What is Centrex?

Centrex is a set of specialized business solutions (primarily, but not exclusively, for voice service) where the equipment providing the call control and service logic functions is owned and operated by the service provider and hence is located on the service provider's premises. Since Centrex frees the customer from the costs and responsibilities of major equipment ownership, Centrex can be thought of as an outsourcing solution.

Call control and service logic refer collectively to the functions needed to process a telephone call and offer telephone features. The following are examples of call control and service logic functions:

• recognizing that a party has gone off hook and that dial tone should be provided
• interpreting the dialed digits to determine where the call is to terminated
• determining whether the called party is available, busy, or has call forwarding, and then applying the appropriate treatment (e.g., ringing the phone, applying busy signal, applying a call waiting tone, delivering the call to voicemail, or forwarding the call to another party)
• recognizing when the called party answers the phone and when either party subsequently hangs up, and recording the appropriate information for billing

In traditional Centrex service (i.e., analog Centrex and ISDN Centrex), call control and service logic reside in a Class 5 switch located in the Central Office. The Class 5 switch is also responsibility for transporting and switching the electrical signals that carry the callers' speech or other information (e.g., faxes). Traditional Centrex service has a number of benefits that are discussed elsewhere on this site.

Packet + Centrex = IP Centrex

In IP telephony, voice conversations can be digitized and packetized for transmission across the network. IP Centrex refers to a number of IP telephony solutions where Centrex service is offered to a customer who transmits its voice calls to the network as packetized streams across a broadband access facility. IP Centrex builds on the traditional benefits of Centrex by combining them with the benefits of IP telephony. One of these IP telephony benefits is increased utilization of access capacity. In IP Centrex, a single broadband access facility is used to carry the packetized voice streams for many simultaneous calls. When calls are not active, more bandwidth is available for high speed data sessions over the LAN, like Internet access. This is a much more efficient use of capacity than traditional Centrex. In analog Centrex, one pair of copper wires is need to serve each analog telephone station, regardless of whether the phone has an active call; one the phone is not engaged in a call, the bandwidth capacity of those wires is unused. An ISDN BRI can support two simultaneous calls (i.e., 128 kbps), but similar to analog lines, an idle BRI's bandwidth capacity cannot be used to increase the corporate LAN's interconnection speed.
See the network architecture page for more details on how IP Centrex works.