ange of and handles data transmission for a customer's business site. Lines features. The addition of displays on these sets provides calling connect each individual telephone and data terminal to the PBX. information such as the called or calling party's name and number, The PBX is connected to the Public Telephone Network Operator's reasons for redirected calls, and digits dialled during call origination (PTO) main exchange (called the Central Office switch) by voice or while activating system features. and data channels called trunks. Voice and data traffic to and from The system offers further facilities, including ACD with sophisti- the public network travels over these trunks, while traffic internal to cated management and centralised message service. A simplified a customer's site is routed by the PBX. Traditionally, trunks do not console enables operators to concentrate on incoming calls. provide any features. Centrex is an alternative for businesses which do not wish to buy and run their own private telecommunications system. As the name 35.2.1.1 User terminals implies its is a Central Office Exchange, with the lines from indi- vidual telephones and data terminals connected directly to the Northern Telecom's terminals support three access interface techno- PTO's exchange rather than a PBX. This exchange is also used for logies, namely: the public telephone service. The software resident in the central office switch is partitioned and programmed to create a virtual PBX 1. Analogue, for plain old telephone service, or POTS. that delivers voice and data services comparable to those on any 2. Business set, for network-wide featured voice services. modern digital PBX. Numerous virtual PBXs can exist on on any 3. ISDN, for network-wide interworking of voice and data capa- one central office switch, all sharing the same processing resources. bilities. Centrex is attractive to corporations, particularly the larger ones, who increasingly find that telecommunications is using up consid- These technologies compleme