Centrex—short for “central office exchange”—is a service that handles business calls at the telephone company’s switch rather than through a customer-owned, premisesbased Private Branch Exchange (PBX). Centrex provides a full complement of station features, remote switching, and network interfaces that provides an economical alternative to owning a PBX.
Centrex offers remote options for businesses with multiple locations, providing features that appear to users and the outside world as if the remote sites and the host switch are one system. Centrex-capable switches now support wireless links, which extends the boundaries of business services. Centrex users have access to direct inward dialing (DID) features, as well as station identification on outgoing calls.
Each station has a unique line appearance in the central office, in a manner similar to residential telecommunications subscriber connections. ACentrex call to an outside line exits the switch in the same manner as a toll call exits a local exchange. Users dial a four- or five-digit number without a prefix to call internal extensions and dial a prefix (usually 9) to access outside numbers.
The telephone company operates, administers, and maintains all Centrex switching equipment for the customers. It also supplies the necessary operating power for the switching equipment, including backup power to ensure uninterrupted service during commercial power failures. Centrex may be offered under different brand names. BellSouth calls it Essex, and SBC Communications calls it Plexar, while Verizon calls it CentraNet.
Centrex is also offered through resellers that buy Centrex lines in bulk from the local exchange carrier. Using its own or commercially purchased software, the reseller packages an offering of Centrex and perhaps other basic and enhanced telecommunications services to meet the needs of a particular business. The customer gets a single bill for all local, long-distance, 800, 900, and calling-card services at a fee that is less than the customer would otherwise pay.
Centrex service offerings typically include direct inward dialing (DID), direct outward dialing (DOD), and automatic identification of outward dialed calls (AIOD). Advanced digital Centrex service provides all the basic and enhanced features of the latest PBXs in the areas of voice communications, data communications, networking, and Integrated Services Digital Network (ISDN) access.
Commonly available features include voice mail, electronic mail, message center support, and modem pooling. For large networks, the Centrex switch can act as a tandem switch, linking a company’s PBXs through an electronic tandem network. Centrex is also compatible with most private switched network applications, including the Federal Telecommunications System (FTS) and the Defense Switched Network (DSN).
Many organizations subscribe to Centrex service primarily because of its networking capabilities, particularly for setting up a virtual citywide network without major cost or management concerns. With city-wide Centrex, a business can set up a network of business locations with a uniform dialing plan, a single published telephone number, centralized attendant service, and full feature transparency for only an incremental cost per month over what a single Centrex site would cost.
A number of value-added services are available through wireless Centrex. Pacific Bell’s Wireless Centrex, for example, combines an existing Centrex service with Ericsson’s Freeset Business Wireless Telephone system to create a private wireless environment at a company’s business location without incurring expensive cellular airtime charges. At the corporate facility multiple, overlapping cells cover assigned areas.
The number of cells required is determined by traffic density at a given location. An on-premises system called the “radio exchange” handles such functions as powering, control, and facilities for connection to Centrex. Base stations relay calls from the radio exchange to the portable telephones. Each base station provides multiple simultaneous speech channels. The coverage of each base station depends on the character of the environment, but it is typically between 8000 and 15,000 square feet.
Portable telephones contain the intelligence needed to accommodate roaming and cell-to-cell handover. When the radio exchange receives an incoming call, it transmits the identification signal of a portable telephone to all base stations. Because the portable telephone communicates with the nearest base station, even in standby mode, it receives the signal and starts ringing.
When the call is answered, the portable telephone selects the channel with the best quality transmission. Pacific Bell’s Wireless Centrex service gives business users full wireless mobility plus the following options:
- Account codes
- Authorization codes
- Automatic callback
- Automatic recall
- Call diversion/call forwarding
- Call diversion override
- Call hold
- Call transfer
- Call waiting
- Call pickup
- Speed calling
- Call park
- Conference/three-way calling
- Remote access to network services
- Distinctive/priority ringing
- Do not disturb
- External call forwarding
- Executive intrusion/executive busy override
- Individual abbreviated dialing/single digit dialing
- Speed dialing
- Last number redial
- Loudspeaker paging
- Message waiting indication
- Remote access to subscriber features
- Select call forwarding
Centrex offers high-quality, dependable, feature-rich telephone service that supports a variety of applications. For many organizations, Centrex offers distinct advantages over on-premises PBX or key/hybrid systems. Centrex can save money over the short term because there is no outlay of cash for an on-premises system. If the service is leased on a month-to-month basis, there is little commitment and no penalty for discontinuing the service.
A company can pick up and move without worrying about reinstalling the system, which may not be right for the new location. With wireless capabilities, businesses get the advantage of mobility without the expense of investing in wireless infrastructure of their own.