Class 4 telephone switch
Encyclopedia
A Class 4, or Tandem, telephone switch is a U.S. telephone company
central office telephone exchange
used to interconnect local exchange carrier
offices for long distance communications in the Public Switched Telephone Network
.
A Class 4 switch doesn't connect directly to any telephones; instead, it connects to other Class 4 switches and to Class 5 telephone switches. The telephones of people who subscribe to telephone service are wired to Class 5 switches. When a call is placed to a telephone that isn't on the same Class 5 switch as the subscriber, the call may be routed through one or more Class 4 switches to reach its destination.
derives from the Latin adverb tandem meaning "at length," and is used in English to mean a group of two people or machines working together, usually in series. A "tandem switch" is used to connect two other switches to each other via trunk lines. Thus, trunk switches are always part of a series of switches and lines that connect telephone callers to each other.
(LATA).
An access tandem switch connects local telephone exchanges to long-distance telephone companies (or interexchange carrier
s, "IXCs"). The point at which an access tandem connects to the IXC's switch is called the point of presence
, or POP.
Modern tandem switches are often located at the center of the areas they serve, and may act as both sector tandems and access tandems.
, Class 4 switches in a telephone office that had operators
present were called "toll centers;" if no operators were present, they were called "toll points." Either type of Class 4 switch might be referred to as a "toll switch." These terms were used because long-distance, or "toll," calls had to pass through Class 4 switches, where the billing for the calls would be handled.
Class 4 switches at that time often had an associated Traffic Service Position System (TSPS) to handle operator-assisted calls. TSPS automated many functions previously handled by the local operator with a "cordboard" telephone switch, such as certain aspects of coin-operated telephone calls. It also allowed the telephone company to route operator calls to remote locations, rather than requiring operators at each switch.
After the divestiture, as human operators became less common, the terms changed. Today, a Class 4 switch that connects Class 5 switches to the long-distance network is called an "access tandem." A Class 4 switch that connects Class 5 switches to each other, but not to the long-distance network, is called a "local tandem."
The majority of Class 4 switches in the Bell System during the 1950s and 1960s used crossbar switch
es, such as the Crossbar Tandem (XBT) variant of the Number One Crossbar Switching System
, or 1XB switch. The Number 4 Crossbar ("4XB") Tandem switch was used in the North American toll network from 1943 until the 1990s, when it was replaced by more modern digital switching equipment, such as the Lucent 4ESS switch
or the Nortel DMS-200
. The last 4XB switch in the United States was installed in 1976.
During the 1980s, Class 4 tandem switches were converted to deal only with high-speed digital four-wire circuit
connections: T1
, T3
, OC-3, etc. The two-wire
local line connections to individual telephones were relegated to the Class 5 switches. By the dawn of the 21st century, almost all other switches also supported four-wire connections.
Modern tandem switches, like other classes of telephone switch, are digital, and use time-division multiplexing
(TDM) to carry circuit-switched telephone calls. Tandems were more quickly converted to TDM than the Class 5 end-offices were. During the transition to digital switching in the 1980s and 1990s, when both TDM and traditional "space division""Space division" is a retronym
used to distinguish traditional telephone trunk lines—where a call would fully occupy a set of wires within a "trunk," or bundle of wires, between switches—from the new TDM trunks, where more than one call could be placed on a pair of wires by digitizing the call and sending the data for each call in pre-defined "timeslots" assigned to the call. switches were in use, American phone company employees often referred tandems as "TDM switches" as a result.
In the past, most of the accounting
, billing management, and call record-keeping
was handled by the tandem switches. During the last third of the 20th century, these tasks were performed by the Class 5 end-office switches.
Other Class 5 digital switches are often used as Class 4 switches for smaller applications.
Telephone 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...
central office telephone exchange
Telephone exchange
In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls...
used to interconnect local exchange carrier
Local exchange carrier
Local Exchange Carrier is a regulatory term in telecommunications for the local telephone company.In the United States, wireline telephone companies are divided into two large categories: long distance and local...
offices for long distance communications in the Public Switched Telephone Network
Public switched telephone network
The public switched telephone network is the network of the world's public circuit-switched telephone networks. It consists of telephone lines, fiber optic cables, microwave transmission links, cellular networks, communications satellites, and undersea telephone cables, all inter-connected by...
.
A Class 4 switch doesn't connect directly to any telephones; instead, it connects to other Class 4 switches and to Class 5 telephone switches. The telephones of people who subscribe to telephone service are wired to Class 5 switches. When a call is placed to a telephone that isn't on the same Class 5 switch as the subscriber, the call may be routed through one or more Class 4 switches to reach its destination.
