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. In Telecommunication, the term inside plant has the following meanings All the cabling and equipment installed in a Telecommunications facility,
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). The United States of America —commonly referred to as the Local access and transport area (LATA is a term used in US telecommunications regulation In United States Telecommunication law Modification of Final Judgment ( MFJ) is the 1982 antitrust suit settlement agreement ( Consent decree
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. Tivadar Puskás ( 17 September, 1844 Pest - 16 March, 1893 Budapest) was a Hungarian Inventor, 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 . 
Later exchanges consisted of one to several hundred plug boards staffed by telephone operators. A switchboard (also called a manual exchange) was a device used to connect a group of Telephones manually to one another or to an outside connection within and between A telephone operator is either a person who provides assistance to a Telephone caller usually in the placing of Operator assisted telephone 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. The subscription business model is a Business model where a customer must pay a subscription price to have access to the product/service A telephone line or telephone circuit (or just line or circuit within the Industry) is a single-user circuit on a Telephone In front of the jack panel lay a horizontal panel containing two rows of patch cords, each pair connected to a cord circuit. In Telecommunication, a cord circuit is a switchboard circuit in which a plug-terminated cord is used to establish connections manually between user When a calling party lifted the receiver, a signal lamp near the jack would light. The calling party (also called caller, call originator or A-party) is a person who (or device that Initiates a Telephone call over 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.  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. 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  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.
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. Events 241 BC - First Punic War: Battle of the Aegates Islands - The Romans sink the Carthaginian fleet bringing Year 1891 ( MDCCCXCI) was a Common year starting on Thursday (link will display the full calendar of the Gregorian calendar (or a Common Almon Brown Strowger (1839 &ndash May 26 1902 gave his name to the electromechanical Telephone exchange technology that his invention and Patent inspired In electrical controls a stepping switch is an electromechanical device which allows an input connection to be connected to one of a number of possible output connections under 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. 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
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". In electrical controls a stepping switch is an electromechanical device which allows an input connection to be connected to one of a number of possible output connections under 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).
Exchanges based on the Strowger switch were challenged by other selectors and by crossbar technology. The panel switching system was an early type of Automatic telephone exchange, first put into urban service by the Bell System in the 1920s and removed during the 1970s A crossbar switch (also known as cross-point switch, crosspoint switch, or matrix switch) is a Switch connecting multiple inputs to multiple 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. Dual-tone multi-frequency ( DTMF) signaling 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.
See Telephone number
This article will use the terms:
Many of the terms in this article have conflicting UK and US usages.
With manual service, the customer lifts the receiver off-hook and asks the operator to connect the call to a requested number. In Telephony, the term off-hook has the following meanings The condition that exists when a Telephone or other user instrument is in A telephone operator is either a person who provides assistance to a Telephone caller usually in the placing of Operator assisted telephone 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. A switchboard (also called a manual exchange) was a device used to connect a group of Telephones manually to one another or to an outside connection within and between 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.
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. In Telephony, the term on-hook has the following meanings The condition that exists when a Telephone or other user instrument is not 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). 
In the largest U. S. cities, it took many years to convert every office to automatic equipment, such as Panel switches. The panel switching system was an early type of Automatic telephone exchange, first put into urban service by the Bell System in the 1920s and removed during the 1970s 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.
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. A dry cell is a galvanic Electrochemical cell with a pasty low-moisture Electrolyte. Magneto systems were in use in one American small town, Bryant Pond, Woodstock, Maine as late as 1983. Woodstock is a town in Oxford County, Maine, United States. The population was 1307 at the 2000 census In general, this type of system had a poorer call quality compared to common-battery systems.
Many small town magneto systems featured party lines, anywhere from two to ten or more subscribers sharing a single line. In 20th century Telephone systems a party line (also multiparty line or Shared Service Line) is an arrangement in which two or more customers are When calling a party, the operator would use a distinctive ringing signal sequence, such as two long rings followed by one short. A ringtone or ring tone is the sound made by a Mobile phone to indicate an incoming call or text message 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.
