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Encyclopedia > Telephone exchange
A telephone operator manually connecting calls with patch cables at a telephone switchboard.
Activity at a manual telephone service exchange
Activity at a manual telephone service exchange

In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls. A central office is the physical building used to house inside plant equipment including telephone switches, which make phone calls "work" in the sense of making connections and relaying the speech information. Look up switch in Wiktionary, the free dictionary. ...  ©  This image is copyrighted. ...  ©  This image is copyrighted. ... A telephone operator at work on a private switchboard 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 (called reversed-charge calls in the UK), calls which... Image File history File links No higher resolution available. ... Image File history File links No higher resolution available. ... Telecommunication involves the transmission of signals over a distance for the purpose of communication. ... In telecommunication, the term inside plant has the following meanings: 1. ...


The term exchange can also be used to refer to an area served by a particular switch (typically known as a wire center in the US telecommunications industry). More narrowly, in some areas it can refer to the first three digits of the local number. In the three-digit sense of the word, other obsolete Bell System terms include office code and NXX. In the United States, the word exchange can also have the legal meaning of a local access and transport area under the Modification of Final Judgment (MFJ). Local access and transport area (LATA) is a term used in U.S. telecommunications regulation. ... In telecommunication, Modification of Final Judgment (MFJ) is the 1982 antitrust suit settlement agreement (consent decree) entered into by the United States Department of Justice and the American Telephone and Telegraph Company (AT&T) that, after modification and upon approval of the United States District Court for the District of...

Contents

Historic perspective

A Verizon Central Office in Lakeland, Florida at night.
A Verizon Central Office in Lakeland, Florida at night.
An AT&T Central Office in Houston, Texas.
An AT&T Central Office in Houston, Texas.

Prior to the telephone, electrical switches were used to switch telegraph lines. One of the first people to build a telephone exchange was Tivadar Puskás in 1877 while he was working for Thomas Edison. George W. Coy designed and built the first commercial telephone exchange which opened in New Haven, Connecticut in January, 1878. The switchboard was built from "carriage bolts, handles from teapot lids and bustle wire" and could handle two simultaneous conversations . [1] Image File history File linksMetadata Download high resolution version (2288x1712, 1427 KB) Summary Photograph of a Verizon Central Office in Lakeland, Florida. ... Image File history File linksMetadata Download high resolution version (2288x1712, 1427 KB) Summary Photograph of a Verizon Central Office in Lakeland, Florida. ... This article or section should include material from Bell Atlantic This article or section should include material from GTE Verizon Communications (NYSE: VZ) is a local exchange telephone company formed by the merger of Bell Atlantic, a former Bell Operating Company, and GTE, which was the largest independant local exchange... Image File history File links No higher resolution available. ... Image File history File links No higher resolution available. ... This article is about the current AT&T. For the 1885-2005 company, see American Telephone & Telegraph. ... Tivadar Puskás (17 September 1844 - 16 March 1893) was a Hungarian inventor, telephone pioneer, inventor of the telephone exchange. ... Edison redirects here. ... New Haven redirects here. ...


Later exchanges consisted of one to several hundred plug boards staffed by telephone operators. Each operator sat in front of a vertical panel containing banks of ¼-inch tip-ring-sleeve (3-conductor) jacks, each of which was the local termination of a subscriber's telephone line. In front of the jack panel lay a horizontal panel containing two rows of patch cords, each pair connected to a cord circuit. When a calling party lifted the receiver, a signal lamp near the jack would light. The operator would plug one of the cords (the "answering cord") into the subscriber's jack and switch her headset into the circuit to ask, "number please?" Depending upon the answer, the operator might plug the other cord of the pair (the "ringing cord") into the called party's local jack and start the ringing cycle, or plug into a trunk circuit to start what might be a long distance call handled by subsequent operators in another bank of boards or in another building miles away. In 1918 the average time to complete a long-distance call was 15 minutes.[2] In the ringdown method, the originating operator called another intermediate operator who would call the called subscriber, or passed it on to another intermediate operator.[3] This chain of intermediate operators could complete the call only if intermediate trunk lines were available between all the centers at the same time. In 1943 when military calls had priority, a cross-country US call might take as long as 2 hours to request and schedule in cities that used manual switchboards for toll calls. Telephone switchboard, 1974 A switchboard (also called a manual branch exchange) is a device used to manually connect a group of telephones from one to another or to an outside connection. ... A telephone operator at work on a private switchboard 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 (called reversed-charge calls in the UK), calls which... Subscriber: In a public switched telecommunications network such as the common telephone system, the ultimate user, customer, of a communications service. ... A telephone line (or just line) is a single-user circuit on a telephone communications system. ... 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. ... The person who (or device that) initiates a telephone call over the public switched telephone network is the calling party. ... Ringdown: In telephony, a method of signaling an operator in which telephone ringing current is sent over the line to operate a lamp or cause the operation of a self-locking relay known as a drop. ...


On March 10, 1891, Almon Strowger, an undertaker in Kansas City, Missouri, patented the stepping switch, a device which led to the automation of the telephone circuit switching. While there were many extensions and adaptations of this initial patent, the one best known consists of 10 levels or banks, each having 10 contacts arranged in a semi-circle. When used with a telephone dial, each pair of numbers caused the shaft of the central contact "hand" first to step up a level per digit and then to swing in a contact row per digit. is the 69th day of the year (70th in leap years) in the Gregorian calendar. ... Year 1891 (MDCCCXCI) was a common year starting on Thursday (link will display the full calendar) of the Gregorian calendar (or a common year starting on Saturday of the 12-day slower Julian calendar). ... Almon Brown Strowger (1839 – May 26, 1902) gave his name to the electromechanical telephone exchange technology that his invention and patent inspired. ... In electrical controls, a stepping switch (also called a uniselector; see Strowger switch, below) is an electromechanical device used, most prominently, in early automatic telephone exchanges to route calls. ... The rotary dial is a device mounted on or in a telephone or switchboard that is designed to send interrupted electrical pulses, known as pulse dialing, corresponding to the number dialled. ...


