Abstract
A trunk circuit is the interface between the trunk and the switching system. The definition still creates confusion because, for example, a “trunk” at a private branch exchange (PBX) installed in an industrial plant or in a business office ends up as a “line” at the central office, while a private circuit or “tie-line” between two PBXs is functionally identical to a trunk between two central office switches. Just as the designer of an electronic telephone set must observe the existing standards and practices, so, too, the designer of the central office equipment must be sure that the circuits will respond properly to either a modern electronic telephone set or conventional telephone set connected to the local loop. These compatibility requirements stem from the fact that it is usually too costly to replace all of the telephones at the time a new switching office is installed, so the new switch interface with the subscriber is designed to allow the use of existing telephones.
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