWA) problem, will be examined in detail in Chapter 10, while the general topic of wavelength-routed networks will be studied in Part III (Chapters 8 through 12).

Returning to our sample networking problem, note that designers of next-generation lightwave networks must be aware of the properties and limitations of optical fiber and devices in order for their corresponding protocols and algorithms to take advantage of the full potential of WDM. Often a network designer may approach the WDM architectures and protocols from an overly simplified, ideal, or traditional-networking point of view. Unfortunately, this may lead an individual to make unrealistic assumptions about the properties of fiber and optical components, and hence may result in an unrealizable or impractical design. The goal of this book is to clarify the properties of WDM optical components and present the WDM networking architectures and challenges.