Later step switches were arranged in banks, beginning with a line-finder which detected that one of up to a hundred subscriber lines had the receiver lifted "off hook". The line finder hooked the subscriber to a "dial tone" bank to show that it was ready. The subscriber's dial pulsed at 10 pulses per second (depending on standards in particular countries). In electrical controls, a stepping switch (also called a uniselector; see Strowger switch, below) is an electromechanical device used, most prominently, in early automatic telephone exchanges to route calls. ...


Exchanges based on the Strowger switch were challenged by other selectors and by crossbar technology. These phone exchanges promised faster switching and would accept pulses faster than the Strowger's typical 10 pps—typically about 20 pps. Many also accepted DTMF "touch tones" or other tone signaling systems. The Panel telephone switch was an early type of automatic telephone exchange, first put into service in the 1920s. ... A crossbar switch is one of the principal architectures used to construct switches of many types. ... Dual-tone multi-frequency (DTMF), also known as Touch Tone® is used for telephone signaling over the line in the voice frequency band to the call switching center. ...


A transitional technology (from pulse to DTMF) had DTMF link finders which converted DTMF to pulse, to feed to older Strowger, panel or crossbar switches. This technology was used as late as mid 2002.


Number plan trivia

See Telephone number A telephone number is a sequence of decimal digits that uniquely indicates the network termination point. ...


Technologies

This article will use the terms:

  • manual service for a condition where a human operator routes calls inside an exchange and a dial is not used
  • dial service for an exchange where calls are routed by a switch interpreting dialed digits
  • telephone exchange for the building housing the switching equipment
  • telephone switch for the switching equipment
  • concentrator for a device that concentrates traffic, be it remote or co-located with the switch
  • off-hook for a tip condition or to describe a circuit that is in use (i.e., when a phone call is in progress)
  • on-hook for an idle circuit (i.e., no phone call is in progress)
  • wire center for the area served by a particular switch or central office

Many of the terms in this article have conflicting UK and US usages. In telephony, the term off-hook has the following meanings: The condition that exists when a telephone or other user instrument is in use, , during dialing or communicating. ... In telecommunication, the term on-hook has the following meanings: 1. ...

  • central office originally referred to switching equipment and its operators. Now it is used generally for the building housing switching and related inside plant equipment.
  • telephone exchange means an exchange building in the UK, and is also the UK name for a telephone switch, and also has a legal meaning in U.S. telecoms.
  • telephone switch is the U.S. term, but is in increasing use in technical UK telecoms usage, to make the CO/switch/concentrator distinction clear.

In telecommunication, the term inside plant has the following meanings: 1. ...

Manual service exchanges

1924 PBX switchboard
1924 PBX switchboard

With manual service, the customer lifts the receiver off-hook and asks the operator to connect the call to a requested number. Provided that the number is in the same central office, the operator connects the call by plugging into the jack on the switchboard corresponding to the called customer's line. If the call is to another central office, the operator plugs into the trunk for the other office and asks the operator answering (known as the "inward" operator) to connect the call. Image File history File links Switchboardof1924. ... Image File history File links Switchboardof1924. ... In telephony, the term off-hook has the following meanings: The condition that exists when a telephone or other user instrument is in use, , during dialing or communicating. ... A telephone operator at work on a private switchboard 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 (called reversed-charge calls in the UK), calls which... Telephone switchboard, 1974 A switchboard (also called a manual branch exchange) is a device used to manually connect a group of telephones from one to another or to an outside connection. ...


Most urban exchanges were common-battery, meaning that the central office provided power for the telephone circuits, as is the case today. In common battery systems, the pair of wires from a subscriber's telephone to the switch (or manual exchange) carry -48VDC (nominal) from the telephone company end, across the conductors. The telephone presents an open circuit when it is on-hook or idle. When the subscriber goes off-hook, the telephone puts a DC resistance short across the line. In manual service, this current flowing through the off-hook telephone flows through a relay coil actuating a buzzer and lamp on the operator's switchboard. The buzzer and lamp would tell an operator the subscriber was off-hook, (requesting service). [4] In telecommunication, the term on-hook has the following meanings: 1. ...


In the largest U.S. cities, it took many years to convert every office to automatic equipment, such as Panel switches. During this transition period, it was possible to dial a manual number and be connected without requesting an operator's assistance. This was because the policy of the Bell System was that customers should not need to know if they were calling a manual or automated office. If a subscriber dialed a manual number, an inward operator would answer the call, see the called number on a display device, and manually connect the call. For instance, if a customer calling from TAylor 4725 dialed a manual number, ADams 1233, the call would go through, from the subscriber's perspective, exactly as a call to LEnnox 5813, in an automated exchange. The Panel telephone switch was an early type of automatic telephone exchange, first put into service in the 1920s. ...


In contrast to the common battery system, smaller towns with manual service often had magneto, or crank, phones. Using a magneto set, the subscriber turned a crank to generate ringing current, to gain the operator's attention. The switchboard would respond by dropping a metal tab above the subscriber's line jack and sounding a buzzer. Dry cell batteries (normally two large "No 6" cells) in the subscriber's telephone provided the DC power for conversation. Magneto systems were in use in one American small town, Bryant Pond, Woodstock, Maine as late as 1983. In general, this type of system had a poorer call quality compared to common-battery systems. A dry cell is a galvanic electrochemical cell with a pasty low-moisture electrolyte. ... Woodstock is a town located in Oxford County, Maine. ...


