t increases rapidly up to a vertical asymptote when p=1. Telecommunication Networks FIGURE 7.23 N = 4 Average latency normalized to the packet duration (T/ts) in a bus-based switch fab- ric for different values of the normalized offered traffic (Ats) and different dimensions of the switch fabric. Normalized offered traffic (0) 7.4 Wavelength Division Switch Fabrics Since switching can be realized both in space and in time domain, it is intuitive that switching is possible also in frequency domain [11,33]. Frequency domain switching could be useful especially for optical applications. In principle an exact equivalent of the time domain switching based on TSI can be real- ized in the wavelength domain by using wavelength interchangers (WI). A WI is a subsystem that is able to interchange two wavelengths belonging to a WDM comb without affecting the other channels. From a functional point of view, a WI is the exact equivalent of the TSI in the time domain, but this similarity does not extend to the physical implementation of the device. As a matter of fact, nothing similar to a random access memory exists in the optical domain and all the processing on a WDM comb have to be performed using analog devices. Thus, the WI has to be realized using an analog scheme. Different solutions have been proposed, all based on the availability of a wavelength converter, that is, a component that is able to move the carrier of a modulated WDM channel from one wavelength to another. The most intuitive scheme for a WI is depicted in Figure 7.24. In the figure it is important to notice that the separation of the WDM channels is operated by a low-loss demux, but the reconstruction of the WDM comb by a high loss coupler. This is due to the fact that the input ports of low-loss demuxes are frequency selective, that is, the frequency arriving at a certain demux port is fixed and cannot be changed. demux coupler FIGURE 7.24 Functional scheme of a wave- length interchanger based on optical wavelength converters. Switching Systems: Architect