plex structure where the radio nect 7 layer model. These cover the physical aspects such as carrier exchange incorporates additional common control and switching frequencies, power levels, modulation characteristics, multiplexing functions to improve roaming and to provide concentration. The of user and signalling information, channel assignment, link set up connection between the radio exchange and the PABX may be by a and control, and network layer protocols for managing calls be- multi-channel digital link, such as ISDN primary rate access. The tween the portables and the central system and external networks. normal services are still provided by the PABX. The radio exchange In addition the requirements for speech transmission performance and the PABX may be built as a single integrated system or as over the air interface are also specified. separate unit. Wired extensions Radio exchange Mobility control Digital Switching Common control stations 'N' handsets Figure 34.16 An improved cordless PABX arrangement Centrex Kanagendra B Tec MBA CDipAF Northern Telecom Europe Contents 35.1 Introduction 35/3 Centrex services 35/3 35.3 Development of the Centrex market 35/6 35.4 References 35/6 This page intentionally left blank Introduction 35/3 35.1 Introduction 35.2.1 Featured voice Centrex is a service offered by telephone companies which gives Centrex has voice features which help ensure that calls are answered their customers the capabilities of a sophisticated voice and data quickly and routed appropriately and cost effectively within the Private Branch Exchange (PBX) without the need to purchase and network. Features provided by typical advanced digital Centrex maintain on-premises switching equipment. include direct dialling inward (DDI), flexible intercept, hunting, A modern digital PBX is a specialised real-time computer which night service and security provisions. Advanced business sets with routes calls and provides calling features such as call forwarding user programmable feature keys enhance access to a r