Many small town magneto systems featured party lines, anywhere from two to ten or more subscribers sharing a single line. When calling a party, the operator would use a distinctive ringing signal sequence, such as two long rings followed by one short. Everyone on the line could hear the rings, and of course could pick up and listen in if they wanted. On rural lines which were not connected to a central office (thus not connected to the outside world), subscribers would crank the correct sequence of rings to reach their party. In older telephone systems, a party line (also multiparty line or Shared Service Line) is an arrangement in which two or more customers are connected directly to the same local loop. ... It has been suggested that this article or section be merged with ring tone, ring (telephone), polyphonic ringtone and Mobile music (Discuss) A ringing signal is an electronic telephony signal that causes a telephone to alert the user to an incoming call. ...


Pre-digital automatic exchanges

A rural telephone exchange building in Australia
A rural telephone exchange building in Australia

Automatic exchanges, or dial service, came into existence in the early 1900s. Their purpose was to eliminate the need for human telephone operators. Before the exchanges became automated, operators had to complete the connections required for a telephone call. Almost everywhere, operators have been replaced by computerized exchanges. A telephone switch is the brains of an automatic exchange. It is a device for routing calls from one telephone to another, generally as part of the public switched telephone network. Image File history File links Metadata Size of this preview: 800 × 600 pixelsFull resolution (2304 × 1728 pixel, file size: 798 KB, MIME type: image/jpeg) Telstra telephone exchange located in Karawinna, Victoria, Australia File historyClick on a date/time to view the file as it appeared at that time. ... Image File history File links Metadata Size of this preview: 800 × 600 pixelsFull resolution (2304 × 1728 pixel, file size: 798 KB, MIME type: image/jpeg) Telstra telephone exchange located in Karawinna, Victoria, Australia File historyClick on a date/time to view the file as it appeared at that time. ... A telephone operator at work on a private switchboard 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 (called reversed-charge calls in the UK), calls which... This article does not adequately cite its references or sources. ... This article is about routing in computer networks. ... For other uses, see Telephone (disambiguation). ... The public switched telephone network (PSTN) is the network of the worlds public circuit-switched telephone networks, in much the same way that the Internet is the network of the worlds public IP-based packet-switched networks. ...


The local exchange automatically senses an off hook (tip) telephone condition, provides dial tone to that phone, receives the pulses or DTMF tones generated by the phone, and then completes a connection to the called phone within the same exchange or to another distant exchange. For other uses, see Telephone (disambiguation). ... For the G.I. Joe character, see List of G.I. Joe ARAH characters. ... Dual-tone multi-frequency (DTMF), also known as Touch Tone® is used for telephone signaling over the line in the voice frequency band to the call switching center. ...


The exchange then maintains the connection until a party hangs up, and the connection is disconnected. This tracking of a connection's status is called supervision. Additional features, such as billing equipment, may also be incorporated into the exchange.


In Bell System dial service, a feature called automatic number identification (ANI) was implemented. ANI allowed services like automated billing, toll-free 800-numbers, and 9-1-1 service. In manual service, the operator knows where a call is originating by the light on the switchboard's jack field. In early dial service, ANI did not exist. Long distance calls would go to an operator queue and the operator would ask the calling party's number, then write it on a paper toll ticket. See also Automatic Message Accounting. Automatic number identification (ANI) is a feature of telephony intelligent network services that permits subscribers to display or capture the telephone numbers of calling parties. ... Automatic Messaging Accounting (AMA) provides detail billing for telephone calls. ...


Early exchanges used motors, shaft drives, rotating switches and relays. In a sense, switches were relay-logic computers. Some types of automatic exchanges were Strowger (also known as Step-By-Step), All Relay, X-Y, Panel and crossbar. These are referred to collectively as electromechanical switches. Relay as used in cars A relay is an electromechanical switch that uses an electromagnet to open or close one or many sets of contacts. ... Almon Brown Strowger (1839 - May 26, 1902) gave his name to the electromechanical telephone exchange technology that his invention and patent inspired. ... The Panel telephone switch was an early type of automatic telephone exchange, first put into service in the 1920s. ... A crossbar switch is one of the principal architectures used to construct switches of many types. ...


Electromechanical signaling

Circuits connecting two switches are called trunks. Before Signalling System 7, Bell System electromechanical switches in the United States communicated with one another over trunks using a variety of DC voltages and signaling tones. It would be rare to see any of these in use today. To meet Wikipedias quality standards, this article or section may require cleanup. ... Signalling System #7 (SS7) is a set of telephony signalling protocols which are used to set up the vast majority of the worlds PSTN telephone calls. ... The Bell System was a trademark and service mark used by the United States telecommunications company American Telephone & Telegraph Company (AT&T) and its affiliated companies to co-brand their extensive circuit-switched telephone network and their affiliations with each other. ...


