Subscriber Loop Design 541 Basic Design Considerations

We speak of the telephone subscriber as the user of the subset. As we mention in Section 1.3, telephone subscribers are connected via a subscriber loop to a local serving switch that can connect a call to another subscriber served by that same switch or via other switches through the PSTN to a distant called subscriber. The conventional subscriber loop is a wire pair. Present commercial telephone service provides for both transmission and reception on the same pair of wires that connect the subscriber to her/his local serving switch. In other words, it is two-wire operation.

The subscriber loop is a dc loop in that it is a wire pair supplying a metallic path2 for the following:

2Metallic path is a path that is "metal," usually copper or aluminum. It may be composed of a wire pair or coaxial cable. We could have a radio path or a fiber-optic path.

1. Talk battery.

2. An ac ringing voltage for the bell or other alerting device on the telephone instrument supplied from a special ringing voltage source.

3. Current to flow through the loop when the telephone subset is taken out of its cradle (off-hook), which tells the switch that it requires "access" and causing line seizure at the local serving switch.

4. The telephone dial3 that, when operated, makes and breaks the dc current on the closed loop, which indicates to the switching equipment the telephone number of the distant telephone with which communication is desired.

The typical subscriber loop is supplied its battery voltage by means of a battery feed circuit illustrated in Figure 5.3. Battery voltages have been standardized at -48 Vdc. It is a negative voltage to minimize cathodic reaction. This is a form of corrosion that can be a thermal noise source.

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