The panel switching system was an early type of automatic 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...

, first put into urban service 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 1920s and removed during the 1970s. The Panel and Rotary systems were developed in parallel by 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...

 before World War I, and had many features in common, though the Rotary system was used in Europe.

The first Panel exchange was installed at the Mulberry Central Office in Newark, New Jersey

Newark, New Jersey

Newark is the largest city in the American state of New Jersey, and the seat of Essex County. As of the 2010 United States Census, Newark had a population of 277,140, maintaining its status as the largest municipality in New Jersey. It is the 68th largest city in the U.S...

. It was placed in service on January 16, 1915. It was a semi-automatic system using non-dial telephones. The next installation was in the Waverly C.O. June 12th. of the same year, also in Newark, NJ.

The Panel Machine Switching System (M.S.S.) was named for its tall panels covered with 500 rows of terminals. Each panel had an electric motor, to drive its (usually sixty) selectors by electromagnetically controlled clutches. The selector was similar in effect 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...

 though it moved continuously rather than in steps. Each selector had five brushes, any of which had 100 terminals among which it could select, arranged in groups. Pulses were sent back from the selector to a Register which had received the dialled digits, rather than forward as in the SXS system, hence the signaling was called "Revertive Pulse".

Telephone numbering

As in the SXS

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...

 system, each installed office could handle up to 10,000 numbered lines, requiring four digits for a line number within the office plus an office code.

The panel system was designed to interconnect the offices of a city or a local calling area. Each office had a two-digit code (later three). Callers dialed the office code followed by the line number within the office. In most situations this led to six-digit numbers (later seven). But from the beginning the panel system handled seven-digit numbers (later eight), for two reasons.

Party line numbers were listed with a J, M, R, or W following the line number. The caller dialed the office code, the line number, and the digit corresponding to the letter.

The panel system was designed to work with manual offices of up to 10,500 lines. Callers dialed the office code followed by the line number within the office. For lines 10,000 and up, callers therefore dialed the office code and a five-digit line number.

Line circuit

As in the divided-multiple 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...

 arrangements with which it served until it replaced them, Panel had an originating section and a terminating one, connected by a line circuit. As in the switchboard's "A Board", the originating section was organized into "panel and jack." At first, Panel was used only to replace the "B Board". All telephone line

Telephone line

A telephone line or telephone circuit is a single-user circuit on a telephone communication system...

s were connected to the incoming section for incoming calls, while operators at the cordboards continued to handle originating calls. The line circuit consisted of a Line Relay to indicate that a customer had gone off-hook

Off-hook

In telephony, the term off-hook has the following meanings:# The condition that exists when a telephone or other user instrument is in use, i.e., during dialing or communicating. Note: off-hook originally referred to the condition that prevailed when telephones had a separate earpiece , which hung...

, and a cutoff relay to keep the line relay from interfering with an established connection. The cutoff relay was controlled by a "sleeve" lead that, as with the multiple switchboard, could be activated by either the incoming section or the outgoing. The incoming final selector performed a sleeve test to detect a busy line. In the original "Ground Cutoff" (GCO) version of Panel, battery on the sleeve lead indicated that "busy tone" was to be returned to the caller. In the later, more fireproof and more numerous BCO offices, a grounded sleeve was the busy indicator.

Supervision or line signalling was supplied by a District Circuit, similar to the plug and light 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...

 that plugged into a line's TRS connector

TRS connector

A TRS connector is a common family of connector typically used for analog signals including audio. It is cylindrical in shape, typically with three contacts, although sometimes with two or four . It is also called an audio jack, phone jack, phone plug, and jack plug...

 on a switchboard. It supervised the calling and called party and, when both had gone on-hook

On-hook

In telephony, the term on-hook has the following meanings:# The condition that exists when a telephone or other user instrument is not in use, i.e., when idle waiting for a call. Note: on-hook originally referred to the storage of an idle telephone receiver, i.e., separate earpiece, on a switchhook...

, released the ground on the sleeve lead, thus releasing all selectors, which returned down to their start position to make ready for further traffic. Some District Frames were equipped with the more complex supervisory and timing circuits required to generate coin collect and return signals and otherwise handle payphone

Payphone

A payphone or pay phone is a public telephone, often located in a phone booth or a privacy hood, with pre-payment by inserting money , a credit or debit card, or a telephone card....

s.

Many of the urban and commercial areas where Panel was first used had mostly Message Rate service rather than flat rate

Flat rate

A flat fee, also referred to as a flat rate or a linear rate, refers to a pricing structure that charges a single fixed fee for a service, regardless of usage. Rarely, it may refer to a rate that does not vary with usage or time of use...

