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63-436: When a telephone call is set up from one subscriber to another, several telephone exchanges could be involved, possibly across international boundaries. To allow a call to be set up correctly, where ISUP is supported, a switch will signal call-related information like called party number to the next switch in the network using ISUP messages. The telephone exchanges may be connected via T1 or E1 trunks which transport

126-545: 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

189-446: 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

252-454: 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. The terminology used in telecommunications has evolved over time, with telephone exchange and central office often used interchangeably,

315-644: A CIC with 14 significant bits instead of the 12 that are shown. When sent using the services of the Signalling Connection Control Part , ISUP messages passed to SCCP in the User Data parameter (NSDU) consist of only the last 4 components ( Message Type, Mandatory fixed part, Mandatory variable part, Optional part ). The routing label and circuit identification code are not included in the user data passed to SCCP. Telephone exchange A telephone exchange , also known as

378-506: A cause code number. Even for a normal ISUP call, a cause code is generated. There are lot of applications developed based on the cause code from ISUP signaling. Similarly Telecom operators trace for Causecodes to debug any call failures. Following are the list of cause codes used. Cause codes only defined by number are effectively undefined, and may be used for proprietary solutions. ... ... ... ... The Signalling Information Field (SIF) for all ISUP Message Signal Units (MSU) contain

441-459: A central office (C.O.) is a common carrier switching center Class 5 telephone switch in which trunks and local loops are terminated and switched. In the UK, a telephone exchange means an exchange building, and is also the name for a telephone switch. With manual service , the customer lifts the receiver off-hook and asks the operator to connect the call to a requested number. Provided that

504-484: A circuit connecting a dialed call through an electromechanical switch had DC continuity within the local exchange area via metallic conductors. The design and maintenance procedures of all systems involved methods to avoid that subscribers experienced undue changes in the quality of the service or that they noticed failures. A variety of tools referred to as make-busy s were plugged into electromechanical switch elements upon failure and during repairs. A make-busy identified

567-412: 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

630-517: A failed switch element. A trouble reporting card system was connected to switch common control elements. These trouble reporting systems punctured cardboard cards with a code that logged the nature of a failure. Electromechanical switching systems required sources of electricity in form of direct current (DC), as well as alternating ring current (AC), which were generated on-site with mechanical generators. In addition, telephone switches required adjustment of many mechanical parts. Unlike modern switches,

693-404: A horizontal panel containing two rows of patch cords, each pair connected to a cord circuit . When a calling party lifted the receiver, the local loop current lit a signal lamp near the jack. The operator responded by inserting the rear cord ( answering cord ) into the subscriber's jack and switched their headset into the circuit to ask, "Number, please?" For a local call, the operator inserted

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756-411: A hundred pair cable between switches, for example. Conductors in one common circuit configuration were named tip, ring, ear (E) and mouth (M). Tip and ring were the voice-carrying pair, and named after the tip and ring on the three conductor cords on the manual operator's console. In two-way trunks with E and M signaling , a handshake took place to prevent both switches from colliding by dialing calls on

819-485: A lamp on the operator's switchboard, signaling the operator to perform service. In the largest cities, it took many years to convert every office to automatic equipment, such as a panel switch . During this transition period, once numbers were standardized to the 2L-4N or 2L-5N format (two-letter exchange name and either four or five digits), it was possible to dial a number located in a manual exchange and be connected without requesting operator assistance. The policy of

882-491: A mandatory fixed-length parameter part, a mandatory variable-length parameter part, and an optional parameter part that are dependent on the type of message being sent. ISUP messages can be sent using the services of the Message Transfer Part , or, less often, the Signalling Connection Control Part . These messages are transmitted in various stages of call setup and release. The most common messages are: This

945-399: A manual office, having listings such as Hillside 834 or East 23, was recognizable by the format in which the second letter was not capitalized. Rural areas, as well as the smallest towns, had manual service and signaling was accomplished with magneto telephones, which had a crank for the signaling generator. To alert the operator, or another subscriber on the same line, the subscriber turned

1008-407: A private telephone exchange is termed a private branch exchange (PBX), which connects to the public switched telephone network. A PBX serves an organization's telephones and any private leased line circuits, typically situated in large office spaces or organizational campuses. Smaller setups might use a PBX or key telephone system managed by a receptionist, catering to the telecommunication needs of

1071-436: A single line. When calling a party, the operator used code ringing, a distinctive ringing signal sequence, such as two long rings followed by one short ring. Everyone on the line could hear the signals, and could pick up and monitor other people's conversations. Automatic exchanges , which provided dial service , were invented by Almon Strowger in 1888. First used commercially in 1892, they did not gain widespread use until

