Central office exchange service (Centrex) is a telephone switch service used by enterprises in which switching occurs at a central (local) office so that enterprises don't need to purchase their own facilities. Centrex service offers an alternative to making the investment in buying and maintaining a private switching system (PBX). Typically, the telephone company owns and manages all the Centrex communications equipment and sells various services to the company. In some cases, the phone company places Centrex equipment on the customer premises.

In many cases, Centrex has replaced the private branch exchange (PBX). The key benefits of Centrex services are:

• Centrex offers feature flexibility for any business and customer do not need to pay for services they do not want.
• Service provider will upgrade the switch with the latest features
• There are no maintenance worries for customers
• The system will not become obsolete
• High availability of the system and services, since the systems are maintained by service providers.

Typical Centrex service includes:

• All dialing features of PBX and public services such call waiting, call forwarding, call hold, conference call,
• Direct inward dialing (DID),
• Sharing of the same system among multiple company locations
• Self-managed line allocation and cost-accounting monitoring

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). IP Centrex, built on the traditional features of Centrex by combining them with the technologies of IP telephony, is replacing the traditional Centrex in the market places.

Related Terms: PBX, IP Telephony, IP Centrex, ISDN Centrex, Class 5 switch

Reference Links:http://www.ip-centrex.org/ : IP Centrex