 a tandem switch from there to another local serving switch if there is no direct route available. If there is a direct route, the tandem is eliminated for that traffic relation. It is unnecessary. Let us define a traffic relation as a connectivity between exchange A and B. The routing on calls for that traffic relation is undetermined. Another connotation for the term traffic relation implies that there would be not only a connectivity capability, but also the BH traffic expected on that connectivity. To carry out these functions, a switch had to have some sort of intelligence. In a manu- ally operated exchange, the intelligence was human, namely, the telephone operator. The operator was replaced by an automatic switch. Prior to the computer age, a switch's intel- ligence was "hard-wired" and its capabilities were somewhat limited. Today, all modern switches are computer-based and have a wide selection of capabilities and services. Our interest here is in the routing of a call. A switch knows how to route a call through the dialed telephone number as we described in Section 1.3.2. There we showed that a basic telephone number consists of seven digits. The last four digits identify the subscriber; the first three digits identify the local serving exchange responsible for that subscriber. The three-digit exchange code is unique inside of an area code. In North America, an area code is a three-digit number identifying a specific geographical area. In many countries, if one wishes to dial a number that is in another area code, an access code is required. In the United States that access code is a 1. 4.3.1 Basic Switching Requirements Conceptually, consider that a switch has inlets and outlets. Inlets serve incoming calls; outlets serve outgoing calls. A call from a calling subscriber enters an exchange through an inlet. It connects to a called subscriber through an outlet. There are three basic switching requirements: 1. An exchange (a switch) must be able to connect any incoming call to one of a multitude of outgoi