be completed. to BT network In adopting a computerised approach to cable design and manage- Gas discharge tubes ment, the design of cabling schemes will become more efficient and management will improve dramatically, thus reducing the problem CCITT Gas discharge tubes of over-crowded cable ways and 'lost' cables. The use of some form BS CECC 42201 Varistors of computerised documentation system may soon become manda- tory for most cabling installations and designing the cabling on the OFTEL NS/G/1235/M/10 Apparatus approval system will ensure the accurate registration of the cables as in- stalled. 15th edition Wiring regulations TCP/IP Simple network management protocol 33.10 References 89/336 Electo-magnetic Butler Cox (1990) Information Technology & Buildings: A Practi- compatibility cal Guide for Designers. PABX and key systems Steve Berrisford Northern Telecom (Section 34.1) David M Davidson Northern Telecom (Sections 34.2-34.7) Fred Howett Northern Telecom (Section 34.8) Contents 34.1 Purpose of key systems and PABXs 34/3 34.2 PABX architecture 34/4 Trunk circuits 34/5 Private networking 34/7 Line circuits 34/11 34.6 Control and administration 34/11 Data transmission 34/14 34.8 Cordless PABX and key systems 34/14 This page intentionally left blank Purpose of key systems and PABXs 34/3 34.1 Purpose of key systems and PABXs 34.1.1 Control system Prior to the existence of key systems and PABXs (also referred to as In very early equipment, the number of services supported were PBX) a company would have a separate telephone line from the very small. There would be an ability to recognise off-hook states, public exchange for each employee requiring a telephone. If a call provide dial tone, recognise dialled digits, route calls, and ring came in to the wrong telephone then either the caller would have to telephones, but not much else. The control system was more of a call back to the correct telephone or the correct person would have concept than a physical reality. Each part of the system controlled to physically c