Some signalling communicated dialed digits. An early form called Panel Call Indicator Pulsing used quaternary pulses to set up calls between a Panel switch and a manual switchboard. Probably the most common form of communicating dialed digits between electromechanical switches was sending dial pulses, equivalent to a rotary dial's pulsing, but sent over trunk circuits between switches. In Bell System trunks, it was common to use 20 pulse-per-second between crossbar switches and crossbar tandems. This was twice the rate of Western Electric/Bell System telephone dials. Using the faster pulsing rate made trunk utilization more efficient because the switch spent half as long listening to digits. DTMF was not used for trunk signaling. Multi-frequency (MF) was the last of the pre-digital methods. It used a different set of tones sent in pairs like DTMF. Dialing was preceded by a special keypulse (KP) signal and followed by a start (ST). Variations of the Bell System MF tone scheme became a CCITT standard. Similar schemes were used in the Americas and in some European countries including Spain. Digit strings between switches were often abbreviated to further improve utilization. For example, one switch might send only the last four or five digits of a telephone number. In one case, seven digit numbers were preceded by a digit 1 or 2 to differentiate between two area codes or office codes, (a two-digit-per-call savings). This improved revenue per trunk and reduced the number of digit receivers needed in a switch. Every task in electromechanical switches was done in big metallic pieces of hardware. Every fractional second cut off of call set up time meant fewer racks of equipment to handle call traffic. The Panel telephone switch was an early type of automatic telephone exchange, first put into service in the 1920s. ... A German Fe TAp 615, a widespread rotary dial telephone of the 1960s to the 1980s The rotary dial is a device mounted on or in a telephone or switchboard that is designed to send interrupted electrical pulses, known as pulse dialing, corresponding to the number dialed. ... In telephony Multi-Frequency (MF) is an outdated, in-band signaling technique. ... The ITU Telecommunication Standardization Sector (ITU-T) coordinates standards for telecommunications on behalf of the International Telecommunication Union (ITU) and is based in Geneva, Switzerland. ... A telephone number is a sequence of decimal digits that uniquely indicates the network termination point. ...


Examples of signals communicating supervision or call progress include E and M signaling, SF signaling, and robbed-bit signaling. In physical (not carrier) E and M trunk circuits, trunks were four wire. Fifty trunks would require a hundred pair cable between switches, for example. Conductors in one common circuit configuration were named tip, ring, ear (E) and mouth (M). In two-way trunks with E and M signaling, a handshake took place to prevent both switches from colliding by dialing calls on the same trunk at the same time. By changing the state of these leads from ground to -48 volts, the switches stepped through a handshake protocol. Using DC voltage changes, the local switch would send a signal to get ready for a call and the remote switch would reply with an acknowledgment to go ahead with dial pulsing. This was done with relay logic and discrete electronics. These voltage changes on the trunk circuit would cause pops or clicks that were audible to the subscriber as the electrical handshaking stepped through its protocol. Another handshake, to start timing for billing purposes, caused a second set of clunks when the called party answered. A second common form of signaling for supervision was called single-frequency or SF signaling. The most common form of this used a steady 2,600 Hz tone to identify a trunk as idle. Trunk circuitry hearing a 2,600 Hz tone for a certain duration would go idle. (The duration requirement reduced falsing). Some systems used tone frequencies over 3,000 Hz, particularly on SSB frequency division multiplex microwave radio relays. On T-carrier digital transmission systems, bits within the T-1 data stream were used to transmit supervision. By careful design, the appropriated bits did not change voice quality appreciably. Robbed bits were translated to changes in contact states (opens and closures) by electronics in the channel bank hardware. This allowed direct current E and M signaling, or dial pulses, to be sent between electromechanical switches over a digital carrier which did not have DC continuity. E&M is a type of supervisory signaling traditionally used in the North American telecommunications industry. ... In telecommunications, falsing, describes a decoder detecting a valid input when one is not present. ... Frequency-division multiplexing (FDM) is a form of signal multiplexing where multiple baseband signals are modulated on different frequency carrier waves and added together to create a composite signal. ... This article or section does not cite its references or sources. ... Two Network Interface Units, one with a single card, the other with two In telecommunications, T-carrier is the generic designator for any of several digitally multiplexed telecommunications carrier systems originally developed by Bell Labs and used in North America and Japan. ...


Sounds

Audio sample:

Step-by-step call Image File history File links Sxs_intra_to_busy. ...

Subscribers hear a different-sounding dialtone in a step-by-step call.
Problems listening to the file? See media help.

A characteristic of electromechanical switching equipment is that the maintenance staff could hear the mechanical clattering of Strowgers or crossbar relays. Most Bell System central offices were housed in reinforced concrete buildings with concrete ceilings and floors. In rural areas, some smaller switching facilities, such as Community Dial Offices (CDOs), were sometimes housed in prefabricated metal buildings. These facilities almost always had concrete floors. The hard surfaces reflected sounds. A Community Dial Office was a small central office equipt for use with telephone switching in rural areas. ...


During heavy use periods, it could be hard to talk over the clatter of calls being processed in a large switch. For example, on Mothers Day in the US, or on a Friday evening around 5pm, the metallic rattling could make raised voices necessary. For Wire spring relay markers these noises resembled hail falling on a metallic roof. A wire spring relay had springs made from drawn wires of phosphor bronze, rather than from flat sheet metal as in the earlier flat spring relay. ... A marker is a type of special purpose control system that was used in electromechanical telephone central office switches. ...


On a pre-dawn Sunday morning, call processing might slow to the point that one might be able to hear individual calls being dialed and set up. There were also noises from whining power inverters and whirring ringing generators. Some systems had a continual, rhythmic "clack-clack-clack" from wire spring relays that made reorder (120 ipm) and busy (60 ipm) signals. In Bell System installations, there were typically alarm bells, gongs, or chimes. These would annunciate alarms calling attention to a failed switch element. Another noisemaker: a trouble reporting card system was connected to switch common control elements. These trouble reporting systems would puncture cardboard cards with a cryptic code that logged the nature of a failure. Remreed technology in Stored Program Control exchanges finally quieted the environment. A wire spring relay had springs made from drawn wires of phosphor bronze, rather than from flat sheet metal as in the earlier flat spring relay. ... Reorder tone (usually referred to simply as reorder or fast busy) is a dual-frequency tone of 480 Hz and 620 Hz at a cadence of 0. ... A CTR census machine, utilizing a punched card system. ... It has been suggested that this article or section be merged into Reed switch. ... Stored Program Control exchange (SPC) is the technical name used for telephone exchanges controlled by a program stored in the memory of the system. ...


Maintenance Tasks

The maintenance of electromechanical systems was partly DC electricity and partly mechanical adjustments. Unlike modern switches, a circuit connecting a dialed call through an electromechanical switch actually had DC continuity. The talking path was a physical, metallic one.


In all systems, subscribers were not supposed to notice changes in quality of service because of failures or maintenance work. A variety of tools referred to as make-busys were plugged into electromechanical switch elements during repairs or failures. A make-busy would identify the part being worked on as in-use, causing the switching logic to route around it. A similar tool was called a TD tool. Subscribers who got behind in payments would have their service temporarily denied (TDed). This was effected by plugging a tool into the subscriber's office equipment (Crossbar) or line group (step). The subscriber could receive calls but could not dial out.


Strowger-based, step-by-step offices in the Bell System were under continual maintenance. They required constant cleaning. Indicator lights on equipment bays in step offices alerted staff to conditions such as blown fuses (usually white lamps) or a permanent signal (stuck off-hook condition, usually green indicators.) Step offices were more susceptible to single-point failures than newer technologies.


Crossbar offices used more shared, common control circuits. For example, a digit receiver (part of an element called an Originating Register) would be connected to a call just long enough to collect the subscriber's dialed digits. Crossbar architecture was more flexible than step offices. Later crossbar systems had punch-card-based trouble reporting systems. By the 1970s, Automatic number identification had been retrofitted to nearly all step-by-step and crossbar switches in the Bell System. Automatic number identification (ANI) is a feature of telephony intelligent network services that permits subscribers to display or capture the telephone numbers of calling parties. ...


Electronic switches

The first Electronic Switching Systems were not entirely digital. The Western Electric 1ESS switch still had reed relay metallic paths. It was stored-program-controlled. Equipment testing, changes to phone numbers, circuit lockouts and similar tasks were accomplished by typing on a terminal. Northern Telecom SP1, Ericsson AKE, Philips PRX/A, ITT Metaconta, British Telecom TXE series and several other designs were similar. These systems could use the old electromechanical signaling methods inherited from crossbar and step-by-step switches. They also introduced a new form of data communications: two 1ESS exchanges could communicate with one another using a data link called Common Channel Interoffice Signaling, (CCIS). This data link was based on CCITT 6, a predecessor to SS7. In telecommunications, an electronic switching system (ESS) is: A telephone switching system based on the principles of time-division multiplexing of digitized analog signals. ... Company Masthead Logo Logo until circa 1969, also current logo on company web site Logo 1969–1983 Hi Dan! Western Electric (sometimes abbreviated WE and WECo) was an American electrical engineering company, the manufacturing arm of AT&T from 1881 to 1995. ... The Number One Electronic Switching System was the first large scale Stored Program Control (SPC) telephone exchange or Electronic Switching System in the Bell System, entering service in the late 1960s. ... It has been suggested that this article or section be merged into Reed switch. ... Stored Program Control exchange (SPC) is the technical name used for telephone exchanges controlled by a program stored in the memory of the system. ... In telecommunications, the term lockout has the following meanings: 1. ... SP-1 (Stored Program 1) was the name of a computerized telephone exchange (a so-called switching office) manufactured by Northern Electric (later Northern Telecom and now Nortel Networks beginning in 1972) in Canada. ... The PRX205 (PRX/A) is a processor controlled reed relay telephone exchange developed by Philips Telecommunicatie Industrie BV (PTI) in Hilversum during the late 1960s and early 1970s. ... TXE, which stands for Telephone eXchange Electronic, was the designation given to a family of telephone exchanges developed by the British General Post Office (GPO), now BT, designed to replace the ageing Strowger systems. ... Common Channel Signaling (CCS) is the transmission of signaling information out of the information band. ... Signalling System #7 is a set of protocols defined by ITU-T, specifically in the Q.7* set of documents, used to set up telephone calls. ...


Digital switches

A typical satellite PBX with front cover removed.
A typical satellite PBX with front cover removed.

Digital switches work by connecting two or more digital circuits together, according to a dialed telephone number. Calls are setup between switches using the Signalling System 7 protocol, or one of its variants. In U.S. and military telecommunication, a digital switch is a switch that performs time division switching of digitized signals. [5] This was first done in a few small and little used systems. The first product using a digital switch system was made by Amtelco. Prominent examples include Nortel DMS-100, Lucent 5ESS switch, Siemens EWSD and Ericsson AXE telephone exchange. With few exceptions, most switches built since the 1980s are digital, so for practical purposes this is a distinction without a difference. This article describes digital switches, including algorithms and equipment. Image File history File links Metadata Size of this preview: 800 × 600 pixelsFull resolution (2816 × 2112 pixel, file size: 1. ... Image File history File links Metadata Size of this preview: 800 × 600 pixelsFull resolution (2816 × 2112 pixel, file size: 1. ... A telephone number is a sequence of decimal digits that uniquely indicates the network termination point. ... Signalling System #7 (SS7) is a set of telephony signalling protocols which are used to set up the vast majority of the worlds PSTN telephone calls. ... Copy of the original phone of Alexander Graham Bell at the Musée des Arts et Métiers in Paris Telecommunication is the assisted transmission of signals over a distance for the purpose of communication. ... Amtelco is a manufacturer of telecommunications equipment and telephone answering service and call center systems, founded in 1976. ... The DMS-100 Switch is Nortel Networks biggest seller of a line of Digital Multiplex System (DMS) telephony switches manufactured by Nortel Networks. ... The 5ESS Switch is the Class 5 telephone Electronic Switching System sold by Alcatel-Lucent. ... EWSD End Office Switch EWSD (Elektronisches Wählsystem Digital in German, Electronic Digital Switching System/Electronic World Switch Digital[1] in English) is one of the most widely installed telephone exchange systems in the world. ... The AXE telephone exchange is a product line of circuit switched digital telephone exchanges manufactured by Ericsson, a Swedish telecom company. ...

A digital exchange (Nortel DMS-100) used by an operator to offer local and long distance services in France. Each switch typically serves 10,000-100,000+ subscribers depending on the geographic area
A digital exchange (Nortel DMS-100) used by an operator to offer local and long distance services in France. Each switch typically serves 10,000-100,000+ subscribers depending on the geographic area

Digital switches encode the speech going on, in 8000 time slices per second. At each time slice, a digital PCM representation of the tone is made. The digits are then sent to the receiving end of the line, where the reverse process occurs, to produce the sound for the receiving phone. In other words, when you use a telephone, you are generally having your voice "encoded" and then reconstructed for the person on the other end. Your voice is delayed in the process by a small fraction of one second — it is not "live", it is reconstructed — delayed only minutely. (See below for more info.) Image File history File links No higher resolution available. ... Image File history File links No higher resolution available. ... Northern Telecommunications Networks, commonly known as Nortel, is a telecommunications equipment manufacturer headquartered in Canada. ... The DMS-100 Switch is Nortel Networks biggest seller of a line of Digital Multiplex System (DMS) telephony switches manufactured by Nortel Networks. ... PCM is an initialism which can have different meanings: Phase Change Material Pulse-code modulation, a way to digitally encode signals representing sound and their video counterparts Potential Cancer Marker Communist Party of Mexico Plug Compatible Manufacturer Power-train control module, a computer in a car which controls the car...


Individual local loop telephone lines are connected to a remote concentrator. In many cases, the concentrator is co-located in the same building as the switch. The interface between remote concentrators and telephone switches has been standardised by ETSI as the V5 protocol. Concentrators are used because most telephones are idle most of the day, hence the traffic from hundreds or thousands of them may be concentrated into only tens or hundreds of shared connections. In telecommunications, the local loop is the wiring between the central office (telephone exchange in British English) and the customers premises demarcation point. ... In modern telephony a remote concentrator is the lowest level in the telephone switch hierarchy. ... The European Telecommunications Standards Institute (ETSI) is a standardization organization of the telecommunications industry (equipment makers and network operators) in Europe, with worldwide projection. ... V5 is a set of protocols defined by ETSI by which a multiplexer in the access network of the PSTN can communicate with a telephone exchange. ...


Some telephone switches do not have concentrators directly connected to them, but rather are used to connect calls between other telephone switches. These complex machine (or series of them) in a central exchange building are referred to as "carrier-level" switches or tandems.


Some telephone exchange buildings in small towns now house only remote or satellite switches, and are homed upon a "parent" switch, usually several kilometres away. The remote switch is dependent on the parent switch for routing and number plan information. Unlike a digital loop carrier, a remote switch can route calls between local phones itself, without using trunks to the parent switch. The local loop is the physical connection between the main distribution frame in the users premises to the telecommunications network provider. ...


Telephone switches are usually owned and operated by a telephone service provider or carrier and located in their premises, but sometimes individual businesses or private commercial buildings will house their own switch, called a PBX, or Private Branch Exchange. PBX redirects here. ...

Map of the Wire Center locations in the US
Map of the Wire Center locations in the US
Map of the Central Office locations in the US
Map of the Central Office locations in the US

Image File history File links Size of this preview: 776 × 600 pixel Image in higher resolution (1056 × 816 pixel, file size: 79 KB, MIME type: image/png) Map of the wire center locations in the United States. ... Image File history File links Size of this preview: 776 × 600 pixel Image in higher resolution (1056 × 816 pixel, file size: 79 KB, MIME type: image/png) Map of the wire center locations in the United States. ... Image File history File links Size of this preview: 776 × 600 pixel Image in higher resolution (1056 × 816 pixel, file size: 99 KB, MIME type: image/png) Map of the central (exchange) office locations of the United States. ... Image File history File links Size of this preview: 776 × 600 pixel Image in higher resolution (1056 × 816 pixel, file size: 99 KB, MIME type: image/png) Map of the central (exchange) office locations of the United States. ...

The switch's place in the system

Telephone switches are a small part of a large network. The majority of work and expense of the phone system is the wiring outside the central office, or the Outside plant. In the middle 20th century, each subscriber telephone number required an individual pair of wires from the switch to the subscriber's phone. A typical central office may have tens-of-thousands of pairs of wires that appear on terminal blocks called the main distributing frame or MDF. A component of the MDF is protection: fuses or other devices that protect the switch from lightning, shorts with electric power lines, or other foreign voltages. In a typical telephone company, a large database tracks information about each subscriber pair and the status of each jumper. Before computerization of Bell System records in the 1980s, this information was handwritten in pencil in accounting ledger books. In telecommunication, the term outside plant has the following meanings: 1. ... In telecommunication, a main distribution frame (MDF) is a distribution frame on one part of which the external trunk cables entering a facility terminate, and on another part of which the internal user subscriber lines and trunk cabling to any intermediate distribution frames terminate. ...


To reduce the expense of outside plant, some companies use "pair gain" devices to provide telephone service to subscribers. These devices are used to provide service where existing copper facilities have been exhausted or by siting in a neighborhood, can reduce the length of copper pairs, enabling digital services such as ISDN or DSL. Pair gain or digital loop carriers (DLCs) are located outside the central office, usually in a large neighborhood distant from the CO. A pair gain system is a transmission system that uses concentrators or multiplexers so that fewer wire pairs may be used than would otherwise be required to provide service to a given number of subscribers. ... ISDN is also short for isosorbide dinitrate Integrated Services Digital Network (ISDN) is a type of circuit switched telephone network system, designed to allow digital (as opposed to analog) transmission of voice and data over ordinary telephone copper wires, resulting in better quality and higher speeds, than available with analog... DSL redirects here. ... The local loop is the physical connection between the main distribution frame in the users premises to the telecommunications network provider. ...


