Telephone switchboard

PBX switchboard, 1975

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 telephone exchanges or private branch exchanges (PBXs). The user was typically known as an operator. Public manual exchanges disappeared during the last half of the 20th century, leaving a few PBXs working in offices and hotels as manual branch exchanges.

The electromechanical automatic telephone exchange, invented by Almon Strowger in 1888, gradually replaced manual switchboards in central telephone exchanges. Manual PBXs have also for the most part been replaced by more sophisticated devices or even personal computers, which give the operator access to an abundance of features.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 avoiding the operator entirely.

Operation

U.S. Air Force operator works a switchboard in the underground command post at Strategic Air Command headquarters, Offutt Air Force Base, Nebraska in 1967.

The switchboard is usually designed to accommodate the operator to sit 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 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.

History

Telephone Operator, circa 1900.

The first telephones in the 1870s were rented in pairs which could only talk to each other, but the example 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 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 mid-1960s when men were once again hired. Early "cord" switchboards were often referred to as "cordboards" by telephone company personnel. Conversion to Panel switch and other automated 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, or direct dial, service, switchboards remained in use for 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 call. In large cities, there was often a special number, such as 112 or 211, which would ring the long-distance operator directly. Elsewhere, the subscriber would ask the local operator to ring the long-distance operator.

When calling long distance, the calling party would give the name and city of the person desired, and hang up, while the operator would begin to set up the call. Many cities had only a limited number of trunks to distant cities, and if those trunks were busy, they'd have to try alternative routings through intermediate cities. The operator would plug into a trunk for the distant 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 local operator would advise him or her to stand by for the calling party, whom she'd then ring back. The calling party's long distance operator would time the call for billing purposes.

Later, 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, one type of operator began to serve both the local and long distance functions. A customer might call to request a collect call, or help getting through on a local or long-distance number which did not ring or might be out of order, for instance. 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, 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. Cordboards for these purposes were replaced in the 1970s and 1980s by TSPS and similar systems.

Virtual switchboard

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 and 3 for support").

References

Notes

Bibliography

• 1911 Britannica article
• Atlanta Telephone History: Part 1 - Early Telephone Service especially the section from 1905 on.