Automatic exchanges, or dial service, came into existence in the early 1900s. Their purpose was to eliminate the need for human telephone operators. A telephone operator is either a person who provides assistance to a Telephone caller usually in the placing of Operator assisted telephone Before the exchanges became automated, operators had to complete the connections required for a telephone call. A telephone call is a connection over a Telephone network between the Calling party and the Called party. 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. Routing is the process of selecting paths in a network along which to send network traffic Basic principle A traditional landline telephone system also known as "plain old telephone service" (POTS, commonly handles both signaling and audio information The public switched telephone network ( PSTN) is the network of the world's public circuit-switched Telephone networks in much the same way that the
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. Basic principle A traditional landline telephone system also known as "plain old telephone service" (POTS, commonly handles both signaling and audio information A dial tone (known in the British Isles as a dialling tone) is a Telephony signal used to indicate that the Telephone exchange is working Dual-tone multi-frequency ( DTMF) signaling 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. Automatic Number Identification (ANI is a feature of Telephony Intelligent network services that permits subscribers to display or capture the Telephone 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 Message Accounting (AMA provides detail billing for telephone calls
Early exchanges used motors, shaft drives, rotating switches and relays. A relay is an electrical Switch that opens and closes under the control of another Electrical circuit. 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. The Strowger switch, also known as Step-by-Step or SXS, is an early electromechanical telephone switching system invented by Almon Brown Strowger. The panel switching system was an early type of Automatic telephone exchange, first put into urban service by the Bell System in the 1920s and removed during the 1970s A crossbar switch (also known as cross-point switch, crosspoint switch, or matrix switch) is a Switch connecting multiple inputs to multiple These are referred to collectively as electromechanical switches.
Circuits connecting two switches are called trunks. In Telecommunication, signalling (UK spelling or signaling (US spelling has the following meanings The use of signals for controlling communications 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. Signaling System #7 (SS7 is a set of Telephony signaling protocols which are used to set up most of the world's Public switched telephone network telephone The Bell System which was named for Alexander Graham Bell, the technologist popularly credited with the invention of the Telephone, was a Trademark and It would be rare to see any of these in use today.
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. The panel switching system was an early type of Automatic telephone exchange, first put into urban service by the Bell System in the 1920s and removed during the 1970s 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. Pulse dialing, dial pulse, or loop disconnect dialing, also called Rotary or Decadic dialling in the United Kingdom (because up to 10 pulses 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 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. In Telephony Multi-Frequency (MF is an outdated in-band signaling technique 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. The ITU Telecommunication Standardization Sector ( ITU-T) coordinates standards for telecommunications on behalf of the International Telecommunication 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. A telephone number or phone number is a sequence of numbers used to call from one Telephone line to another in a Telephone network. 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.
Examples of signals communicating supervision or call progress include E and M signaling, SF signaling, and robbed-bit signaling. E&M is a type of supervisory line signaling that uses separate leads called the "E" lead and "M" lead traditionally used in the North American telecommunications 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). In Telecommunications, falsing describes a Decoder detecting a valid input when one is not present Some systems used tone frequencies over 3,000 Hz, particularly on SSB frequency division multiplex microwave radio relays. Frequency-division multiplexing ( FDM) is a form of signal Multiplexing where multiple Baseband signals are Modulated on different frequency Microwave radio relay is a technology for transmitting digital and analog signals, such as long-distance Telephone calls and the relay of On T-carrier digital transmission systems, bits within the T-1 data stream were used to transmit supervision. In Telecommunications T-carrier, sometimes abbreviated as T-CXR, is the generic designator for any of several digitally multiplexed telecommunications 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.
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. A "Community Dial Office" ( CDO) was a small Class 5 Telephone exchange in a rural area These facilities almost always had concrete floors. The hard surfaces reflected sounds.
