re depend- to justify the purchase of a PBX, yet need the sophisticated voice ent on what the terminal can support. and data services an expensive PBX can offer, such as Automatic The POTS interface can transmit data only with a modem at Call Distribution (ACD), and integrated Voice Mail (IVM). limited data rates. It is, however, the most cost-effective interface for basic telephone service. The second access technology, the business set interface, offers 35.2 Centrex services significant advantages over the POTS interface. The benefits result from an out of band signalling channel which carries the feature The network and PBX approaches offer comparable features and control signalling information independently of the voice call over benefits. When both options are available, as in North America, the same pair of wires. hybrid networks are frequently found, with a vast majority of This capability to carry both voice and signalling information Centrex users using a PBX somewhere in their network. However, allows multiple line appearance on the terminal and provides users PTOs can offer integrated access to a much wider range of service with easier access to advanced Centrex voice features. The out-of- with Centrex, for example ACD, advanced data switching such as band signalling capability also allows the switch to constantly Frame Relay for LAN to LAN interconnection, and private leased update the terminal on the status of the line or any feature being lines. A typical deployment of Centrex and hybrid networking on invoked. the Northern Telecom DMS is shown in Figure 35.1. This technology was first introduced by Northern Telecom in the early 1980s. One of the most recent additions is the M5000 series of Site 2 Customer Customer Group A Group A Remote PRA Proprietary C7 Backbone Site 1 Customer Group B PRA Proprietary Remote International Gateway Site 2 Figure 35.1 Typical Centrex architecture Centrex services 35/5 advanced business sets, offering single key access to DMS digital 35.2.3 High speed digital data Cen