In this section our concern is telephone switching, the switching of voice channels. We will deal with some switching concepts and with several specifics. Switching was defined in Section 4.1 in contraposition with transmission.
Actual connectivity is carried out by the switching function. A connectivity may involve more than one switch. As we pointed out in Chapter 1, there are local switches, tandem switches, and transit switches. A transit switch is just a tandem switch that operates in the long distance or "toll" service.
A local switch has an area of responsibility. We call this its serving area. All subscriber loops in a serving area connect to that switch responsible for the area. Many calls in a local area traverse no more than one switch. These are calls to neighbors. Other calls, destined for subscribers outside of that serving area, may traverse 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 manually 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 intelligence 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.
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