Centrex
Encyclopedia
Telephone exchange
In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls...
.
In the United Kingdom
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
, British Telecom markets this service as FeatureLine (although refers to it internally as Centrex).
Use of Centrex
Centrex is a PBX-like service providing switching at the central office instead of at the customer's premises. Typically, the telephone companyTelephone company
A telephone company is a service provider of telecommunications services such as telephony and data communications access. Many were at one time nationalized or state-regulated monopolies...
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's premises
Customer-premises equipment
Customer-premises equipment or customer-provided equipment is any terminal and associated equipment located at a subscriber's premises and connected with a carrier's telecommunication channel at the demarcation point...
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 by multiplexing
Multiplexing
The multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred...
a number of lines over a single fiber optic or copper link. 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 the need for separate exchange lines delivered to a site for use with a 1A2 Key System
1A2 Key System
The 1A2 Key System is an analog multiline business key telephone system. Unlike more modern multiline systems, every telephone line serving a particular phone is wired into that phone, and electromechanical switches switch the lines in the phone itself...
or similar, or PBX. Instead, telephone extensions
Extension (telephone)
An extension telephone is an additional telephone wired to the same telephone line as another. In middle 20th century telephone jargon, the first telephone on a line was a "Main Station" and subsequent ones "Extensions". Such extension phones allow making or receiving calls in different rooms,...
, called Centrex lines, are delivered directly from the local exchange to the user. Some customers, however, still like to use a key telephone system
Key telephone system
A business telephone system is any of a range of a multiline telephone systems typically used in business environments, encompassing systems ranging from small key systems to large scale private branch....
for a small office within the large corporate Centrex, in an arrangement called "key behind 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
IP PBX
An IP PBX is a business telephone system designed to deliver voice or video over a data network and interoperate with the normal Public Switched Telephone Network ....
systems also allow phones at any location with a WAN
Wide area network
A wide area network is a telecommunication network that covers a broad area . Business and government entities utilize WANs to relay data among employees, clients, buyers, and suppliers from various geographical locations...
or Internet
Internet
The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...
connection to act as a local extension. Facilities such as Direct Inward Dialing
Direct Inward Dialing
Direct inward dialing , also called direct dial-in in Europe and Oceania, is a feature offered by telephone companies for use with their customers' private branch exchange systems...
(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 line
Private line
In wired telephony, a private line or tie line is a service that involves dedicated circuits, private switching arrangements, and/or predefined transmission paths, whether virtual or physical, which provide communications between specific locations. Most private lines connect only two locations...
s 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.
Birth of Centrex
Centrex was invented in the mid 1960s by the Engineering Department of New York TelephoneNew York Telephone
The New York Telephone Company was organized in 1896, taking over the New York City operations of the American Bell Telephone Company.-Predecessor companies:...
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 marker
Marker (telecommunications)
A marker is a type of special purpose control system that was used in electromechanical telephone central office switches. Central office switches are the large devices that telephone companies use to make the connections that support telephone calls...
s. The redesigned equipment was so expensive that usually a separate 5XB switch was used just for Centrex customers, while POTS
Plain old telephone service
Plain old telephone service is the voice-grade telephone service that remains the basic form of residential and small business service connection to the telephone network in many parts of the world....
(Plain Old Telephone Service) 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 desired extension. The feature was implemented in Centrex with a timing circuit to detect this hook flash
Hook Flash
Hook flash or flash is a button on a telephone that simulates quickly hanging up then picking up again . This action can signal the telephone exchange to do something. A common use of hook flash is to switch to another incoming call with the call waiting service...
and set up a three way connection among outside party, inside party and operator. For this purpose the incoming trunk also needed its own data storage in the form of reed relay
Reed relay
A reed relay is a type of relay that uses an electromagnet to control one or more reed switches. The contacts are of magnetic material and the electromagnet acts directly on them without requiring an armature to move them...
packs to store the identity of the connected line.
The operator or "attendant" had a large and complex attendant console
Attendant console
An attendant console is a telephone station that is generally part of a private branch exchange or Centrex or other private telephone system...
