Number One Crossbar Switching System
Encyclopedia
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 Divided Multiple Switchboard
. Thus, it had separate incoming and outgoing sections. However, lines appeared only on the Line Link Frame (LLF), rather than requiring a multi-wire connection to two different sections as in a panel switch. The LLF uniting the telephone line
circuit at its "column and switch" simplified administration. The first 1XB was the PR7 exchange at Troy Avenue in Brooklyn, New York in 1937.
, which corresponded to the cord circuit
of the old cord telephone switchboard
. As in the panel switch, the Sender then found the chosen junctor and supplied dial tone
. Like the panel switch, the 1XB common control
was based on a complex, versatile Sender circuit. The Sender was able to accept Dial Pulses and (in later models) dual-tone multi-frequency signaling (DTMF). A large number of senders used a common translator circuit to detect a call going to a nearby area code to be stored in abbreviated form. It called in an auxiliary sender when necessary to implement Direct Distance Dialing
. Like the panel switch, two or more offices with separate incoming sections could share an outgoing section for more efficient trunking. Unlike the panel switch, it was rare to combine more than two this way.
es using the link principle required Incoming and Outgoing Marker
s to find an idle path and set up the switch train for each call. Earlier crossbar exchanges had used the crossbar switch according to the selector principle, with one input and typically 100 or 200 outputs, similar to a stepping switch
. 1XB pioneered the link principle, with each switch able to handle as many phone calls as it had inputs or outputs, typically ten. This innovation diminished the cost of switches, at the expense of more complex controls. The complexity of the circuitry challenged the ability of drawings to make it clear, leading to the later development of Detached Contact drawings, which in turn led to the application of Boolean algebra and Karnaugh map
s.
An Outgoing Marker, being a complex control instrument with a short holding time, had the additional job of decoding the first three digits of the seven digit telephone number
to determine the routing to the distant exchange. A cross connect field had a terminal for each office code, which was cross connected to the coil of a Route Relay. When the office code point was grounded, it operated the Route Relay, whose contacts were wired in another cross connect or data field. This told the Outgoing Marker which two Office Frames to search for idle trunks to the destination, and provided pulsing information to the Sender. The Sender sent the phone number to the distant terminating office. When one office was constructed, retired or changed, staff in other offices received a Routing Letter, ordering the cross connect fields to be changed at a particular date and time (usually after midnight) to accommodate the change in the network. Translation cross connect fields such as these were among the first to be converted from soldered terminals to wire wrap
.
If all trunks were busy, the Marker did a Route Advance to operate a different Route Relay to select an alternate route
via a Tandem. This feature allowed trunk groups to be smaller and more heavily loaded with traffic, thus saving cost in outside plant
.
to receive only the last four digits. Multi-frequency
Terminating Senders were introduced in the 1950s as part of Direct Distance Dialing
, and also used for incoming traffic from some local crossbar exchanges. The Terminating Sender activated an Incoming Marker, which then used a Number Group Circuit to find the line, marked an idle path, and operated the crossbar switches to use the links to connect the incoming trunk to the line.
The Revertive Pulse system as used in 1XB had a "High Five" feature by which the Incoming Brush parameter could be incremented by five. Thus, the new IB numbers 6 through 10 designated a second ten thousand telephone numbers, known as a "B" office. This allowed each 1XB incoming section to handle twenty thousand lines.
Multifrequency Terminating Senders accepted a fifth digit to discriminate among office codes served by the same office. The originating marker told the sender to delete the first two of seven digits in these cases. Sometimes the two office codes had the same third digit, in which case the first three digits for the "B" office were deleted and replaced with a single digit, indicated as AR for Arbitrary, usually a zero. The potential for five incoming digits to address a 100,000 line office was not exploited.
(XBT) replaced a Panel Sender Tandem, because its multi-frequency senders were able to receive seven digits and some were modified to accept ten. Sometimes it replaced an Office Select Tandem as well, since its Revertive Pulse senders were able to accept Office Brush and Group parameters. In big cities, some XBT were strictly incoming Class 4 telephone switch
es, some outgoing, a few both-way, and some only for tandem traffic within the metropolitan area. Specialization was less marked in less dense areas. XBT served local telephone companies until the late 20th Century when they were replaced by 4ESS switch
es or other digital switches.
or 1AESS Stored Program Control exchange
s.
