ABSTRACT
The word ‘telecommunication’ is derived fromtwo Greek words, which can be translated as ‘the passage of information at a distance’. Historically the only means of communication available were visual communications, e.g. beacons, smoke signals and semaphore signalling.Modern telecommunications networks enable different types of data to be passed: Voice (by telephony), Paper images (by facsimile), Computer systems (by using digital data networks), Plain text (by telex), Formatted Text (by electronic mail and related systems) and Video Systems (by Integrated Services Digital Network — ISDN). The different forms of telecommunication are distinguished in two ways: firstly by the type of device used (telephone, fax machine, telex machine, etc.) and secondly by the kind of network used to interconnect the devices (telephone network, telex network, data network, etc.) For example, telephones, fax and modems would use the Public Switched Telephone Network (PSTN) and telex machines would use a dedicated telex network. Digital data would have to use a digital network like X.25, Frame Relay, Switched Multimegabit Data Service (SMDS) or Asynchronous Transfer Mode (ATM), to name a few. All PTTs (Post, Telephone and Telegraph) and all other Licensed Operators in the UK operate a digital network. The bandwidth of PSTN is determined by various speech components. The speech input from a PSTN telephone consists of a sum of a number of different frequencies components, from about 50Hz to about 7 kHz depending on who is speaking. The speech output from a PSTN telephone also consists of a number of frequency components, but this time only from 300 to 3400Hz. Thus the telephone network cuts out all frequency components below 300Hz and above 3400Hz. Hence the bandwidth of PSTN is 3.1 kHz (3.4 kHz to 300Hz). Voice being transmitted through the PSTN is called 3.1 k or ‘commercial speech’. Music frequencies can be in the range of 30 kHz (Double Bass) to 20 kHz (Flute). The telephone service is a result of this compromise. Transferring all speech components would result in a high quality but expensive service; progressively reducing the bandwidth lowers both the quality and cost.
