ce the data has to wait in a buffer for the time slot assigned to. Although buffer delays exist in every digital network, modern switches have very high processing power and perform all these operations very rapidly. Therefore, delays introduced by digital transmission are negligible for most applications. For instance, for a voice application, transmission delays higher than 300 ms round-trip make it difficult to maintain a normal conversation. The delay introduced by digital switching is far below this threshold. A more problematic disadvantage of digital systems for transmitting voice signal is the complexity of the terminal, which has to include a coder- decoder to convert the acoustic signal into a digital stream. This mechanism makes digital terminals trickier and more expensive than analog ones. To avoid terminal complexity, many digital networks implement an analog interface towards the terminal. The interface between the terminal and the local exchange, which is called a local loop or subscriber line (see Fig- ure 1-11), enables the user to retain his or her trusty analog terminal and still connect to a digital local exchange. the coding and decoding of analog signals takes place within the local exchanges of the parties conversing. Reading between the lines, it can be seen that at the time systems were migrating from analog to digital, the philosophy of the PSTN already was to design simple user equipment and complex networks. There were some additional reasons for that back then, including backwards-compatibility and the price of the terminals, but undeniably this design style persists into the present. Time Division Multiplexing FDM, besides being a expensive technology, also limits multiplexing capac- ity. Different channels have to use frequencies sufficiently separated to avoid interference, and the usable frequency range is defined by the band- width of the medium and by the attenuation. (Attenuation is a function of the frequency: the higher the frequency, the more severe the attenuation.) Becaus