, replacing the old, larger and even more complex switchboard and giving additional control of a connection. Its buttons and lights needed approximately fifty pairs of wires. Incoming calls arrived on an Attendant Trunk which, for customers with more than one console, waited for one to become idle and then connected to it, initially via Step by Step
Strowger switch
The Strowger switch, also known as Step-by-Step or SXS, is an early electromechanical telephone switching system invented by Almon Brown Strowger...
switches arranged as a Trunk Finder and Position Finder making up a small Automatic call distributor
Automatic call distributor
In telephony, an Automatic Call Distributor , also known as Automated Call Distribution, is a device or system that distributes incoming calls to a specific group of terminals that agents use. It is often part of a computer telephony integration system.Routing incoming calls is the task of the ACD...
some years before that term was invented. Customers who preferred the old cord switchboards could use an adapted "608" board. In either case the console communicated with the incoming trunk with coded plus and minus 130 V
Volt
The volt is the SI derived unit for electric potential, electric potential difference, and electromotive force. The volt is named in honor of the Italian physicist Alessandro Volta , who invented the voltaic pile, possibly the first chemical battery.- Definition :A single volt is defined as the...
DC
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
signals on Tip, Ring or both wires.
Bell Labs
Bell Labs
Bell Laboratories is the research and development subsidiary of the French-owned Alcatel-Lucent and previously of the American Telephone & Telegraph Company , half-owned through its Western Electric manufacturing subsidiary.Bell Laboratories operates its...
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. The unreliable Step switches were replaced by a Centrex Position Link Circuit (CXPL), a small, dedicated two stage crossbar network. In the early 1970s Centrex III arrived, a complete redesign bringing more versatility to the old 5XB system. Rather than incoming trunks having appearances on the ordinary Line Link Frame for transfer purposes, Centrex III (or Phase 3 Centrex) had a separate four stage Transfer Network of six-wire crossbar switches to connect to a Transfer Trunk. CXPL continued in use for connecting to consoles. Among other changes, the Phase 3 consoles used DTMF rather than multifrequency pulsing. The Centrex product line was also extended down-market, with Centrex Small Business for customers having mere dozens of lines and no attendant console.
Meanwhile Stored Program Control
Stored Program Control exchange
Stored Program Control exchange is the technical name used for telephone exchanges controlled by a computer program stored in the memory of the system. Early exchanges such as Strowger, panel, rotary, and crossbar switches were electromechanical and had no software 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
1ESS switch
The Number One Electronic Switching System, the first large-scale Stored Program Control telephone exchange or Electronic Switching System in the Bell System, was introduced in Succasunna, New Jersey, in May 1965. The switching fabric was composed of reed matrixes controlled by wire spring relays...
. 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 known as Nortel
Nortel
Nortel Networks Corporation, formerly known as Northern Telecom Limited and sometimes known simply as Nortel, was a multinational telecommunications equipment manufacturer headquartered in Mississauga, Ontario, Canada...
) called their version IBN or Integrated Business Network and enhanced the service with their proprietary P-phone sets in the 1980s.
Types of Centrex Service
The Centrex customer is not restricted to using the features available to POTSPlain old telephone service
Plain old telephone service is the voice-grade telephone service that remains the basic form of residential and small business service connection to the telephone network in many parts of the world....
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".
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.
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.
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
Least cost routing
In voice telecommunications, least-cost routing is the process of selecting the path of outbound communications traffic based on cost. Within a telecoms carrier, an LCR team might periodically choose between routes from several or even hundreds of carriers for destinations across the world...
or Time of Day Routing), and customized codes for invoking features.
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.
Services
- Call transferCall transferA call transfer is a telecommunications mechanism that enables a user to relocate an existing call to another phone or attendant console by using the transfer button and dialing the required location. The transferred call is either announced or unannounced....
- Call DivertCall forwardingCall forwarding , in telephony, is a feature on some telephone networks allowing an incoming call to a called party to be redirected to a third party. For example, the third party may be a mobile telephone, voicemail box or other telephone number where the desired called party is situated. It was...
– on no reply /on busy - Call WaitingCall waitingCall waiting , in telephony, is a feature on some telephone networks. If a calling party places a call to a called party which is otherwise engaged, and the called party has the call waiting feature enabled, the called party is able to suspend the current telephone call and switch to the new...
- Three Party Conference
- Call pickup (Group)
- Ring Back
- Reminder / Alarm call
- Last Number Redial
- Centrex HotlineHotlineIn telecommunication, a hotline is a point-to-point communications link in which a call is automatically directed to the preselected destination without any additional action by the user when the end instrument goes off-hook...
(non-dialed connection) - Centrex Warm Line (delayed Hotline)
- Centrex Hunt Groups, with optional bypass numbers