arketing and the digital telephone (terminal) located in the user's premises and financial services, and those using direct order entry, such as express the local exchange. The 192kbit/s is divided into a B1 channel delivery services, airline reservations, as well as such public service (64kbit/s), a B2 channel (64kbit/s) and a D channel (16kbit/s). The organisations as police, fire and ambulance are all making use of B channels can be used for either voice or data. A digital telephone headsets to overcome the problem, and at the same time increasing will consist of a handset and possibly handsfree loudspeaker and their employee's productivity and efficiency. microphone connected to an integrated circuit, which will provide the functions of send, receive gain, frequency response, sidetone generation and the codec functions. Additional circuits are necess- 58.12 Headset aesthetics ary to provide the physical drive to the line, extract power from the line and operate the link access procedure on the D channel (LAPD). Most headsets today are light and efficient and offer excellent sound The functionality of the telephone can be built into the micropro- quality. It is important, however, that operators using these sets, cessor software (functional model) or respond to stimulus signalling often for most of the day, should look right and feel right when from the exchange (stimulus model). In the former all the call wearing them. When a headset is worn, there is a tendency to regard processing logic is designed into the telephone, whilst in the latter it as an article of clothing, and if the user doesn't like the headset it is held centrally in the exchange and the telephone responds only then he or she will find a reason for not wearing it. So aesthetics as to the messages from the exchange. well as comfort are of prime importance. Headsets are frequently perceived to be expensive but, in com- parison with the cost of the telephone systems they are used with, 58.10 Telephone standards they represent but a fraction of t