Taxonomy
TandemTandem
Tandem is an arrangement where a team of machines, animals or people are lined up one behind another, all facing in the same direction....
derives from the Latin adverb tandem meaning "at length," and is used in English to mean a group of two people or machines working together, usually in series. A "tandem switch" is used to connect two other switches to each other via trunk lines. Thus, trunk switches are always part of a series of switches and lines that connect telephone callers to each other.
Sector and access tandems
A sector tandem switch connects local telephone exchanges (Class 5 switches) and carries traffic within the local access and transport areaLocal access and transport area
Local access and transport area is a term used in U.S. telecommunications regulation. It represents a geographical area of the United States under the terms of the that precipitated the breakup of the original AT&T into the "Baby Bells" or created since that time for wireline...
(LATA).
An access tandem switch connects local telephone exchanges to long-distance telephone companies (or interexchange carrier
Interexchange carrier
An Interexchange Carrier is a U.S. legal and regulatory term for a telecommunications company, commonly called a long-distance telephone company, such as MCI , Sprint and the former AT&T in the United States...
s, "IXCs"). The point at which an access tandem connects to the IXC's switch is called the point of presence
Point of presence
A point of presence is an artificial demarcation point or interface point between communications entities. It may include a meet-me-room.In the US, this term became important during the court-ordered breakup of the Bell Telephone system...
, or POP.
Modern tandem switches are often located at the center of the areas they serve, and may act as both sector tandems and access tandems.
History
Before the Bell System divestitureBell System divestiture
The Bell System divestiture, or the breakup of AT&T, was initiated by the filing in 1974 by the U.S. Department of Justice of an antitrust lawsuit against AT&T. The case, United States v...
, Class 4 switches in a telephone office that had operators
Telephone operator
A telephone operator is either* a person who provides assistance to a telephone caller, usually in the placing of operator assisted telephone calls such as calls from a pay phone, collect calls , calls which are billed to a credit card, station-to-station and person-to-person calls, and certain...
present were called "toll centers;" if no operators were present, they were called "toll points." Either type of Class 4 switch might be referred to as a "toll switch." These terms were used because long-distance, or "toll," calls had to pass through Class 4 switches, where the billing for the calls would be handled.
Class 4 switches at that time often had an associated Traffic Service Position System (TSPS) to handle operator-assisted calls. TSPS automated many functions previously handled by the local operator with a "cordboard" telephone switch, such as certain aspects of coin-operated telephone calls. It also allowed the telephone company to route operator calls to remote locations, rather than requiring operators at each switch.
After the divestiture, as human operators became less common, the terms changed. Today, a Class 4 switch that connects Class 5 switches to the long-distance network is called an "access tandem." A Class 4 switch that connects Class 5 switches to each other, but not to the long-distance network, is called a "local tandem."
The majority of Class 4 switches in the Bell System during the 1950s and 1960s used crossbar switch
Crossbar switch
In electronics, a crossbar switch is a switch connecting multiple inputs to multiple outputs in a matrix manner....
es, such as the Crossbar Tandem (XBT) variant of the Number One Crossbar Switching System
Number One Crossbar Switching System
The Number One Crossbar Switching System, or 1XB switch, was the primary urban local telephone exchange design used by the Bell System in the mid-20th century. Its switch fabric topology was based on the earlier urban panel switch system, which, in turn, was based on the turn of the century...
, or 1XB switch. The Number 4 Crossbar ("4XB") Tandem switch was used in the North American toll network from 1943 until the 1990s, when it was replaced by more modern digital switching equipment, such as the Lucent 4ESS switch
4ESS switch
The 4ESS switch is a Class 4 telephone Electronic Switching System that was the first digital electronic toll switch introduced by Western Electric for long distance switching. It was introduced in 1976 in Chicago, Illinois to replace the 4a crossbar switch. The last of 145 in the AT&T network was...
or the Nortel DMS-200
Digital Multiplex System
Digital Multiplex System is the name shared among several different telephony product lines from Nortel Networks for wireline and wireless operators...
. The last 4XB switch in the United States was installed in 1976.
During the 1980s, Class 4 tandem switches were converted to deal only with high-speed digital four-wire circuit
Four-wire circuit
In telecommunication, a four-wire circuit is a two-way circuit using two paths so arranged that the respective signals are transmitted in one direction only by one path and in the other direction by the other path...
connections: T1
T-carrier
In telecommunications, T-carrier, sometimes abbreviated as T-CXR, is the generic designator for any of several digitally multiplexed telecommunications carrier systems originally developed by Bell Labs and used in North America, Japan, and South Korea....
