Marker (telecommunications)
Encyclopedia
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 call
s. The switch makes voice connections between users, and to control equipment such as that used to detect dialed numbers.
Markers were sometimes referred to as special purpose computer
s but, lacking stored program control
, they were not computers according to the understanding of the middle 20th Century. After unfruitful German efforts in the 1920s, they were successfully developed at Bell Labs
in the 1930s to support the then new generation of crossbar switch
es which were replacing the Step-by-Step switch
es and Panel switch
es of the first generation of automatic switching.
Markers were built from relays (wire spring relay
s and other kinds). Different types of markers performed various specialized hard-wired operations. For example, 1XB switches had separate markers for incoming and outgoing calls. 5XB switches had dial tone markers to select one of a number of shared digit receivers (termed originating registers) and connect it to a subscriber who wished to make a telephone call. The digit receiver would collect the digits of the call and make them available to other markers which would use them for routing purposes. In this case the Completing Marker would mark a proper path of idle links for the call to make through the mechanical voice switching matrix.
Markers were used in the design of switches from the 1930s until the late 1960s when they were replaced with software controlled electronic computers of modern design.
The term marker came from its use to mark a path of links through the switching fabric
. A marker's comprehensive view of the switching fabric allowed it to find and assemble a path from one terminal to another, if the links were available, unlike the earlier graded progressive systems in which a path might not be found.
During the middle 20th Century markers in Bell System
exchanges, being complex common control
circuits with short holding time, acquired other functions that were only needed once or twice per telephone call
, including outgoing digit translation and enforcement of different policies upon different classes of service
in the provision of features to customers. This practice evolved into Customer Groups, allowing the addition of Centrex
features to 5XB switch. These were the most complex markers made, and were abandoned in the 1970s and 1980s when Stored Program Control
became mature.
Markers were mostly associated with crossbar switches, but many non Bell System crossbar exchanges did not use them. Where those exchanges had markers, for example in the British TXK
or the Ericsson ARM, they were simpler, the digit translation jobs that were added to Bell System markers being handled by other equipment.
Telephone exchange
In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls...
are the large devices that telephone companies use to make the connections that support telephone call
Telephone call
A telephone call is a connection over a telephone network between the calling party and the called party.-Information transmission:A telephone call may carry ordinary voice transmission using a telephone, data transmission when the calling party and called party are using modems, or facsimile...
s. The switch makes voice connections between users, and to control equipment such as that used to detect dialed numbers.
Markers were sometimes referred to as special purpose computer
Computer
A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...
s but, lacking 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...
, they were not computers according to the understanding of the middle 20th Century. After unfruitful German efforts in the 1920s, they were successfully developed at 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...
in the 1930s to support the then new generation of crossbar switch
Crossbar switch
In electronics, a crossbar switch is a switch connecting multiple inputs to multiple outputs in a matrix manner....
es which were replacing the Step-by-Step switch
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...
es and Panel switch
Panel switch
The panel switching system was an early type of automatic telephone exchange, first put into urban service by the Bell System in the 1920s and removed during the 1970s...
es of the first generation of automatic switching.
Markers were built from relays (wire spring relay
Wire spring relay
A wire spring relay is a type of relay, primarily manufactured by the Western Electric Company for use by the Bell System in electromechanical telephone exchanges. It was licenced for use around the world, and was commonplace in Japan...
s and other kinds). Different types of markers performed various specialized hard-wired operations. For example, 1XB switches had separate markers for incoming and outgoing calls. 5XB switches had dial tone markers to select one of a number of shared digit receivers (termed originating registers) and connect it to a subscriber who wished to make a telephone call. The digit receiver would collect the digits of the call and make them available to other markers which would use them for routing purposes. In this case the Completing Marker would mark a proper path of idle links for the call to make through the mechanical voice switching matrix.
Markers were used in the design of switches from the 1930s until the late 1960s when they were replaced with software controlled electronic computers of modern design.
The term marker came from its use to mark a path of links through the switching fabric
Switched fabric
Switched fabric, switching fabric, or just fabric, is a network topology where network nodes connect with each other via one or more network switches . The term is popular in telecommunication, Fibre Channel storage area networks and other high-speed networks, including InfiniBand...
. A marker's comprehensive view of the switching fabric allowed it to find and assemble a path from one terminal to another, if the links were available, unlike the earlier graded progressive systems in which a path might not be found.
During the middle 20th Century markers in Bell System
Bell System
The Bell System was the American Bell Telephone Company and then, subsequently, AT&T led system which provided telephone services to much of the United States and Canada from 1877 to 1984, at various times as a monopoly. In 1984, the company was broken up into separate companies, by a U.S...
exchanges, being complex common control
Common control
In telecommunication, a common control is an automatic telephone exchange arrangement in which the control equipment necessary for the establishment of connections is shared by being associated with a given call only during the period required to accomplish the control function for the given call...
circuits with short holding time, acquired other functions that were only needed once or twice per telephone call
Telephone call
A telephone call is a connection over a telephone network between the calling party and the called party.-Information transmission:A telephone call may carry ordinary voice transmission using a telephone, data transmission when the calling party and called party are using modems, or facsimile...
, including outgoing digit translation and enforcement of different policies upon different classes of service
Class of service
-Class of service :As related to network technology, CoS is a 3-bit field within an Ethernet frame header when using 802.1Q tagging. The field specifies a priority value of between 0 and 7 inclusive that can be used by quality of service disciplines to differentiate traffic.While CoS operates only...
in the provision of features to customers. This practice evolved into Customer Groups, allowing the addition of Centrex
Centrex
Centrex is a portmanteau of central exchange, a kind of telephone exchange.In the United Kingdom, British Telecom markets this service as FeatureLine .-Use of Centrex:...
features to 5XB switch. These were the most complex markers made, and were abandoned in the 1970s and 1980s when 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...
became mature.
Markers were mostly associated with crossbar switches, but many non Bell System crossbar exchanges did not use them. Where those exchanges had markers, for example in the British TXK
TXK
TXK was a range of Crossbar exchanges used by the British Post Office telephone network, subsequently BT, between 1964 and 1994. TXC was used as the designation at first, but this was later changed as TXC sounded too much like TXE the code used for later electronic exchanges...
or the Ericsson ARM, they were simpler, the digit translation jobs that were added to Bell System markers being handled by other equipment.