7.9 Centrex

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For businesses that do not have the resources or the desire to manage their own voice systems, there is the alternative of letting the telephone company do it through a service offering called Centrex.

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, premises-based PBX. Centrex provides a full complement of station features, remote switching, and network interfaces that offer 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 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. A Centrex 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 a prefix (usually 9) to access outside numbers.

The telephone company operates, administers, and maintains all Centrex switching equipment for 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, SBC Communications calls it Plexar, and 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.

7.9.1 Centrex Features

Centrex service offerings typically include DID, direct outward dialing (DOD), and automatic identification of outward dialed calls (AIOD). Advanced digital Centrex service provides all of the basic and enhanced features of the latest PBXs in the areas of voice communications, data communications, networking, and 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 city-wide 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.

7.9.2 Customer Premises Equipment

Centrex CPE is available for lease or purchase from a number of vendors, including the local telephone company. Centrex CPE combines the advanced features of a PBX with the convenience and flexibility of Centrex. Popular Centrex CPE products include multi-line phones, PC-based attendant workstations, attendant consoles, call reporting and management packages, voice processing equipment, line monitors, and tip-and-ring scanners.

Centrex Telephones Phones made specifically for Centrex service allow users to access a wealth of Centrex features with the touch of a button—without having to memorize codes. Many even provide single-button transfer capability to remote sites. When users have the proper equipment, the service is used more efficiently and the features are used more frequently, resulting in better value for the company’s Centrex investment dollars.

PC-Based Attendant Workstations Screen-based attendant consoles combine Centrex access and improved calling options with Windows databases. This allows an attendant to work in a Windows word processing application, for example, and hot-key over to answer an incoming call.

Some PC attendant workstations come equipped with database directories that can support multiple telephone, fax, and paging numbers for each entry and enable single-keystroke dialing. Those that support ANI and directory name lookup can do “screen pops” of caller information to enable organizations to personalize call handling.

Centrex workstations that interface with LANs also allow attendants to do more than just handle calls. Hot-keying lets administrative personnel answer and transfer calls and still complete computer-based tasks over the local network.

Centrex Answering Consoles

Centrex answering consoles can take the form of multi-button telephones, conventional-looking attendant consoles, or PC-based platform systems. Centrex attendant consoles allow call handlers to perform single-button call transfers. They also have line-status displays that let the attendant know when a line is in use or idle. In addition, many consoles allow the attendant to reprogram extensions, access features, and make other system rearrangements without the help of the telephone company.

Message Desk

As an option, the telephone company can provide a data line to the customer, which is called a station message desk interface (SMDI). Centrex CPE is available that interfaces with the SMDI link to give full voice mail and Centrex integration. It allows a call to an unanswered station to be rerouted directly to that person’s voice mailbox without the caller having to re-enter the extension number.

Call Accounting System

Many options are available for businesses that use Centrex to obtain SMDR data in order to increase system efficiency, just like a PBX. The telephone company can provide call detail information to their customers or, alternatively, customers can use CPE line scanners and PC-based telecommunications management products to obtain the same functionality at a lower price. Some systems record SMDR right from the switch and store the information until it can be transferred to a PC for processing.

Administration Systems

In the past, Centrex customers had to go through the telephone company to change an extension on their Centrex service. Now Centrex administration systems let users reconfigure their Centrex service. PC-based consoles allow users to turn enhanced CO services (e.g., CO voice mail) on and off; activate or deactivate lines; add or reassign trunks; and assign extensions for least-cost routing of long-distance calls.

7.9.3 IP Centrex

Centrex can be enhanced to support telephone calls over IP packet networks. The advantage of integrating Centrex with IP packet-based nets is to enable the telephone company to reduce facility costs by using the inherent efficiencies of IP networks. It also enables the telephone company to offer value-added services to its Centrex customers, such as VPNs, telecommuter access, and virtual call centers. Telephone companies can also use the IP network to extend the market reach for their Centrex offerings by serving corporate locations out of their local serving area.

The integration of Centrex with IP is accomplished with a gateway that gives the existing Class 5 CO switch the ability to offer the full spectrum of voice, data, and Centrex features over an IP network (see Figure 7.10). The gateway connects to the Class 5 switch using the Telcordia Technologies (formerly, Bellcore) standard GR-303 interface. The gateway uses the existing stable and secure Class 5 infrastructure for billing, OAM&P, signaling (SS7), and trunking. The telephone company can increase revenues with new IP applications that can be deployed immediately and minimize expenditures by providing these new services using existing equipment.

Figure 7.10: An IP gateway connects to the Class 5 central office switch using the industry standard GR-303 interface to extend the reach of Centrex features via the IP.

At the customer premises, a variety of equipment can be used to get voice and data traffic through the IP network to the gateway and then to the Centrex switch. A multiport access gateway is a concentration device that connects multiple legacy key sets to an IP network. Standard phone wiring is needed only between desktop key sets and the gateway.

Terminal adapters connect a single legacy phone to an IP network. They perform coding and decoding and packetization, using digital signal processors (DSPs) for high performance. Some terminal adapters come bundled with a PC user interface. Others have a PSTN connection as a backup.

Soft phones are telephony applications that run on a PC. Soft phones typically use the PC’s main processor to perform coding and decoding and packetization, in addition to any other work being done on the PC. There are also IP phones that replace conventional phones and connect directly to the IP network, usually via an Ethernet jack. The coding and decoding and packetization functions are performed in the key set. There are also wireless IP phones, which perform the coding and decoding function in the handset.

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 or 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.

Centrex systems are easily expanded to accommodate customer growth by adding communication paths, memory, intercom lines, tie trunks, and CO lines as needed. Now, with IP connectivity, Centrex can be used for VPNs, virtual call centers, and economical telecommuter access. If there is a Centrex problem, repair is immediate and inexpensive. There is no need for a company to invest in spare-parts inventory, test equipment, or technical staff to take care of a PBX—the telephone company is responsible for all of that. And, as the CO switching equipment is updated, the Centrex services are also updated.

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