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 neighbours. 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.
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 outgoing circuits.
2. It has the ability not only to establish and maintain (or hold) a physical connection between a caller and the called party for the duration of the call, but also to be able to disconnect (i.e., “clear”) it after call termination.
3. It also has the ability to prevent new calls from intruding into circuits that are already in use. To avoid this, a new call must be diverted to another circuit that is free or it must be temporarily denied access where the caller will hear a “busy back” (i.e., a tone cadence indicating that the line is busy) or an “all trunks busy” tone cadence signal or voice announcement (i.e., indicating congestion or blockage).
Let’s differentiate local and tandem/transit exchanges. A local exchange connects lines (subscriber loops) to other lines or to trunks. A tandem/transit exchange switches trunks. Local exchanges concentrate and expand. Tandem and transit exchanges do not.
Concentration and Expansion
Trunks are expensive assets. Ideally, there should be one trunk available for every subscriber line (loop). Then there never would be a chance of blockage. Thus, whenever a subscriber wished to connect to a distant subscriber, there would be a trunk facility available for that call. Our knowledge of telephone calling habits of subscribers tells us that during the busy hour, on the order of 30% of subscriber lines will be required to connect to trunks for business customers and some 10% for residential customers. Of course, these values are rough estimates. We’d have to apply the appropriate traffic formula based on a grade of service, for refined estimates.
Based on these arguments, a local exchange serving residential customers might have 10,000 lines, and only 1000 trunks would be required. This is concentration. Consider that those 1000 incoming trunks to that exchange must expand out to 10,000 subscribers. This is expansion. It provides all subscribers served by the switch with access to incoming trunks and local switching paths. The concentration/expansion concept of a local serving exchange is illustrated in the following diagram:
