Panel call indicator

Last updated

Panel Call Indicator, or PCI, is a form of signalling used between two telephone offices. It was also originally called Relay Call Indicator (RCI).

Contents

Originally designed along with the panel type telephone office, PCI was intended to allow subscribers in fully automated exchanges to dial numbers in manual offices the same way they dialed numbers in their own exchange. For PCI to achieve its purpose, the panel office sent the requested number to the manual office, where the number was lit on an operator's display. The switchboard operator at a PCI-aware manual office reads the number from the call indicator display and completes the call in the usual way. As a format of interoffice signaling, PCI is one of multiple options retained for compatibility in the #5 Crossbar switch, a later system that served as the platform for the initial DTMF push-button telephone services.

In the British Director telephone system, Coded-Call Indicator working (CCI) filled a similar role, as it displayed dialed telephone numbers at the local manual exchange in a mixed (automatic and manual) linked-number area. [1]

What is sent

A Call Indicator on display at the Museum of Communications Panel Call Indicator Display.jpg
A Call Indicator on display at the Museum of Communications

The usual seven-digit US telephone number consists of a three-digit office code and a four-digit line number within the given office. After a caller in a panel office dialed the office code, the sender looked up the type of the called office (panel, manual, etc.), and whether PCI was to be used. Then the sender found a trunk to the called office. Once a trunk had been found and the decision to use PCI had been made, the sender outpulsed the digits to the manual office where they were displayed for the operator.

Because the operator only needed the last four digits of the phone number to complete the call, the first three dialed digits were not sent to the manual office. However, for two reasons, PCI always sends five-digit numbers rather than four (and the caller may need to dial eight digits rather than seven).

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. For offices with fewer than 10,000 lines, callers dialed four digits but PCI sends a leading 0.

Also, when the panel office was designed, many people used party lines. 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. Party letters replace the ten-thousands digit described above. That is, PCI sends the letter followed by the four-digit line number. So party letters and line numbers above 10,000 can't be used together.

In special cases, calls through a tandem office also used PCI signaling. This originated with manual operator tandems, and was later carried on to machine-based tandems as well. For this special tandem class, all seven dialed digits were sent to the distant office. In the case of a manual tandem, operators had a special seven segment display which displayed the entire telephone number. In later mechanical tandems, PCI was used because it was faster than revertive pulse signaling, and would thus allow for quicker call completion times.

PCI encoding

Digits are sent in order from most to least significant. The party letter is sent first, since it replaces the ten-thousands digit.

Generally, each digit is sent as a group of four bits having the place values 1, 2, 4, and 5. This could be considered a form of biquinary code. The thousands digit is sent using binary-coded decimal, but with the 1 bit moved to the end of the group. Except for the 1 bit in the thousands digit, all bits are sent in order from least to most significant. An entire transmission uses twenty bits.

Looking at the transmission another way, it consists of twenty "time slots" which may be filled by DC pulses. The first and third pulse times in a group of four contain either a positive DC current or no current. The second and fourth pulse times in a group of four contain either a light (high resistance) negative DC pulse or a heavy (low resistance) negative DC pulse. The pattern of current flow (positive, negative, positive, negative) keeps the receiver synchronized with the sender. This pattern never varies so it contains no information. The existence of a current (in the first and third pulse times) or the resistance of the current (in the second and fourth times) carries the information.

In the below table, + represents a positive pulse, - represents a light negative pulse, and ▭ represents a heavy negative pulse. Note that the thousands digit (as transmitted) is coded differently than the hundreds, tens, and units digits.

Panel-Call-Indicator-Code.png

Below, is a graph of PCI pulses transmitted from the Panel switch at the Museum of Communications in Seattle. The positive and negative pulses occur on physically separate leads, but are overlaid on the same graph for simplicity, so the values of the Y axis are not absolute. Indicated on the graph are the start and end of the transmission (bottom), and the actual digits being transmitted (top).

A graph of PCI pulses transmitted from the Panel switch at the Museum of Communications in Seattle. The positive and negative pulses occur on physically separate leads, but are overlaid on the same graph for simplicity, so the values of the Y axis are not absolute. PCI-signaling-graph.png
A graph of PCI pulses transmitted from the Panel switch at the Museum of Communications in Seattle. The positive and negative pulses occur on physically separate leads, but are overlaid on the same graph for simplicity, so the values of the Y axis are not absolute.

CCI encoding

The Director telephone exchange was based on a store-and-forward handling of dialled numbers, originally with seven-digit numbers. The director would receive all seven digits, use the first three to determine the destination exchange, then send the last four digits onward. For CCI working, the director would wait until the equipment at the destination was ready and then send each digit as a series of current pulses. Much like PCI signalling, each pulse had three possible levels (weak positive current, weak negative current, strong negative current) and a digit consisted of at most four pulses.

Related Research Articles

<span class="mw-page-title-main">Rotary dial</span> Component that allows dialing numbers

A rotary dial is a component of a telephone or a telephone switchboard that implements a signaling technology in telecommunications known as pulse dialing. It is used when initiating a telephone call to transmit the destination telephone number to a telephone exchange.

Direct distance dialing (DDD) is a telecommunications service feature in North America by which a caller may, without operator assistance, call any other user outside the local calling area. Direct dialing by subscribers typically requires extra digits to be dialed as prefixes to the directory telephone number of the destination. International Direct Distance Dialing (IDDD) extends the system beyond the geographic boundaries of the North American Numbering Plan (NANP).

<span class="mw-page-title-main">Emergency telephone number</span> Telephone number that allows caller to contact local emergency services for assistance

An emergency telephone number is a number that allows a caller to contact local emergency services for assistance. The emergency number differs from country to country; it is typically a three-digit number so that it can be easily remembered and dialed quickly. Some countries have a different emergency number for each of the different emergency services; these often differ only by the last digit.