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 is a type of Relay, primarily manufactured by the Western Electric Company for use by the Bell System in electromechanical For other meanings see the disambiguation page Marker A marker is a type of special purpose control system that was used in electromechanical
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. A wire spring relay is a type of Relay, primarily manufactured by the Western Electric Company for use by the Bell System in electromechanical The reorder tone, also known as the fast busy tone or the congestion tone, is a dual- Frequency tone of 480 Hz and 620 Hz at a cadence of 0 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 reed relay is one or more Reed switches controlled by an Electromagnet. Stored Program Control exchange (SPC is the technical name used for Telephone exchanges controlled by a computer program stored in the memory of the system
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
The first Electronic Switching Systems were not entirely digital. In Telecommunications an electronic switching system ( ESS) is A Telephone exchange based on the principles of Time-division multiplexing The Western Electric 1ESS switch still had reed relay metallic paths. Western Electric Company (sometimes abbreviated WE and WECo) was an American Electrical engineering company the manufacturing arm The Number One Electronic Switching System, the first large scale Stored Program Control (SPC Telephone exchange or Electronic Switching System in the A reed relay is one or more Reed switches controlled by an Electromagnet. It was stored-program-controlled. Stored Program Control exchange (SPC is the technical name used for Telephone exchanges controlled by a computer program stored in the memory of the system Equipment testing, changes to phone numbers, circuit lockouts and similar tasks were accomplished by typing on a terminal. In Telecommunications the term lockout has the following meanings In Telephone systems treatment of a user 's line or Northern Telecom SP1, Ericsson AKE, Philips PRX/A, ITT Metaconta, British Telecom TXE series and several other designs were similar. 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 The PRX205 (PRX/A is a processor controlled Reed relay Telephone exchange developed by Philips Telecommunicatie Industrie BV (PTI in Hilversum TXE, which stands for T elephone e' X' change E lectronic was the designation given to a family of telephone exchanges developed by the British General 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). See also Signaling (telecommunications In telephony Common Channel Signaling (CCS or in the US Common Channel Interoffice Signaling (CCIS is the transmission This data link was based on CCITT 6, a predecessor to SS7.
Digital switches work by connecting two or more digital circuits together, according to a dialed telephone number. A telephone number or phone number is a sequence of numbers used to call from one Telephone line to another in a Telephone network. Calls are setup between switches using the Signalling System 7 protocol, or one of its variants. Signaling System #7 (SS7 is a set of Telephony signaling protocols which are used to set up most of the world's Public switched telephone network telephone In U. S. and military telecommunication, a digital switch is a switch that performs time division switching of digitized signals.  This was first done in a few small and little used systems. The first product using a digital switch system was made by Amtelco. Amtelco is a manufacturer of Telecommunications equipment and telephone answering service and call center systems founded in 1976 Prominent examples include Nortel DMS-100, Lucent 5ESS switch, Siemens EWSD and Ericsson AXE telephone exchange. The DMS-100 Switch is the biggest seller of a line of Digital Multiplex System (DMS Telephone exchange switches manufactured by Nortel Networks. The 5ESS Switch is the Class 5 telephone Electronic switching system sold by Alcatel-Lucent. EWSD ( E lektronisches W ähl' s' ystem D igital in German, Electronic Digital Switching System/Electronic World Switch Digital The AXE telephone exchange is a product line of circuit switched digital Telephone exchanges manufactured by Ericsson, a Swedish telecom company 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.
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. )
Individual local loop telephone lines are connected to a remote concentrator. In Telephony, the local loop (also referred to as a subscriber line) is the physical link or circuit that connects from the Demarcation point of the In modern Telephony a remote concentrator or Remote Line Concentrator (RLC is the lowest level in the Telephone switch hierarchy 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. The European Telecommunications Standards Institute ( ETSI) is an independent non-for-profit Standardization organization of the Telecommunications V5 is a family of telephone network protocols defined by ETSI which allow communications between the Telephone exchange, also known in the specifications 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.
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. A digital loop carrier (DLC is a system which uses Digital transmission to extend the range of the Local loop farther than would be possible using only
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. A telephone company (or telco) provides Telecommunications services such as Telephony and Data communications Most of the largest telcos whatever
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 Telecommunication, the term outside plant has the following meanings In civilian Telecommunications, all cables conduits ducts poles 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. In Telephony, a Main Distribution Frame ( MDF or Main Frame) is a signal Distribution frame for connecting equipment ( Inside plant) 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.
To reduce the expense of outside plant, some companies use "pair gain" devices to provide telephone service to subscribers. Telephony, pair gain is a method of transmitting multiple POTS signals over the Twisted pairs traditionally used for a single traditional Subscriber 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 digital loop carrier (DLC is a system which uses Digital transmission to extend the range of the Local loop farther than would be possible using only
DLCs are often referred to as Subscriber Loop Carriers (SLCs), after Lucent's proprietary name for their pair gain products. Subscriber Loop Carrier refers to equipment providing central office-like Telephone interface functionality Lucent Technologies was a technology company composed of what was formerly AT&T Technologies, which included Western Electric and Bell Labs. 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). A line card or Digital Line Card is a modular electronic circuit on a Printed circuit board, the electronic circuits on the card interfacing the telecommunication 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.
