Centrex

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Typical standard phone used with Centrex. Note the "Recall" button and the Message Waiting Lamp.

Centrex is a contraction of central exchange, a kind of telephone exchange.

1 Use of Centrex
2 Birth of Centrex
3 Types of Centrex Service
4 Example Applications
5 Services

[edit] Use of Centrex

A PBX-like service providing switching at the central office instead of at the customer's premises. Typically, the telephone company owns and manages all the communications equipment and software necessary to implement the Centrex service and then sells various services to the customer.

No switching equipment resides on the customer premise as the service is supplied and managed directly from the phone company's exchange site, with lines being delivered to the premises either as individual lines over traditional copper pairs, or delivering many lines multiplexed to the premises over fibre optic or copper links, eliminating the need to bring many individual pairs from the exchange to the premises. In effect, Centrex provides an emulation of a hardware PBX, by using special software programming at the central office, which can be customized to meet a particular customer's needs. As with a PBX, stations inside the group can call each other with 3, 4 or 5 digits, depending on how large the group, instead of an entire telephone number.

Centrex obviates separate exchange lines delivered to a site for use with a Key telephone system or PBX. Instead, telephone extensions, called Centrex lines, are delivered directly from the local exchange to the user. Some customers, however, still like to use a key system for a small office within the large corporate Centrex. Unlike with a conventional PBX, it is a simple matter to have extensions at different locations while allowing them to function as if they were within the same building. Newer IP PBX systems also allow phones at any location with a WAN or Internet connection to act as a local extension. Facilities such as Direct Inward Dialing (DID), where individual extensions are offered a direct and unique telephone number for incoming calls, are standard features in a Centrex environment. Stations may also be part of a hunt group, allowing for automatic distribution of incoming calls to two or more extensions.

With the high price of Long Distance service, many large companies had their own network of private lines crossing the country or to distant countries. Managing these networks of 'Tie Lines' and connecting users to them was also an important part of Centrex.

[edit] Birth of Centrex

Centrex was invented in the middle 1960s by the Engineering Department of New York Telephone to replace the PBX switchboards of large customers. It was a feature package of the 5XB crossbar system. Much equipment had to be redesigned, including incoming trunks and markers. The redesigned equipment was so expensive that usually a separate 5XB switch was used just for Centrex customers, while POTS customers were wired to an unmodified exchange.

The PBX had the important feature that someone who received a call in error could jiggle the switchhook, causing a light to flash to recall the operator to connect the caller to the right extension. This feature was implemented in Centrex with a timing circuit to detect a hook flash and set up a three way connection among outside party, inside party and operator. The operator or "attendant" had a large and complex console, replacing the old, larger and even more complex switchboard, providing additional control over a connection. Customers who preferred the old cord switchboards could use an adapted "608" board.

Bell Labs took over Centrex development in the late 1960s. The NYTel version was designated Centrex I, and the Bell Labs revision Centrex II with additional features. In the early 1970s Centrex III arrived, a complete redesign bringing more versatility to the old 5XB system. The Centrex product line was extended down-market, with Centrex Small Business for customers having mere dozens of lines and no attendant console.

Meanwhile Stored Program Control came to live up to its promise of versatility, and Centrex customers were among the first in the middle 1970s to be removed from 5XB to 1ESS switch. Complex logic and storage was cheaper in a central computer than in individual trunk circuits, hence outgoing calls could be transferred as easily as incoming. Other manufacturers produced similar services, usually calling them something else due to trademark considerations. Northern Electric, later Nortel, called their version IBN or Integrated Business Network.

[edit] Types of Centrex Service

The Centrex customer is not restricted to using the features available to POTS (Plain Old Telephone Service) customers, but can choose from a wide variety of special services and features. In fact, telecommunications companies generally offer numerous types of Centrex service, including "Packaged Centrex", "Centrex Data", and "Customized Centrex".

[edit] Packaged Centrex

Packaged Centrex customers could be offered a fixed set of features in package "A", a different set of features in package "B", and so on. These packages can be offered at a relatively low cost as little or no customization is permitted, minimizing the operational costs of programming and maintaining the services for the telephone company.

[edit] Centrex Data

Centrex Data services can provide relatively low speed (56 and 64 kbit/s) data services utilizing the circuit-switched telephone network. Although they are now overshadowed by the internet and other data networks, Centrex Data services can offer very flexible and wide-reaching network configurations since connections can be made almost anywhere within the reach of the telephone network.

[edit] Customized Centrex

Customized Centrex is the most flexible (and most expensive) Centrex service as it offers a highly customizable set of options that require specialized programming and troubleshooting skills to maintain. A typical Customized Centrex setup will allow 4 digit dialing between "locals" (perhaps even if they are located in different parts of the city), customized routing through the telephone network (such as Least Cost Routing or Time of Day Routing), and customized codes for invoking features.

[edit] Example Applications

Small business start-ups (growth and costs)
Banks and financial institutions (branch offices/multi-departmental branches)
Professional offices (reliability, connectivity and customer service)
Local government (reliability, cost, multi-location)
Hotels and guest houses (customer service)
Colleges and Universities (reliability, cost, multi-location, growth)
Temporary Locations where permanent PBX services are impractical, such as a campaign office.