WDM Networking Evolution

Point-to-Point WDM Systems

WDM technology is being deployed by several telecommunication companies for point-to-point communications. This deployment is being driven by the increasing demands on communication bandwidth. When the demand exceeds the capacity in existing fibers, WDM is turning out to be a more cost-effective alternative compared to laying more fibers. A recent study [MePD95] compared the relative costs of upgrading the transmission capacity of a point-to-point transmission link from OC-48 (2.5 Gbps) to OC-192 (10 Gbps) via three possible solutions:

1. installation/burial of additional fibers and terminating equipment (the ``multifiber'' solution);
2. a four-channel ``WDM solution'' (see Fig. 1.2) where a WDM multiplexer (mux) combines four independent data streams, each on a unique wavelength, and sends them on a fiber; and a demultiplexer (demux) at the fiber's receiving end separates out these data streams; and
3. OC-192, a ``higher-electronic-speed'' solution.

The analysis in [MePD95] shows that, for distances lower than 50 km for the transmission link, the ``multi-fiber'' solution is the least expensive; but for distances longer than 50 km, the ``WDM'' solution's cost is the least with the cost of the ``higher-electronic-speed'' solution not that far behind.

WDM mux/demux in point-to-point links is now available in product form from several vendors such as IBM, Pirelli, and AT&T [Gree96]. Among these products, the maximum number of channels is 20 today, but this number is expected to increase soon.

  
Figure 1.2: A four-channel point-to-point WDM transmission system with amplifiers.

  
Figure 1.3: A Wavelength Add/Drop Multiplexer (WADM).

Wavelength Add/Drop Multiplexer (WADM)

A Wavelength Add/Drop Multiplexer (WADM) is shown in Fig. 1.3. It consists of a demux, followed by a set of 2 x 2 switches -- one switch per wavelength -- followed by a mux. The WADM can be essentially ``inserted'' on a physical fiber link. If all of the 2 x 2 switches are in the ``bar'' state, then all of the wavelengths flow through the WADM ``undisturbed.'' However, if one of the 2 x 2 switches is configured into the ``cross'' state (as is the case for the lambda_i switch in Fig. 1.3) via electronic control (not shown in Fig. 1.3), then the signal on the corresponding wavelength is ``dropped'' locally, and a new data stream can be ``added'' on to the same wavelength at this WADM location. More than one wavelength can be ``dropped and added'' if the WADM interface has the necessary hardware and processing capability.

Fiber and Wavelength Crossconnects -- Passive Star, Passive Router, and Active Switch

In order to have a ``network'' of multiwavelength fiber links, we need appropriate fiber interconnection devices. These devices fall under three broad categories:

• passive star (see Fig. 1.4),
• passive router (see Fig. 1.5), and
• active switch (see Fig. 1.6).

    
Figure 1.4: A 4 x 4 passive router (four wavelengths).

Figure 1.5: A 4 x 4 passive star.

  
Figure 1.6: A 4 x 4 active switch (four wavelengths).

The passive star is a ``broadcast'' device, so a signal that is inserted on a given wavelength from an input fiber port will have its power equally divided among (and appear on the same wavelength on) all output ports. As an example, in Fig. 1.4, a signal on wavelength lambda_1 from Input Fiber 1 and another on wavelength lambda_4 from Input Fiber 4 are broadcast to all output ports. A ``collision'' will occur when two or more signals from the input fibers are simultaneously launched into the star on the same wavelength. Assuming as many wavelengths as there are fiber ports, an N x N passive star can route N simultaneous connections through itself.

A passive router can separately route each of several wavelengths incident on an input fiber to the same wavelength on separate output fibers, e.g., wavelengths lambda_1, lambda_2, lambda_3, and lambda_4 incident on Input Fiber 1 are routed to the same corresponding wavelengths to Output Fibers 1, 2, 3, and 4, respectively, in Fig. 1.5. Observe that this device allows wavelength reuse, i.e., the same wavelength may be spatially reused to carry multiple connections through the router. The wavelength on which an input port gets routed to an output port depends on a ``routing matrix'' characterizing the router; this matrix is determined by the internal ``connections'' between the demux and mux stages inside the router (see Fig. 1.5). The routing matrix is ``fixed'' and cannot be changed. Such routers are commercially available, and are also known as Latin routers, waveguide grating routers (WGRs), wavelength routers (WRs), etc. Again, assuming as many wavelengths as there are fiber ports, a N x N passive router can route N^2 simultaneous connections through itself (compared to only N for the passive star); however, it lacks the broadcast capability of the star.

The active switch also allows wavelength reuse, and it can support N^2 simultaneous connections through itself (like the passive router). But the active star has a further enhancement over the passive router in that its ``routing matrix'' can be reconfigured on demand, under electronic control. However the ``active switch'' needs to be powered and is not as fault-tolerant as the passive star and the passive router which don't need to be powered. The active switch is also referred to as a wavelength-routing switch (WRS), wavelength selective crossconnect (WSXC), or just crossconnect for short. (We will refer to it as a WRS in this book.)

The active switch can be enhanced with an additional capability, viz., a wavelength may be converted to another wavelength just before it enters the mux stage before the output fiber (see Fig. 1.6). A switch equipped with such a wavelength-conversion facility is more capable than a WRS, and it is referred to as a wavelength-convertible switch, wavelength interchanging crossconnect (WIXC), etc.

The passive star is used to build local WDM networks, while the active switch is used for constructing wide-area wavelength-routed networks. The passive router has mainly found application as a mux/demux device.