. For this reason the Line Finder had, besides the "tip and ring

Tip and ring

"Tip" and "Ring" are common terms in the telephone service industry referring to the two wires or sides of an ordinary telephone line. Tip is the ground side and Ring is the battery side of a phone circuit. In the UK these are referred to as the 'A' and 'B' wires...

" leads for talking and the "sleeve" lead for control, a fourth wire for the District Circuit to send metering pulse

Metering pulse

In telecommunications signalling, metering pulses are signals sent by telephone exchanges to metering boxes and payphones aimed at informing the latter of the cost of ongoing telephone calls....

s to control the message register. The introduction 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...

 in the 1950s required the addition of Automatic Number Identification

Automatic number identification

Automatic number identification is a feature of telephony intelligent network services that permits subscribers to display or capture the billing telephone number of a calling party. In the United States it is part of Inward Wide Area Telephone Service . ANI service was created by AT&T for...

 equipment to allow Centralized Automatic Message Accounting

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...

.

The incoming section of the office, being fixed to the MCDU structure of the last four digits of the 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...

, had a limit of 10,000 phone numbers, but in some of the urban areas where Panel was used, even a single square mile might have three or five times that many. Thus the incoming selectors of several separate switching entities would share floor space and staff, but required separate incoming trunk groups from distant offices. Sometimes an Office Select Tandem was used to distribute incoming traffic among the offices. This was a Panel office with no senders or other common equipment; just one stage of selectors and accepting only the Office Brush and Office Group parameters. Panel Sender Tandems were also used, when their greater capabilities were worth their additional cost.

Outgoing

The various switching entities in the building could share the same outgoing section, and this was particularly advantageous for trunks to particularly distant parts of the city, consolidating traffic that would otherwise be scattered among smaller and less efficient trunk groups or require using a tandem switch office.

Idle outgoing trunks were picked by the traditional "sleeve test" method, as lines were, except that hunting was the usual practice for trunks rather than a special service feature. The selector moved upward through twenty terminals, checking for one with an ungrounded sleeve lead, then selecting and grounding it (this in the Battery Cutoff version, which was the later, more fireproof and more widespread one). If no trunk was idle, the selector sent back an All Circuits Busy tone (reorder tone

Reorder tone

The reorder tone, also known as the fast busy tone, is the congestion tone or all trunks busy tone of a public switched telephone network. It varies from country to country; in the USA it is a dual-frequency tone of 480 Hz and 620 Hz at a cadence of 0.25 seconds on, 0.25 off; that is two beeps per...

). There was no provision for alternate routing as in earlier manual systems and later more sophisticated mechanical ones.

Sender

While the non-director Strowger

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...

 (step-by-step or SXS) switch moves synchronously with the dial pulses

Pulse dialing

Pulse dialing, dial pulse, or loop disconnect dialing, also called rotary or decadic dialling in the United Kingdom , is pulsing in which a direct-current pulse train is produced by interrupting a steady signal according to a fixed or formatted code for each digit and at a standard pulse repetition...

 that come from the telephone dial, the more sophisticated Panel switch had sender

Sender

A sender was a circuit in a 20th century electromechanical telephone exchange which sent telephone numbers and other information to another exchange. In some American exchange designs, for example 1XB switch the same term was also used to refer to the circuit that received this information...

s, similar to the directors of later Director telephone system

Director telephone system

The Director System was a system which made it possible to call subscribers at other telephone exchanges without operator intervention in large multi-exchange cities, and to have a mixture of automatic and manual exchanges within these cities...

 Strowger exchanges. The sender first translated the received digits into numbers appropriate for the selectors: District Brush, District Group, Office Brush, Office Group, Incoming Brush, Incoming Group, Final Brush, Final Tens, Final Units. Decoders helped by translating the first three digits of the phone number into four "District" and "Office" selecting numbers. Decoders also determined the proper rate at which to operate the message register and gave this information to the sender, which set this rate in the District Junctor. Auxiliary senders were added in the mid 20th century 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...

.

When the selector had activated the correct brush or group, the sender sent a brief open circuit signal to command the selector to stop there and prepare for the next number for the next stage. District and Office parameters were variable translations supplied by the decoder, while Incoming parameters and Final Brush were a fixed translation from the Thousands and Hundreds digits of the phone number, merely to adapt efficiently to the capabilities of the Panel selector.

In the 1930s when the 1XB switch crossbar switching system was introduced, it used the same Revertive Pulse Register signaling

Register signaling

In telecommunications, register signaling provides addressing information, such as the calling and/or called telephone number. R2 register signaling is an example.This is contrasted with line signaling....

 system, not only to control panel selectors but to signal within itself and with similar exchanges. Later 5XB switch, 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...

 and other systems included RP equipment in order to maintain compatibility, in some cases decades after the last Panel switch in the city had been scrapped.