1134-526: A telephone with a dial tone . Telecommunication carriers also define rate centers for business and billing purposes, which in large cities, might encompass clusters of central offices to specify geographic locations for distance measurement calculations. In the 1940s, the Bell System in the United States and Canada introduced a nationwide numbering system that identified central offices with

1197-466: 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 binary digital transmission systems, bits within the T-1 data stream were used to transmit supervision. By careful design,

1260-476: A unique three-digit code, along with a three-digit numbering plan area code (NPA code or area code), making central office codes distinctive within each numbering plan area. These codes served as prefixes in subscriber telephone numbers. The mid-20th century saw similar organizational efforts in telephone networks globally, propelled by the advent of international and transoceanic telephone trunks and direct customer dialing. For corporate or enterprise applications,

1323-402: A year later. In 1887 Puskás introduced the multiplex switchboard . . Later exchanges consisted of one to several hundred plug boards staffed by switchboard 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

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1386-724: Is a very basic call flow involving only two telecom switches which exchange the ISUP messages. The subscriber interfaces are not covered here and are only listed for a better understanding. Detailed call flows are provided in ITU-T Recommendation Q.784.1. Below is a detailed exchange of ISUP messages involving two ISDN telecom switches. The report was from an Alcatel S12 digital switch. Release cause codes are used to identify and debug any events occurring in ISDN User Part signaling. Every event in ISUP signaling generates

1449-403: Is backwards compatible with ISUP Blue Book and Q.767 for basic call procedures and supplementary services except for some procedures (e.g. number portability). Additionally the compatibility features introduced in this version ensure forward compatibility with newer versions. An ISUP message contains a fixed header containing the circuit identification code and the ISUP message type, followed by

1512-468: The Bell System stated that customers in large cities should not need to be concerned with the type of office, whether they were calling a manual or an automatic office. When a subscriber dialed the number of a manual station, an operator at the destination office answered the call after seeing the number on an indicator , and connected the call by plugging a cord into the outgoing circuit and ringing

1575-737: The Bell Telephone Company in Boston in 1877. The world's first state-administered telephone exchange opened on November 12, 1877 in Friedrichsberg close to Berlin under the direction of Heinrich von Stephan . George W. Coy designed and built the first commercial US telephone exchange which opened in New Haven, Connecticut in January, 1878, and the first telephone booth was built in nearby Bridgeport . The switchboard

1638-799: The North American Numbering Plan ; however, some countries under the NANP differ in their support of some procedures (for example, LATA is meaningless within Canada. Also, RBOCs support Telcordia procedures not fully specified by ANSI.) Some countries outside the NANP support ANSI-based variants (e.g. Mexico). While these variations of ISUP differ in subtle ways, the vast majority of ISUP message type, parameter type, and parameter field code-points, and related fundamental call processing procedures, agree across all variants. According to ITU-T Q.761 section 2.4.1 ISUP interworking ISUP'92

1701-470: The Western Electric 1ESS switch , Northern Telecom SP1 , Ericsson AXE, Automatic Electric EAX-1 & EAX-2, Philips PRX /A, ITT Metaconta, British GPO/BT TXE series and several other designs were similar. Ericsson also developed a fully computerized version of their ARF crossbar exchange called ARE. These used a crossbar switching matrix with a fully computerized control system and provided

1764-546: The crossbar switch . Circuits interconnecting switches are called trunks . Before Signalling System 7 , Bell System electromechanical switches in the United States originally communicated with one another over trunks using a variety of DC voltages and signaling tones. Today, those simple digital signals have been replaced by more modern coded digital signals (typically using binary code). Some signaling communicated dialed digits. An early form called Panel Call Indicator Pulsing used quaternary pulses to set up calls between

1827-415: The telephone when the user removes the handset from the switchhook or cradle. The exchange provides dial tone at that time to indicate to the user that the exchange is ready to receive dialed digits. The pulses or DTMF tones generated by the telephone are processed and a connection is established to the destination telephone within the same exchange or to another distant exchange. The exchange maintains

1890-409: The Bell System required continuous maintenance, such as cleaning. Indicator lights on equipment bays 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,

1953-414: The Bell System. Electronic switching systems gradually evolved in stages from electromechanical hybrids with stored program control to the fully digital systems. Early systems used reed relay -switched metallic paths under digital control. Equipment testing, phone numbers reassignments, circuit lockouts and similar tasks were accomplished by data entry on a terminal. Examples of these systems included