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. Telcordia Technologies, formerly Bell Communications Research Inc
Switches are used in both local central offices and in long distance centers. The public switched telephone network ( PSTN) is the network of the world's public circuit-switched Telephone networks in much the same way that the There are two major types in the Public switched telephone network (PSTN):
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. The Network Time Protocol ( NTP) is a protocol for distributing the Coordinated Universal Time (UTC by means of synchronizing the clocks of computer systems Time-standards equipment may include Rubidium- or Caesium-based standards and a Global Positioning System receiver. Basic concept of GPS operation A GPS receiver calculates its position by carefully timing the signals sent by the constellation of GPS Satellites high above the Earth
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. A Class 5 switch in United States telephony jargon refers to a Telephone switch or exchange located at the local telephone company's central office directly Central offices have more than 90% of their channel capacity unused.
While traditionally, telephone switches connected physical circuits (e. g. , wire pairs), modern telephone switches use a combination of space- and time-division switching. In Electronics, a multiplexer or mux ( occasionally the term muldex is also found for a combination multiplexer-demultiplexer In other words, each voice channel is represented by a time slot (say 1 or 2) on a physical wire pair (A or B). Time-Division Multiplexing ( TDM) is a type of Digital or (rarely analog Multiplexing in which two or more signals or bit streams are transferred 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 structure of a switch is an odd number of layers of smaller, simpler subswitches. A nonblocking minimal spanning switch is a device that implements a "switch" which is capable of connecting N inputs to N outputs in any combination (it is non-blocking in 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 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 nonblocking minimal spanning switch is a device that implements a "switch" which is capable of connecting N inputs to N outputs in any combination (it is non-blocking in Crossover switches are complex array matrices designed to switch any one input path to any one (or more output path(s
One way is to have enough switching fabric to assure that the pairwise allocation will always succeed by building a fully-connected mesh network. Switched fabric, switching fabric, or just fabric, is a Network topology where network nodes connect with each other via one or more Network switches Network topology is the study of the arrangement or mapping of the elements ( links, nodes, etc This is the method usually used in central office switches, which have low utilization of their resources.
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. In Engineering, Fault-tolerant design, also known as fail-safe design, is a design that enables a system to continue operation possibly at a reduced level (also known 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 algorithms in modern industry. A nonblocking minimal spanning switch is a device that implements a "switch" which is capable of connecting N inputs to N outputs in any combination (it is non-blocking in
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. FIFO is an Acronym for First In First Out, an abstraction in ways of organizing and manipulation of data relative to time and prioritization 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. LIFO is an Acronym which stands for last in first out. In Computer science and Queueing theory this refers to the way items stored
In US telecommunication jargon, a central office (C. The history of telecommunication began with the use of Smoke signals and drums in Africa, the Americas and parts of Asia. This is intended to be a list of the more common Central office ( Telephone company operated Telephone switches This list is nowhere near complete Telephony, pair gain is a method of transmitting multiple POTS signals over the Twisted pairs traditionally used for a single traditional Subscriber When customers of the PSTN make Telephone calls they commonly make use of a Telecommunications network called a switched-circuit network 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 computer program stored in the memory of the system A telephone number or phone number is a sequence of numbers used to call from one Telephone line to another in a Telephone network. A Digital Subscriber Line Access Multiplexer ( DSLAM, often pronounced dee-slam) allows Telephone lines to make faster connections to the Internet The Plesiochronous Digital Hierarchy ( PDH) is a technology used in Telecommunications networks to transport large quantities of data over digital transport equipment A private branch exchange (PBX is a Telephone exchange that serves a particular business or office as opposed to one that a Common carrier or telephone company During the early years of Telephone service communities that required more than 10000 Telephone numbers whether dial service was available or not utilized exchange names to O. ) is a common carrier switching center Class 5 telephone switches in which trunks and local loops are terminated and switched. A common carrier is a business that transports people goods or services and offers its services to the general public under license or authority provided by a regulatory body 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 A Class 5 switch in United States telephony jargon refers to a Telephone switch or exchange located at the local telephone company's central office directly In Telephony, the local loop (also referred to as a subscriber line) is the physical link or circuit that connects from the Demarcation point of the 
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. The Defense Switched Network (DSN is a primary information transfer network for the Defense Information Systems Network (DISN AUTOVON, short for Automatic Voice Network, was an American military phone system built in 1963 to survive nuclear attacks Deprecated synonym switch. A switch is a mechanical device used to connect and disconnect an electric Circuit at will