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 telephone 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). It is sometimes confused with other concepts of telephone geography, such as NPA or area code. 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
Prior to the telephone, electrical switches were used to switch telegraph lines. One of the first people to build a telephone exchange was Hungarian 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 .
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 the connection for 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. 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 Brown Strowger, an undertaker in Kansas City, Missouri, patented the stepping switch, a device which led to 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 semi-circle. 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 the first digit and then to swing horizontally in a contact row with one small rotation for each pulse in the next 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 mid 2002.
Number plan details
See Telephone number
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.
- 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.
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).
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.
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.
Many small town magneto systems featured party lines, anywhere from two to ten or more subscribers sharing a single line. When c
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