Stuck sender

Revertive Pulse was faster than dial pulse, but the greater advantage was when something went wrong. In earlier systems when a worn pawl or other problem in a Strowger selector caused it to fail to advance, nobody knew except the calling party

Calling party

The calling party is a person who initiates a telephone call over the public switched telephone network, usually by dialing a telephone number....

, who only knew nothing was happening. The caller eventually lost patience, redialled, and might easily get stuck again on the same selector, or another caller could get stuck there. One bad Strowger selector could block dozens of calls per hour until subscriber complaints led staff to discover it.

With RP, the pulses were going backwards to the sender, a complex and sophisticated piece of hardware. If a selector failed to advance, it stopped sending pulses to the sender. A timer

Timeout (telecommunication)

In telecommunication and related engineering , the term timeout or time-out has several meanings, including...

 in the sender detected the failure, returned a trouble tone to the caller, held the switch train out of service with a grounded sleeve lead so no other caller would use the faulty circuit, and sounded an alarm. Staff could then trace the stuck sender, and identify and repair the defect while the caller tried again and usually succeeded.

Interoffice signaling

For panel-office-to-panel-office calls, the two offices communicated by revertive pulsing. For calls within a single panel office, the two halves of the office also communicated by revertive pulsing.

For compatibility with manual offices, Panel Call Indicator

Panel call indicator

Panel Call Indicator, or PCI, is a form of signalling used between two telephone offices.PCI was designed along with the panel type of telephone office...

 (PCI) signaling was used. PCI used multilevel DC pulses, for a bit to baud

Baud

In telecommunications and electronics, baud is synonymous to symbols per second or pulses per second. It is the unit of symbol rate, also known as baud rate or modulation rate; the number of distinct symbol changes made to the transmission medium per second in a digitally modulated signal or a...

 ratio of 2:1. PCI signalling lit lamps on the B operator

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...

's desk at the terminating manual office. Another type of signaling, Call Annunciator, used speech recorded on strips of photographic film

Photographic film

Photographic film is a sheet of plastic coated with an emulsion containing light-sensitive silver halide salts with variable crystal sizes that determine the sensitivity, contrast and resolution of the film...

 to announce the called number to the answering operator.

PCI continued in use for tandem purposes, decades after its original purpose had disappeared. In the 1950s Auxiliary Senders were added, to allow storing more than eight digits, and sending by MF

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...

 for 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...

. MF was also used by Crossbar and ESS exchanges.

Compatibility with manual offices

Panel was installed in cities where many people still had manual (non-dial) service. Calls from panel offices to manual offices required PCI signals to tell the "B Board Machine Incoming" operator or incoming operator the phone number. The number appeared on a lit display (or indicator). The operator completed the call in the usual way.

Calls from manual offices to panel offices required the "A board" or outgoing operator to request the number from the caller, connect to an idle trunk to the distant exchange and relay to the "B Board Manual Incoming Call" operator the desired number. The "B Board Manual Incoming Call" operator would then key in the desired number. The Panel machine would then setup the incoming and final frames to the called telephone number.

Motor

Panel is an example of a power drive system. Strowger or crossbar systems, in contrast, use individual electromagnets for operation: the power available from an electromagnet limits the maximum size of the switch element it can move. Panel having no such restriction, the dimensions of the panel were determined solely by the needs of the switch and the design of the exchange, as the driving electric motor can be made as large as is necessary to move the switch elements. Thus, most calls required only about half as many stages as in earlier and later systems. Motors used on Panel frames operated on A.C. or D.C. however they could only be started on D.C. In the event of an A.C. power failure the motor would switch over to its D.C. windings.

External links

• Short movie of a Panel call from Seattle Museum of Communications.
• Short movie about early Panel Archive Footage.
• Photos & descriptions last working Panel Office Seattle Museum of Communications.
• More Panel photos can be found at WECo Panel Office.
• A good description of the Panel switch can be found in the Survey of Telephone Switching.
• A Panel Final Selector frame is on display at the Museum of Communications.
• Some 1970s-era recordings of telephone calls involving panel switches by Evan Doorbell can be found at Phone Trips, including one presentation devoted exclusively to "panel pulsing".
• The sounds of a rotary dial call originating in a Crossbar office (old dialtone) going through a XB Tandem office MF and terminating in a Panel office (MP3 Format)(hear the revertive pulsing) followed by machine ringing and the clicks of the Panel test line.
• Machine-switching in 1911 Britannica