Centrex service is regarded as an "outsourcing" of telephony call services. Centrex does not maintain a PBX on customer premises. Instead, Centrex service removes the PBX function from the customer premises, provides a Centrex trunk to the customer, and provides the telephony services over the trunk. Typically, the Centrex trunk is arranged as a TDM channel associated signaling (CAS) circuit or as an ISDN Q.931 connection. Customer billing for this service is similar to the billing for outsourcing services.
IP Centrex performs the same job as a PBX but delivers the service over an IP network instead of a circuit-switched network. Service is accessed via an IP network and delivered to customers in a private or multitenant installation. The call control functions that Centrex delivers include:
Cisco delivers IP Centrex through the use of a call agent, such as the Cisco BTS 10200 Softswitch. Figure B-5 illustrates the use of a call agent for the Centrex services that are delivered to telephones with the trunking gateway serving as a path to the PSTN. Notice that Media Gateway Control Protocol (MGCP) acts as the gateway control protocol because the call routing intelligence resides in the call agent, which is maintained by the service provider.
Figure B-5. IP Centrex