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2016-474: The Strowger's typical 10 pps—typically about 20 pps. At a later date many also accepted DTMF "touch tones" or other tone signaling systems. A transitional technology (from pulse to DTMF) had converters to convert DTMF to pulse, to feed to older Strowger, panel, or crossbar switches. This technology was used as late as mid-2002. Many terms used in telecommunication technology differ in meaning and usage among

2079-471: 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 pure digital carrier which did not have DC continuity. Bell System installations typically had alarm bells, gongs, or chimes to announce alarms calling attention to

2142-407: The automation of 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 semicircle. When used with a rotary telephone dial , each pair of digits caused the shaft of the central contact "hand" of the stepping switch to first step (ratchet) up one level for each pulse in

2205-474: 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. On March 10, 1891, Almon Brown Strowger , an undertaker in Kansas City, Missouri , patented the stepping switch , a device which led to

2268-424: The called party number) to determine which inbound and outbound circuits should be connected together to provide an end to end speech path. In addition to call related information, ISUP is also used to exchange status information for, and permit management of, the available circuits. In the case of no outbound circuit being available on a particular exchange, a release message is sent back to the preceding switches in

2331-474: The chain. Different ISUP variants exist. ITU-T specifies the variant used in the international network. In Europe ETSI releases its own ISUP specification which is close that of the ITU-T. ITU-T ISUP is used for international connections and is the base for some national ISUP variants. Most countries have their own variation of ISUP to cover national requirements. ANSI specifies variations of ISUP utilized under

2394-437: The circuit, allowing them to handle another call, while the caller heard an audible ringback signal, so that that operator would not have to periodically report that they were continuing to ring the line. 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. This chain of intermediate operators could complete

2457-419: The connection until one of the parties hangs up. This monitoring of connection status is called supervision. Additional features, such as billing equipment, may also be incorporated into the exchange. The Bell System dial service implemented a feature called automatic number identification (ANI) which facilitated services like automated billing, toll-free 800-numbers , and 9-1-1 service. In manual service,

2520-581: The crank to generate ringing current. The switchboard responded by interrupting the circuit, which dropped a metal tab above the subscriber's line jack and sounded a buzzer. Dry cell batteries, normally two large N°. 6 cells in the subscriber's telephone, provided the direct current for the transmitter. Such magneto systems were in use in the US as late as 1983, as in the small town, Bryant Pond, Woodstock, Maine . Many small town magneto systems featured party lines , anywhere from two to ten or more subscribers sharing

2583-468: The destination station. For example, if a dial customer calling from TAylor 4725 dialed a number served by a manual exchange, e.g., ADams 1383-W, the call was completed, from the subscriber's perspective, exactly as a call to LEnnox 5813, in an automated exchange. The party line letters W, R, J, and M were only used in manual exchanges with jack-per-line party lines. In contrast to the listing format MAin 1234 for an automated office with two capital letters,

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2646-473: The enterprise. In the era of the electrical telegraph, its principal users were post offices, railway stations, the more important governmental centers (ministries), stock exchanges, very few nationally distributed newspapers, the largest internationally important corporations, and wealthy individuals. Despite the fact that telephone devices existed before the invention of the telephone exchange, their success and economical operation would have been impossible on

2709-493: 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-computerized 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

2772-435: The first decade of the 20th century. They eliminated the need for human switchboard operators who completed the connections required for a telephone call . Automation replaced human operators with electromechanical systems, and telephones were equipped with a dial by which a caller transmitted the destination telephone number to the automatic switching system. A telephone exchange automatically senses an off-hook condition of

2835-424: The first digit and then to swing horizontally in a contact row with one small rotation for each pulse in the next digit. Later stepping switches were arranged in banks, the first stage of which was a linefinder . If one of up to a hundred subscriber lines (two hundred lines in later linefinders) had the receiver lifted "off hook", a linefinder connected the subscriber's line to a free first selector, which returned

2898-584: The following components: The Routing Label indicates the Point Codes of the originating and destination nodes in the network; it also includes the Signalling Link Selection field that is used to select between the multiple routes an MSU could take between two nodes. The Circuit Identification Code is used to specify which trunk between two switches is used to carry a particular call. Note that some versions of ANSI ISUP permit

2961-399: The front cord of the pair ( ringing cord ) into the called party's local jack and started the ringing cycle. For a long-distance call, the operator plugged into a trunk circuit to connect to another operator in another bank of boards or at a remote central office. In 1918, the average time to complete the connection for a long-distance call was 15 minutes. Early manual switchboards required