<span class="mw-page-title-main">Blue box</span> Device for hacking telephone networks

A blue box is an electronic device that produces tones used to generate the in-band signaling tones formerly used within the North American long-distance telephone network to send line status and called number information over voice circuits. During that period, charges associated with long-distance calling were commonplace and could be significant, depending on the time, duration and destination of the call. A blue box device allowed for circumventing these charges by enabling an illicit user, referred to as a "phreaker" to place long-distance calls, without using the network's user facilities, that would be billed to another number or dismissed entirely by the telecom company's billing system as an incomplete call. A number of similar "color boxes" were also created to control other aspects of the phone network.

<span class="mw-page-title-main">North American Numbering Plan</span> Integrated telephone numbering plan of twenty North American countries

The North American Numbering Plan (NANP) is a telephone numbering plan for twenty-five regions in twenty countries, primarily in North America and the Caribbean. This group is historically known as World Zone 1 and has the telephone country code 1. Some North American countries, most notably Mexico, do not participate with the NANP.

Automatic number identification (ANI) is a feature of a telecommunications network for automatically determining the origination telephone number on toll calls for billing purposes. Automatic number identification was originally created by the American Telephone and Telegraph Company (AT&T) for long distance service in the Bell System, eliminating the need for telephone operators to manually record calls.

A toll-free telephone number or freephone number is a telephone number that is billed for all arriving calls. For the calling party, a call to a toll-free number from a landline is free of charge. A toll-free number is identified by a dialing prefix similar to an area code. The specific service access varies by country.

111 is the emergency telephone number in New Zealand. It was first implemented in Masterton and Carterton on 29 September 1958, and was progressively rolled out nationwide with the last exchanges converting in 1988.

A telephone numbering plan is a type of numbering scheme used in telecommunication to assign telephone numbers to subscriber telephones or other telephony endpoints. Telephone numbers are the addresses of participants in a telephone network, reachable by a system of destination code routing. Telephone numbering plans are defined in each of the administrative regions of the public switched telephone network (PSTN) and in private telephone networks.

<span class="mw-page-title-main">Director telephone system</span> Telephone switching system used in the UK

The director telephone system was a development of the Strowger or step-by-step (SXS) switching system used in London and five other large cities in the UK from the 1920s to the 1980s.

Seven-digit dialing is a telephone dialing procedure customary in some territories of the North American Numbering Plan (NANP) for dialing telephone numbers in the same numbering plan area (NPA). NANP telephone numbers consist of ten digits, of which the leading three are the area code. In seven-digit dialing it is not necessary to dial the area code. The procedure is also sometimes known as local format or network format.

In telecommunications, a long-distance call (U.S.) or trunk call is a telephone call made to a location outside a defined local calling area. Long-distance calls are typically charged a higher billing rate than local calls. The term is not necessarily synonymous with placing calls to another telephone area code.

<span class="mw-page-title-main">Telephone exchange names</span> Alphabetic telephone numbering plan

A telephone exchange name or central office name was a distinguishing and memorable name assigned to a central office. It identified the switching system to which a telephone was connected, and facilitated the connection of telephone calls between switching systems in different localities.

<span class="mw-page-title-main">Telephone numbers in New Zealand</span> New Zealand numbering plan

The New Zealand telephone numbering plan describes the allocation of telephone numbers in New Zealand and the Pitcairn Islands.

Automatic message accounting (AMA) provides detailed accounting for telephone calls. When direct distance dialing (DDD) was introduced in the US, message registers no longer sufficed for dialed telephone calls. The need to record the time and phone number of each long-distance call was met by electromechanical data processing equipment.

The Number One Crossbar Switching System (1XB), was the primary technology for urban telephone exchanges served by the Bell System in the mid-20th century. Its switch fabric used the electromechanical crossbar switch to implement the topology of the panel switching system of the 1920s. The first No. 1 Crossbar was installed in the PResident-2 central office at Troy Avenue in Brooklyn, New York which became operational in February 1938.

<span class="mw-page-title-main">Telephone number</span> Sequence of digits assigned to a telephone subscription

A telephone number is a sequence of digits assigned to a landline telephone subscriber station connected to a telephone line or to a wireless electronic telephony device, such as a radio telephone or a mobile telephone, or to other devices for data transmission via the public switched telephone network (PSTN), or other public and private networks. Modern smart phones have added a built-in layer of abstraction whereby individuals or businesses are saved into a contacts application and the numbers no longer have to be written down or memorized.

<span class="mw-page-title-main">Telephone exchange</span> Interconnects telephones for calls

A telephone exchange, also known as a telephone switch or central office, is a crucial component in the public switched telephone network (PSTN) or large enterprise telecommunications systems. It facilitates the interconnection of telephone subscriber lines or digital system virtual circuits, enabling telephone calls between subscribers.

In conventional landline telephony, a non-dialable toll point or toll station was a lone station or line serving a rural subscriber many miles from the nearest central office. As it had no home telephone exchange and therefore no local calling area, no customer could dial its number; all connections to it had to be obtained manually by the long distance operator.

<span class="mw-page-title-main">Panel switch</span>

The Panel Machine Switching System is a type of automatic telephone exchange for urban service that was used in the Bell System in the United States for seven decades. The first semi-mechanical types of this design were installed in 1915 in Newark, New Jersey, and the last were retired in the same city in 1983.

References

  1. "Archived copy" (PDF). Archived from the original (PDF) on 2016-03-04. Retrieved 2015-01-15.{{cite web}}: CS1 maint: archived copy as title (link)