DLCs are often referred to as Subscriber Loop Carriers (SLCs), after Lucent's proprietary name for their pair gain products. Early SLC systems (SLC-1) used an analog carrier for transport between the remote site and the central office. Later systems (SLC-96, SLC-5) and other vendors' DLC products contain line cards that convert the analog signal to a digital signal (usually PCM). This digital signal can then be transported over copper, fiber, or other transport medium to the central office. Other components include ringing generators to provide ringing current and battery backups. Subscriber Loop Carrier refers to equipment providing central office-like telephone interface functionality. ... In 1996, AT&T spun off its Systems and Technology units, along with the famous Bell Laboratories, to form a new company named Lucent Technologies (NYSE: LU). ... Telecomunications The Line Card is an electronic printed circuit board in an access network element of a telecommunication network. ... PCM is an initialism which can have different meanings: Phase Change Material Pulse-code modulation, a way to digitally encode signals representing sound and their video counterparts Potential Cancer Marker Communist Party of Mexico Plug Compatible Manufacturer Power-train control module, a computer in a car which controls the car...


DLCs can be configured as universal (UDLCs) or integrated (IDLCs). Universal DLCs have two terminals, a central office terminal (COT) and a remote terminal (RT), that function similarly. Both terminals interface with analog signals, convert to digital signals, and transport to the other side where the reverse is performed. Sometimes, the transport is handled by separate equipment. In an Integrated DLC, the COT is eliminated. Instead, the RT is connected digitally to equipment in the telephone switch. This reduces the total amount of equipment required. Several standards cover DLCs, including Telcordia's TR/GR-008 & TR/GR-303. ...


Switches are used in both local central offices and in long distance centers. There are two major types in the Public switched telephone network (PSTN): Office classification: Prior to divestiture, numbers that were assigned to offices according to their hierarchical function in the U.S. public switched telephone network. ... The public switched telephone network (PSTN) is the network of the worlds public circuit-switched telephone networks, in much the same way that the Internet is the network of the worlds public IP-based packet-switched networks. ...

  1. Class 4 telephone switches designed for toll or switch-to-switch connections.
  2. Class 5 telephone switches or subscriber switches, which manage connections from subscriber telephones. Since the 1990s, hybrid Class 4/5 switching systems that serve both functions have become common.

Another element of the telephone network is time and timing. Switching, transmission and billing equipment may be slaved to very high accuracy 10 MHz standards which synchronize time events to very close intervals. Time-standards equipment may include Rubidium- or Caesium-based standards and a Global Positioning System receiver. A Class 4 or Tandem switch is U.S. telephone company (telco) central office telephone exchange used for long distance communications in the Public Switched Telephone Network (PSTN) to interconnect telephone company offices. ... A Class 5 switch, in United States telephony jargon refers to a telephone switch or exchange located at the local telephone companys central office, directly serving subscribers. ... The Network Time Protocol (NTP) is a protocol for synchronizing the clocks of computer systems over packet-switched, variable-latency data networks. ... GPS redirects here. ...


Switch design

Long distance switches may use a slower, more efficient switch-allocation algorithm than local central offices, because they have near 100% utilization of their input and output channels. Central offices have more than 90% of their channel capacity unused. A Class 5 switch, in United States telephony jargon refers to a telephone switch or exchange located at the local telephone companys central office, directly serving subscribers. ...


While traditionally, telephone switches connected physical circuits (e.g., wire pairs), modern telephone switches use a combination of space- and time-division switching. In other words, each voice channel is represented by a time slot (say 1 or 2) on a physical wire pair (A or B). In order to connect two voice channels (say A1 and B2) together, the telephone switch interchanges the information between A1 and B2. It switches both the time slot and physical connection. To do this, it exchanges data between the time slots and connections 8000 times per second, under control of digital logic that cycles through electronic lists of the current connections. Using both types of switching makes a modern switch far smaller than either a space or time switch could be by itself. The term multiplexer has uses in several fields of application: Electronics In electronics, a multiplexer or mux is a device that combines several electrical signals into a single signal. ... Time-division multiplexing (TDM) is a type of digital multiplexing in which two or more apparently simultaneous channels are derived from a given frequency spectrum, i. ...


The structure of a switch is an odd number of layers of smaller, simpler subswitches. Each layer is interconnected by a web of wires that goes from each subswitch, to a set of the next layer of subswitches. In most designs, a physical (space) switching layer alternates with a time switching layer. The layers are symmetric, because in a telephone system callers can also be callees. A substitute for a 16x16 crossbar switch made from 12 4x4 crossbar switches. ...


A time-division subswitch reads a complete cycle of time slots into a memory, and then writes it out in a different order, also under control of a cyclic computer memory. This causes some delay in the signal.


A space-division subswitch switches electrical paths, often using some variant of a nonblocking minimal spanning switch, or a crossover switch. A substitute for a 16x16 crossbar switch made from 12 4x4 crossbar switches. ... Crossover Switches are complex array matrixes to switch any one input path to any one(or more) output path(s). ...


Switch control algorithms

Fully-connected mesh network

One way is to have enough switching fabric to assure that the pairwise allocation will always succeed by building a fully-connected mesh network. This is the method usually used in central office switches, which have low utilization of their resources. Switching Fabric: A high speed decision unit ,which comprises of specific software and hardware, in order to provide the switching process more reliably and swiftly in a routing/switching device. ... For other uses of topology, see topology (disambiguation). ...