3024-404: The impetus for the creation of a new industrial sector. As with the invention of the telephone itself, the honour of "first telephone exchange" has several claimants. One of the first to propose a telephone exchange was Hungarian Tivadar Puskás in 1877 while he was working for Thomas Edison . The first experimental telephone exchange was based on the ideas of Puskás, and it was built by

3087-408: The latter term originating from the Bell System . A central office typically refers to a facility that houses the inside plant equipment for one or several telephone exchanges, each catering to a specific geographical region. This region is sometimes known as the exchange area. In North America, the term wire center may be used to denote a central office location, indicating a facility that provides

3150-399: The number is in the same central office, and located on the operator's switchboard, the operator connects the call by plugging the ringing cord into the jack corresponding to the called customer's line. If the called party's line is on a different switchboard in the same office, or in a different central office, the operator plugs into the trunk for the destination switchboard or office and asks

3213-476: 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. 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

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3276-432: The operator answering (known as the "B" operator) to connect the call. Most urban exchanges provided common-battery service, meaning that the central office provided power to the subscriber telephone circuits for operation of the transmitter, as well as for automatic signaling with rotary dials . In common-battery systems, the pair of wires from a subscriber's telephone to the exchange carry 48V (nominal) DC potential from

3339-543: The operator knows where a call is originating by the light on the switchboard jack field. Before ANI, long-distance calls were placed into an operator queue and the operator asked the calling party's number and recorded it on a paper toll ticket. Early exchanges were electromechanical systems using motors, shaft drives, rotating switches and relays . Some types of automatic exchanges were the Strowger switch or step-by-step switch, All Relay, panel switch , Rotary system and

3402-408: The operator to operate listening keys and ringing keys, but by the late 1910s and 1920s, advances in switchboard technology led to features which allowed the call to be automatically answered immediately as the operator inserted the answering cord, and ringing would automatically begin as soon as the operator inserted the ringing cord into the called party's jack. The operator would be disconnected from

3465-427: The part being worked on as in-use, causing the switching logic to route around it. A similar tool was called a TD tool. Delinquent subscribers had their service temporarily denied (TDed). This was effected by plugging a tool into the subscriber's office equipment on Crossbar systems or line group in step-by-step switches. The subscriber could receive calls but could not dial out. Strowger-based, step-by-step offices in

3528-679: The same schema and structure of the contemporary telegraph, as prior to the invention of the telephone exchange switchboard, early telephones were hardwired to and communicated with only a single other telephone (such as from an individual's home to the person's business ). A telephone exchange is a telephone system for a small geographic area that provides the switching (interconnection) of subscriber lines for calls made between them. Telephone exchanges replaced small telephone systems that connected its users with direct lines between each and every subscriber station. Exchanges made telephony an available and comfortable technology for everyday use and it gave

3591-414: 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 (a wink) to go ahead with dial pulsing. This was done with relay logic and discrete electronics. These voltage changes on

3654-422: The speech from the calls. These trunks are divided into 64 kbit/s timeslots, and one timeslot can carry exactly one call. Regardless of what facilities are used to interconnect switches, each circuit between two switches is uniquely identified by a circuit identification code (CIC) that is included in the ISUP messages. The exchange uses this information along with the received signaling information (especially

3717-457: The subscriber a dial tone to show that it was ready to receive dialled digits. The subscriber's dial pulsed at about 10 pulses per second, although the speed depended on the standard of the particular telephone administration. Exchanges based on the Strowger switch were eventually challenged by other exchange types and later by crossbar technology. These exchange designs promised faster switching and would accept inter-switch pulses faster than

3780-400: The telephone company end across the conductors. The telephone presents an open circuit when it is on-hook or idle. When a subscriber's phone is off-hook, it presents an electrical resistance across the line which causes current to flow through the telephone and wires to the central office. In a manually operated switchboard, this current flowed through a relay coil, and actuated a buzzer or

3843-434: 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

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3906-451: The various English speaking regions. For the purpose of this article the following definitions are made: A central office originally was a primary exchange in a city with other exchanges service parts of the area. The term became to mean any switching system including its facilities and operators. It is also used generally for the building that houses switching and related inside plant equipment. In United States telecommunication jargon,

3969-593: Was built from "carriage bolts, handles from teapot lids and bustle wire" and could handle two simultaneous conversations. Charles Glidden is also credited with establishing an exchange in Lowell, MA. with 50 subscribers in 1878. In Europe other early telephone exchanges were based in London and Manchester , both of which opened under Bell patents in 1879. Belgium had its first International Bell exchange (in Antwerp )

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