, T3
T-carrier
In telecommunications, T-carrier, sometimes abbreviated as T-CXR, is the generic designator for any of several digitally multiplexed telecommunications carrier systems originally developed by Bell Labs and used in North America, Japan, and South Korea....
, OC-3, etc. The two-wire
Twisted pair
Twisted pair cabling is a type of wiring in which two conductors are twisted together for the purposes of canceling out electromagnetic interference from external sources; for instance, electromagnetic radiation from unshielded twisted pair cables, and crosstalk between neighboring pairs...
local line connections to individual telephones were relegated to the Class 5 switches. By the dawn of the 21st century, almost all other switches also supported four-wire connections.
Modern tandem switches, like other classes of telephone switch, are digital, and use time-division multiplexing
Time-division multiplexing
Time-division multiplexing is a type of digital multiplexing in which two or more bit streams or signals are transferred apparently simultaneously as sub-channels in one communication channel, but are physically taking turns on the channel. The time domain is divided into several recurrent...
(TDM) to carry circuit-switched telephone calls. Tandems were more quickly converted to TDM than the Class 5 end-offices were. During the transition to digital switching in the 1980s and 1990s, when both TDM and traditional "space division""Space division" is a retronym
Retronym
A retronym is a type of neologism that provides a new name for an object or concept to differentiate the original form or version of it from a more recent form or version. The original name is most often augmented with an adjective to account for later developments of the object or concept itself...
used to distinguish traditional telephone trunk lines—where a call would fully occupy a set of wires within a "trunk," or bundle of wires, between switches—from the new TDM trunks, where more than one call could be placed on a pair of wires by digitizing the call and sending the data for each call in pre-defined "timeslots" assigned to the call. switches were in use, American phone company employees often referred tandems as "TDM switches" as a result.
In the past, most of the accounting
FCAPS
FCAPS is the ISO Telecommunications Management Network model and framework for network management. FCAPS is an acronym for fault, configuration, accounting, performance, security, the management categories into which the ISO model defines network management tasks...
, billing management, and call record-keeping
Automatic Message Accounting
Automatic message accounting provides detail billing for telephone calls. When direct distance dialing was introduced in the US, message registers no longer sufficed for dialed telephone calls...
was handled by the tandem switches. During the last third of the 20th century, these tasks were performed by the Class 5 end-office switches.
Switching equipment
- The Lucent 4ESS4ESS switchThe 4ESS switch is a Class 4 telephone Electronic Switching System that was the first digital electronic toll switch introduced by Western Electric for long distance switching. It was introduced in 1976 in Chicago, Illinois to replace the 4a crossbar switch. The last of 145 in the AT&T network was...
is a digital switch widely used as a Class 4 switch in the United States. It was developed by AT&T's Western ElectricWestern ElectricWestern Electric Company was an American electrical engineering company, the manufacturing arm of AT&T from 1881 to 1995. It was the scene of a number of technological innovations and also some seminal developments in industrial management...
division, before that division was spun off as Lucent. The first 4ESS was installed in ChicagoChicagoChicago is the largest city in the US state of Illinois. With nearly 2.7 million residents, it is the most populous city in the Midwestern United States and the third most populous in the US, after New York City and Los Angeles...
in 1976. The last 4ESS in the AT&T Long Lines network was installed in 1999. - The Lucent 5ESS5ESS SwitchThe 5ESS Switch is a Class 5 telephone electronic switching system sold by Alcatel-Lucent. This digital central office telephone circuit switching system is used by many telecommunications service providers.-History:...
, a Class 5 switching system, is sometimes used as a Class 4 switch (or as a mixed Class 4/5 switch) in markets that are too small to justify a 4ESS switch. - The Nortel DMS-250Digital Multiplex SystemDigital Multiplex System is the name shared among several different telephony product lines from Nortel Networks for wireline and wireless operators...
, a larger variant of the DMS-100DMS-100The DMS-100 Switch is a line of Digital Multiplex System telephone exchange switches manufactured by Nortel Networks.The purpose of the DMS-100 Switch is to provide local service and connections to the PSTN public telephone network. It is designed to deliver services over subscribers' telephone...
, is a popular competitor to Lucent's 4ESS, especially among telephone companies that were not previously a part of AT&T. Other DMS switches can also be used as tandems. - The Nortel SP1 4-Wire was an early electronic switch used as a Class 4 switch.
Other Class 5 digital switches are often used as Class 4 switches for smaller applications.