Clos's nonblocking switch algorithm

The scarce resources in a telephone switch are the connections between layers of subswitches. The control logic has to allocate these connections, and most switches do so in a way that is fault tolerant. See nonblocking minimal spanning switch for a discussion of Charles Clos's algorithm, used in many telephone switches, and arguably one of the most important[citation needed] algorithms in modern industry. Fault-tolerance or graceful degradation is the property of a system that continues operating properly in the event of failure of some of its parts. ... A substitute for a 16x16 crossbar switch made from 12 4x4 crossbar switches. ...


Fault tolerance

Composite switches are inherently fault-tolerant. If a subswitch fails, the controlling computer can sense it during a periodic test. The computer marks all the connections to the subswitch as "in use". This prevents new calls, and does not interrupt old calls that remain working. After all in progress calls have ended, the subswitch then becomes unused. Some time later, a technician can replace the circuit board. When the next test succeeds, the connections to the repaired subsystem are marked "not in use", and the switch returns to full operation.


To prevent frustration with unsensed failures, all the connections between layers in the switch are allocated using first-in-first-out lists. As a result, if a connection is faulty or noisy and the customer hangs up and redials, they will get a different set of connections and subswitches. A last-in-first-out allocation of connections might cause a continuing string of very frustrating failures. This article is about FIFOs in computing and electronic design. ... In a stack, the topmost item, which is added last, is taken out first. ...


See also

In US telecommunication jargon, a central office (C.O.) is a common carrier switching center Class 5 telephone switches in which trunks and local loops are terminated and switched. [6] The history of telecommunication predates what is commonly thought of as modern ideas and the systems currently in place today. ... This is intended to be a list of the more common central office (telephone company operated) phone switches. ... In telecommunications, a pair-gain system is a network amplifier system used to increase the strength of signals on paired wires installed to reach distances up to about five miles (8 km). ... This article needs to be wikified. ... A softswitch is a central device in a telephone network which connects calls from one phone line to another, entirely by means of software running on a computer system. ... Stored Program Control exchange (SPC) is the technical name used for telephone exchanges controlled by a program stored in the memory of the system. ... A telephone number is a sequence of decimal digits that uniquely indicates the network termination point. ... Siemens DSLAM SURPASS hiX 5625 A Digital Subscriber Line Access Multiplexer (DSLAM) allows telephone lines to make faster connections to the Internet. ... DSL redirects here. ... ISDN redirects here. ... The Plesiochronous Digital Hierarchy (PDH) is a technology used in telecommunications networks to transport large quantities of data over digital transport equipment such as fibre optic and microwave radio systems. ... PBX redirects here. ... During the early years of telephone service, communities that required more than 10,000 telephone numbers, whether dial service was available or not, utilized exchange names to distinguish identical numerics for different customers. ... Copy of the original phone of Alexander Graham Bell at the Musée des Arts et Métiers in Paris Telecommunication is the assisted transmission of signals over a distance for the purpose of communication. ... A common carrier is an organization that transports persons or goods, and offers its services to the general public. ... A switching center is a node in a telecommunications Circuit switching network which is connected to either another switching center and/or to end user devices. ... A Class 5 switch, in United States telephony jargon refers to a telephone switch or exchange located at the local telephone companys central office, directly serving subscribers. ... In telecommunications, the local loop is the wiring between the central office (telephone exchange in British English) and the customers premises demarcation point. ...


Note: In the DOD, "common carrier" is called "commercial carrier." Synonyms exchange, local central office, local exchange, local office, switching center (except in DOD Defense Switched Network (formerly AUTOVON) usage), switching exchange, telephone exchange. Deprecated synonym switch.[7] The Defense Switched Network (DSN) is a primary information transfer network for the Defense Information Systems Network (DISN). ... Autovon, the Automatic Voice Network, was an American military phone system built to survive nuclear attacks. ... Electrical switches. ...


Notes

  1. ^ See National Park Service "first switchboard" page.
  2. ^ Calvert, J. B. (2003-09-07). Basic Telephones. Retrieved on 2006-09-13.
  3. ^ Calvert, J. B. (2003-09-07). Basic Telephones, The Switchboard (ringdown is near bottom). Retrieved on 2006-09-13.
  4. ^ Connected to a switch, an off-hook condition operates a relay to connect a dial tone and a device to collect dialed digits.
  5. ^ Source: from Federal Standard 1037C and from MIL-STD-188.
  6. ^ Source: from Federal Standard 1037C.
  7. ^ Source: from Federal Standard 1037C.

Year 2003 (MMIII) was a common year starting on Wednesday of the Gregorian calendar. ... is the 250th day of the year (251st in leap years) in the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 256th day of the year (257th in leap years) in the Gregorian calendar. ... Year 2003 (MMIII) was a common year starting on Wednesday of the Gregorian calendar. ... is the 250th day of the year (251st in leap years) in the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 256th day of the year (257th in leap years) in the Gregorian calendar. ... Federal Standard 1037C, entitled Telecommunications: Glossary of Telecommunication Terms is a United States Federal Standard, issued by the General Services Administration pursuant to the Federal Property and Administrative Services Act of 1949, as amended. ... MIL-STD-188 is a series of U.S. military standards relating to telecommunications. ... Federal Standard 1037C, entitled Telecommunications: Glossary of Telecommunication Terms is a United States Federal Standard, issued by the General Services Administration pursuant to the Federal Property and Administrative Services Act of 1949, as amended. ... Federal Standard 1037C, entitled Telecommunications: Glossary of Telecommunication Terms is a United States Federal Standard, issued by the General Services Administration pursuant to the Federal Property and Administrative Services Act of 1949, as amended. ...

External links

Wikimedia Commons has media related to:
The GPOs first telephone exchange to open in London. ...

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