rformance. Signalling. Loudspeaking telephones. Digital measurements. Data communications network measurements. telephones. Telephone standards. Headsets. Headset aesthetics. ISDN test equipment. Introduction to cellular radio. Analogue cel- Headset technical considerations. The growing need for headsets. lular test equipment. Digital cellular test equipment. The OSI Model Headset approval process. Headset design criteria. and GSM. The radio path or Physical Layer (Layer 1). Data Link Layer (Layer 2). Network Layer (Layer 3). A.F. and speech codec measurements. Test set up configurations. References. 59 Telex communications Introduction. Characteristics of telex. Telex signalling. Telex proto- 62 Glossary col. Telex terminals. Introduction. Acronyms. Glossary of terms. Foreword Telecommunications is a prime industry, like electricity, water controls and competitive forces to govern its operation. The charac- and gas and, like these industries, it impacts on every individual, teristics of this network are fairly clear already. organisation and country. Telecommunications is, however, much First, in wideband access electronics will migrate much closer to younger and as such it is more dynamic and vibrant. It is growing the customer in the coming months and years, driven not only by and changing at a phenomenal rate. technology but more importantly by demands for service respon- Politically, telecommunications is going through an exciting siveness and lower operating costs. In many cases a fibre will period. Old state owned monopolies are being privatised, and lib- terminate in the basement of the business building. Almost all large eralisation has resulted in many new entrants. As organisations have businesses and many medium and small locations will have direct become more multinational, so have these telecommunication oper- access to a fibre based connection. This will lead to wideband access ators. at up to 2 Mbits/s becoming widely available, with short provision- Competition has quickened the pace of technolog