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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. 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 computers 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 switches which were replacing the Step-by-Step switches and Panel switches of the first generation of automatic switching.
Markers were built from relays (wire spring relays 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.