Telephone exchange
In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls...
design used by the 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...
in the mid-20th century. Its switch fabric topology was based on the earlier urban 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...
system, which, in turn, was based on the turn of the century Divided Multiple Switchboard
Telephone switchboard
A switchboard was a device used to connect a group of telephones manually to one another or to an outside connection, within and between telephone exchanges or private branch exchanges . The user was typically known as an operator...
. Thus, it had separate incoming and outgoing sections. However, lines appeared only on the Line Link Frame (LLF), rather than requiring a multi-wire connection to two different sections as in a panel switch. The LLF uniting the telephone line
Telephone line
A telephone line or telephone circuit is a single-user circuit on a telephone communication system...
circuit at its "column and switch" simplified administration. The first 1XB was the PR7 exchange at Troy Avenue in Brooklyn, New York in 1937.
Subscriber sender
For outgoing calls, the Line Link Frame acted like the Line Finder of the panel switch, autonomously connecting the line to a junctorJunctor
A junctor is a circuit used in analog telephone exchanges, including the Number One Crossbar Switching System, Number Five Crossbar Switching System, Panel switch, 1ESS switch and other switches....
, which corresponded to the cord circuit
Cord circuit
In telecommunication, a cord circuit is a switchboard circuit in which a plug-terminated cord is used to establish connections manually between user lines or between trunks and user lines. A number of cord circuits are furnished as part of the switchboard position equipment. The cords may be...
of the old cord telephone switchboard
Telephone switchboard
A switchboard was a device used to connect a group of telephones manually to one another or to an outside connection, within and between telephone exchanges or private branch exchanges . The user was typically known as an operator...
. As in the panel switch, the Sender then found the chosen junctor and supplied dial tone
Dial tone
A dial tone is a telephony signal used to indicate that the telephone exchange is working, has recognized an off-hook, and is ready to accept a call. The tone stops when the first numeral is dialed...
. Like the panel switch, the 1XB 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...
was based on a complex, versatile Sender circuit. The Sender was able to accept Dial Pulses and (in later models) dual-tone multi-frequency signaling (DTMF). A large number of senders used a common translator circuit to detect a call going to a nearby area code to be stored in abbreviated form. It called in an auxiliary sender when necessary to implement Direct Distance Dialing
Direct distance dialing
Direct distance dialing or direct dial is a telecommunications term for a network-provided service feature in which a call originator may, without operator assistance, call any other user outside the local calling area. DDD requires more digits in the number dialed than are required for calling...
. Like the panel switch, two or more offices with separate incoming sections could share an outgoing section for more efficient trunking. Unlike the panel switch, it was rare to combine more than two this way.
Marker
Unlike the motor driven, clutch controlled panel switch selectors, crossbar switchCrossbar switch
In electronics, a crossbar switch is a switch connecting multiple inputs to multiple outputs in a matrix manner....
es using the link principle required Incoming and Outgoing 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 to find an idle path and set up the switch train for each call. Earlier crossbar exchanges had used the crossbar switch according to the selector principle, with one input and typically 100 or 200 outputs, similar to a stepping switch
Stepping switch
In electrical controls, a stepping switch, also known as a stepping relay, is an electromechanical device which allows an input connection to be connected to one of a number of possible output connections, under the control of a series of electrical pulses. It can step on one axis , or on two axes...
. 1XB pioneered the link principle, with each switch able to handle as many phone calls as it had inputs or outputs, typically ten. This innovation diminished the cost of switches, at the expense of more complex controls. The complexity of the circuitry challenged the ability of drawings to make it clear, leading to the later development of Detached Contact drawings, which in turn led to the application of Boolean algebra and Karnaugh map
Karnaugh map
The Karnaugh map , Maurice Karnaugh's 1953 refinement of Edward Veitch's 1952 Veitch diagram, is a method to simplify Boolean algebra expressions...
s.
