 category of telephone services, but the first and most important one is the transmission of voice between users. Any user of the network has to be able to call any other user attached to it. All telephone networks must fulfill this requirement. The first telephone network that enabled two users to communicate con- sisted of two telephone sets joined by a cable. That system provided both users of the network with adequate voice service, but as soon as more users wanted to be able to use the network, the number of telephone sets increased. The best way to add a new user to the network was to install new cables from the telephone set of the newcomer to all the already-installed telephone sets, creating what is known as a fully meshed topology, where every telephone set is directly connected to every other telephone (see Figure 1-1). Figure 1-1 A fully meshed topology. Chapter 1 Unfortunately, a fully meshed topology presents a number of disadvan- tages in the telephone context. Failure to scale is a very important draw- back. An individual connection from each user to any other user who potentially can be called has to be established. A physical connection to every single user is affordable as long as the system is small, but as the number of users grows, the number of connections grows in an exponential manner. Cables are expensive and difficult to install. Therefore, adding new users to the network becomes very expensive very quickly. Cost aside, if a single telephone set has as many cables connected to it as there are users in the network, routing becomes complicated to manage. Each telephone set has to maintain a huge routing table that indicates which cable corresponds to which destination in order to route the calls. Due to their limitations of scalability and management, early systems evolved from a fully meshed topology to a star topology. In a star topology, all telephone sets are connected to a single central unit called a switch (see Figure 1-2) and all calls are routed through it from source to destinat