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manually connecting calls with patch cables at a telephone switchboard.]] telephone 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. 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). HISTORIC PERSPECTIVE Central Office in Lakeland, Florida at night.]] Central Office in Houston, Texas.]] The first telephone exchange opened in New Haven, Connecticut in 1878. The switchboard was built from "carriage bolts, handles from teapot lids and bustle wire" and could handle two simultaneous conversations. See National Park Service "first switchboard" page . Later exchanges consisted of one to several ''hundred'' Plug Boards staffed by Telephone Operator s. 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.1 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.2 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, MO 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. 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). 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. 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 the mid to late 1990s. Number plan trivia See Telephone Number TECHNOLOGIES This article will use the terms:
Many of the terms in this article have conflicting UK and US usages.
Manual service exchanges 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. 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). Connected to a switch, an off-hook condition operates a relay to connect a dial tone and a device to collect dialed digits. In the largest U.S. cities, it took many years to convert every office to automatic equipment, such as Panel Switch es. 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 at the subscriber's home provided the DC power for conversation. Magneto systems were in use in some small towns in the U.S. as late as the 1980s. In general, this type of system had a poorer call quality compared to common-battery systems. Many small town magneto systems featured Party Line s, 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. Pre-digital automatic exchanges Automatic exchanges, or '''dial service''', came into existence in the early 1900s. Their purpose was to eliminate the need for human Telephone Operator s. 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 . 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. 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 . 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. Electromechanical signaling See Also: Signalling (telecommunications) 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. 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. 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 radios. 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. Sounds
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