Broadcast-and-Select Networks

Broadcast-and-select networks are based on a passive star coupler device connected to several stations in a star topology. This device is a piece of glass that splits the signal it receives on any of its ports to all the ports. As a result it offers an optical equivalent of radio systems: each transmitter broadcasts its signal on a different wavelength, and the receivers can tune to receive the desired signal (see Fig. 5 for a schematic drawing of such a system).

Figure 5 - A broadcast-and-select system.

The main networking challenge in such networks pertains to the coordination of a pair of stations in order to agree and tune their systems to transmit and receive on the same wavelength. One design issue that must be determined before deciding on these protocols is the tuneable part of the system. It is possible to either have the transmitters each fixed on a different wavelength and have tuneable receivers, have fixed receivers and tuneable transmitters, or have tuning abilities in both components. It has been shown that it is more advantageous to have tuneable receivers and fixed transmitters than the other way around. The advantage of these networks is in their simplicity and natural multicasting capability. However, they have severe limitations since they do not enable reuse of wavelengths and are thus not scalable beyond the number of supported wavelengths.

Another factor that hinders the scalability of this solution and disables it from spanning long distances is the splitting of the transmitted energy to all the ports. For these reasons the main application for broadcast-and-select is high-speed local and metropolitan area networks. However, the relatively high costs of WDM transmitters and receivers compared to the low costs of other technologies (e.g., ATM and switched Ethernet) do not enable broadcast-and-select networks to be competitive in this arena currently. Due to these reasons we will ignore broadcast-and-select networks for the rest of the discussion.