To services EXCHANGE Originating Outgoing To other exchanges Transit Own Exchange From other Terminating Incoming exchanges Figure 31.3 Call routing 31/4 Principles of switching systems data terminals and the public switched telephone network (PSTN), which was originally established to carry voice traffic, is increas- ingly being used to carry data traffic and to provide a variety of services beyond plain old telephone service (POTS). 31.2 Structure of telephone systems 200 S2 31.2.1 Telephone instrument Exchange battery The speech transmission elements of a telephone instrument, Figure 200 O 31.4, consist of a receiver and transmitter (microphone) usually assembled in a common mounting to form a handset and connected to the line through an anti-sidetone induction coil (hybrid transfor- mer) or an equivalent electronic circuit. The complete instrument <1800 (2) also incorporates signalling elements, the exact form of which depends upon the type of exchange with which it is designed to interwork. Generally they comprise a dial or push buttons and associated gravity switch for signalling to the exchange and a ringer (bell) or an electronic detection circuit and sound transducer for Figure 31.5 Principle of transmission bridge receiving calling (ringing) signals from the exchange. ASTIC combination of d.c. feed, battery decoupling and speech coupling components is known as a transmission bridge or feeding bridge. The relays providing the battery decoupling also serve to detect line (315T) (540T) (900T) signals to control the holding and releasing of the connection. The microphone resistance is nonlinear and is subject to wide variations due to temperature, movement of granules and, of course, the effects of sound. Typically it is between 40 and 400 ohms under normal working conditions. Some telephone instruments employ other types of microphone (e.g. moving coil) which do not themselves require a d.c. feed. Such microphones, however, lack the inherent amplification of the carb- on microphone. They are usually suppor