WDM takes optical signals (each carrying information at a certain bit rate), gives them a color (a wavelength or specific frequency), and then sends them down the same fiber (see Figure 6). Each piece of equipment sending an optical signal has the illusion of having its own fiber. WDM gets more cars to travel, not by increasing their speed but by getting them to travel in parallel in their own dedicated lanes. Traffic in each lane can travel at different speeds, as each lane is independent. The wavelengths used for WDM are chosen in a certain range of frequencies (around 1550 nm), also called a window.

Figure 6. Multiplexing Optical Signals

TDM is there to generate the fastest bit stream possible or economical in a part of the network (see Figure 7). The fastest stream deployed now is 10 Gbps. This stream can then be fed into a WDM system, creating the greatest capacity on a fiber—which is now 160 wavelengths at 10 Gbps or 1.6–Tbps capacity on a single fiber.

Figure 7. Relation between TDM and WDM

The development of the next TDM transmission speed (40 Tbps) is now underway. Trials of WDM at this speed have already been completed. If the TDM (synchronous) multiplexer cannot generate the proper color needed for the WDM system, an adaptor (called a wavelength converter) can be used.

The synchronous multiplexers generate the correct colors for the WDM system, saving this extra piece of equipment. However, this wavelength converter is still useful to pick up traffic from other sources.

Similar functions in the TDM and WDM environment may be identified. In fact, TDM simply manipulates bit streams, while WDM manipulates wavelengths (or streams of light).