 coaxial cable, or optical fibre link, without inter- fering with other calls. This process is known as multiplexing, and refers to the process whereby several signals are simultaneously transmitted together using a shared medium. Several types of multiplexing exist. Frequency division multiplexing (FDM) is when user sig- nals are modulated to different frequency bands. The frequency bands are distinct and as a result signals do not interfere with each other. Another method is time division multiplexing (TDM) where multiple signals are each sent at a different time slot. FDM technology is widely used for transmission of voice signals over high capacity coaxial cables between exchanges. The FDM process is illustrated in Figure 3.16, where three modu- lators operating at three different carrier frequencies are shown. The resulting multiplexed output depicts the voice signal spectrum side-by-side. The FDM process may be likened to a freight train that loads cargo containers, and carries them side by side between major train stations. At the destination train station the container is downloaded and carried to its final destination individually. Similarly on the receiver local exchange, each modulated signal is transferred back to its baseband signal and delivered to the intended recipient. Teletraffic Engineering High capacity media which connect exchanges need to be dimensioned in the number of calls they may carry. An important parameter in inter-exchange capacity is the necessary number of lines to ensure a high degree of connectivity. For example, if an exchange serves 1000 local subscribers, the number of links that connect the exchange to other parts of the network is an important technological, business and indeed policy parameter. One factor of interest is the probability that a subscriber will not be able to make a call because all the inter-exchange links are in use by other subscribers, and is therefore blocked. Under- dimensioning, or too few links, requires small infrastructure investment. However, it wi