An Outgoing Marker, being a complex control instrument with a short holding time, had the additional job of decoding the first three digits of the seven digit telephone number
Telephone number
A telephone number or phone number is a sequence of digits used to call from one telephone line to another in a public switched telephone network. When telephone numbers were invented, they were short — as few as one, two or three digits — and were given orally to a switchboard operator...
to determine the routing to the distant exchange. A cross connect field had a terminal for each office code, which was cross connected to the coil of a Route Relay. When the office code point was grounded, it operated the Route Relay, whose contacts were wired in another cross connect or data field. This told the Outgoing Marker which two Office Frames to search for idle trunks to the destination, and provided pulsing information to the Sender. The Sender sent the phone number to the distant terminating office. When one office was constructed, retired or changed, staff in other offices received a Routing Letter, ordering the cross connect fields to be changed at a particular date and time (usually after midnight) to accommodate the change in the network. Translation cross connect fields such as these were among the first to be converted from soldered terminals to wire wrap
Wire wrap
Wire wrap is a technology used to assemble electronics. It is a method to construct circuit boards without having to make a printed circuit board. Wires can be wrapped by hand or by machine, and can be hand-modified afterwards. It was popular for large-scale manufacturing in the 60s and early 70s,...
.
If all trunks were busy, the Marker did a Route Advance to operate a different Route Relay to select an alternate route
Routing in the PSTN
Routing in the PSTN is the process used to route telephone calls across the public switched telephone network. This process is the same whether the call is made between two phones in the same locality, or across two different continents....
via a Tandem. This feature allowed trunk groups to be smaller and more heavily loaded with traffic, thus saving cost in outside plant
Outside plant
In telecommunication, the term outside plant has the following meanings:*In civilian telecommunications, outside plant refers to all of the physical cabling and supporting infrastructure , and any associated hardware located between a demarcation point in a switching facility and a demarcation...
.
Terminating sender
At the terminating office, a Terminating Sender Link circuit connected a terminating sender to the trunk, in order to receive the phone number, typically using Revertive Pulse as in the panel switchPanel 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...
to receive only the last four digits. Multi-frequency
Multi-frequency
In telephony, multi-frequency signaling is an outdated, in-band signaling technique. Numbers were represented in a two-out-of-five code for transmission from a multi-frequency sender, to be received by a multi-frequency receiver in a distant telephone exchange...
Terminating Senders were introduced in the 1950s as part of Direct Distance Dialing
Direct distance dialing
Direct distance dialing or direct dial is a telecommunications term for a network-provided service feature in which a call originator may, without operator assistance, call any other user outside the local calling area. DDD requires more digits in the number dialed than are required for calling...
, and also used for incoming traffic from some local crossbar exchanges. The Terminating Sender activated an Incoming Marker, which then used a Number Group Circuit to find the line, marked an idle path, and operated the crossbar switches to use the links to connect the incoming trunk to the line.
The Revertive Pulse system as used in 1XB had a "High Five" feature by which the Incoming Brush parameter could be incremented by five. Thus, the new IB numbers 6 through 10 designated a second ten thousand telephone numbers, known as a "B" office. This allowed each 1XB incoming section to handle twenty thousand lines.
Multifrequency Terminating Senders accepted a fifth digit to discriminate among office codes served by the same office. The originating marker told the sender to delete the first two of seven digits in these cases. Sometimes the two office codes had the same third digit, in which case the first three digits for the "B" office were deleted and replaced with a single digit, indicated as AR for Arbitrary, usually a zero. The potential for five incoming digits to address a 100,000 line office was not exploited.
Tandem
One version of 1XB omitted the incoming section and Line Link Frames and replaced the junctor circuits with incoming trunks, leaving only the ability to connect those incoming trunks to outgoing trunks. In many cases the resulting Crossbar Tandem switchClass 4 telephone switch
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....
(XBT) replaced a Panel Sender Tandem, because its multi-frequency senders were able to receive seven digits and some were modified to accept ten. Sometimes it replaced an Office Select Tandem as well, since its Revertive Pulse senders were able to accept Office Brush and Group parameters. In big cities, some XBT were strictly incoming Class 4 telephone switch
Class 4 telephone switch
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....
es, some outgoing, a few both-way, and some only for tandem traffic within the metropolitan area. Specialization was less marked in less dense areas. XBT served local telephone companies until the late 20th Century when they were replaced by 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...
es or other digital switches.
Successor
1XB inspired the later 5XB switch which intensified the trend towards greater efficiency and complexity. 5XB installations seldom replaced 1XB but were operated side by side with them, or in towns too small to pay for 1XB. Most 1XB were replaced in the 1970s by 1ESS switch1ESS 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...
or 1AESS Stored Program Control exchange
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...
s.