||This article needs additional citations for verification. (June 2011)|
A telephone switchboard is a telecommunications system used in the public switched telephone network or in enterprises to interconnect circuits of telephones to establish telephone calls between the subscribers or users, or between other exchanges. The switchboard was an essential component of a manual telephone exchange, and was operated by one or more persons, called operators who either used electrical cords or switches to establish the connections.
The electromechanical automatic telephone exchange, invented by Almon Strowger in 1888, gradually replaced manual switchboards in central telephone exchanges starting in 1919 when the Bell System adopted automatic switching, but many manual branch exchanges remained operational during the last half of the 20th century in offices, hotels, or other enterprises. Later electronic devices and computer technology gave the operator access to an abundance of features. In modern businesses, a PBX often has an attendant console for the operator, or an auto-attendant, which bypasses the operator entirely.
The switchboard is usually designed to accommodate the operator, who sits facing it. It has a high backpanel which consists of rows of female jacks, each jack designated and wired as a local extension of the switchboard (which serves an individual subscriber) or as an incoming or outgoing trunk line. The jack is also associated with a lamp.
On the table or desk area in front of the operator are columns of keys, lamps and cords. Each column consists of a front key and a rear key, a front lamp and a rear lamp, followed by a front cord and a rear cord, making up together a cord circuit. The front key is the "talk" key allowing the operator to speak with that particular cord pair. The rear key on older "manual" boards and PBXs is used to physically ring a telephone. On newer boards, the back key is used to collect (retrieve) money from coin telephones. Each of the keys has three positions: back, normal and forward. When a key is in the normal position an electrical talk path connects the front and rear cords. A key in the forward position (front key) connects the operator to the cord pair, and a key in the back position sends a ring signal out on the cord (on older manual exchanges). Each cord has a three-wire TRS phone connector: tip and ring for testing, ringing and voice; and a sleeve wire for busy signals.
When a call is received, a jack lamp lights on the back panel and the operator responds by placing the rear cord into the corresponding jack and throwing the front key forward. The operator then converses with the caller, who informs the operator to whom he or she would like to speak. If it is another extension, the operator places the front cord in the associated jack and pulls the front key backwards to ring the called party. After connecting, the operator leaves both cords "up" with the keys in the normal position so the parties can converse. The supervision lamps light to alert the operator when the parties finish their conversation and go on-hook. Either party could "flash" the operator's supervision lamps by depressing their switch hook for a second and releasing it, in case they needed assistance with a problem. When the operator pulls down a cord, a pulley weight behind the switchboard pulls it down to prevent it from tangling.
On a trunk, on-hook and off-hook signals must pass in both directions. In a one-way trunk, the originating or A board sends a short for off-hook, and an open for on-hook, while the terminating or B board sends normal polarity or reverse polarity. This "reverse battery" signaling was carried over to later automatic exchanges.
The first telephones in the 1870s were rented in pairs which were limited to conversation between those two instruments. The use of a central exchange was soon found to be even more advantageous than in telegraphy. Small towns typically had the switchboard installed in the operator's home so that he or she could answer calls on a 24 hour basis. In 1894, New England Telephone and Telegraph installed the first battery-operated switchboard on January 9 in Lexington, Massachusetts.
Early switchboards in large cities usually were mounted floor to ceiling in order to allow the operators to reach all the lines in the exchange. The operators were boys who would use a ladder to connect to the higher jacks. Late in the 1890s this measure failed to keep up with the increasing number of lines, and Milo G. Kellogg devised the Divided Multiple Switchboard for operators to work together, with a team on the "A board" and another on the "B." These operators were almost always women until the early 1970s, when men were once again hired. Cord switchboards were often referred to as "cordboards" by telephone company personnel. Conversion to Panel switch and other automated switching systems first eliminated the "B" operator and then, usually years later, the "A". Rural and suburban switchboards for the most part remained small and simple. In many cases, customers came to know their operator by name.
As telephone exchanges converted to automatic (dial) service, switchboards continued to serve specialized purposes. Before the advent of direct-dialed long distance calls, a subscriber would need to contact the long-distance operator in order to place a toll call. In large cities, there was often a special number, such as 112, which would ring the long-distance operator directly. Elsewhere, the subscriber would ask the local operator to ring the long-distance operator.
The long distance operator would record the name and city of the person to be called, and the operator would advise the calling party to hang up and wait for the call to be completed. Each toll center had only a limited number of trunks to distant cities, and if those circuts were busy, the operator would try alternate routings through intermediate cities. The operator would plug into a trunk for the destination city, and the inward operator would answer. The inward operator would obtain the number from the local information operator, and ring the call. Once the called party answered, the originating operator would advise him or her to stand by for the calling party, whom she'd then ring back, and record the starting time, once the conversation began.
In the 1940s, with the advent of dial pulse and multi-frequency operator dialing, the operator would plug into a tandem trunk and dial the NPA (area code) and operator code for the information operator in the distant city. For instance, the New York City information operator was 212-131. If the customer knew the number, and the point was direct-dialable, the operator would dial the call. If the distant city did not have dialable numbers, the operator would dial the code for the inward operator serving the called party, and ask her to ring the number.
In the 1960s, once most phone subscribers had direct long-distance dialing, a single type of operator began to serve both the local and long distance functions. A customer might call to request a collect call, a call billed to a third number, or a person-to-person call. All toll calls from coin phones required operator assistance. The operator was also available to help complete a local or long-distance number which did not complete. For example, if a customer encountered a reorder tone (a fast busy signal), it could indicate "all circuits busy," or a problem in the destination exchange. The operator might be able to use a different routing to complete the call. If the operator could not get through by dialing the number, she could call the inward operator in the destination city, and ask her to try the number, or to test a line to see if it was busy or out of order.
Cord switchboards used for these purposes were replaced in the 1970s and 1980s by TSPS and similar systems, which greatly reduced operator involvement in calls. The customer would, instead of simply dialing "0" for the operator, dial 0+NPA+7digits, after which an operator would answer and provide the desired service (coin collection, obtaining acceptance on a collect call, etc.), and then release the call to be automatically handled by the TSPS.
Before the late 1970s and early 1980s, it was common for many smaller cities to have their own operators. An NPA (area code) would usually have its largest city as its primary toll center, with smaller toll centers serving the secondary cities scattered throughout the NPA. TSPS allowed telephone companies to close smaller toll centers and consolidate operator services in regional centers which might be hundreds of miles from the subscriber.
A virtual switchboard is an automated system used to connect an incoming caller with an agent or staff member. The virtual switchboard user normally has the option of controlling how incoming calls are routed via a web interface. For example calls could be routed to different destinations according to certain criteria such as the time of day etc.
Interactive voice response (IVR) functionality is also a common feature with virtual switchboards. IVR enables incoming callers to a virtual switchboard to hear prerecorded announcements. Popular announcements instruct callers to press a number on their key pad to select which department they want to reach (for example, "press 1 for sales, 2 for accounts, 3 for support, and so on").
- 1911 Britannica article
- Atlanta Telephone History: Part 1 - Early Telephone Service especially the section from 1905 on.
|Wikimedia Commons has media related to: Manual telephone exchanges|