It was observed, from the earliest days of the telephone, that it was more practical to connect different telephone instruments by running wires from each instrument to a central switching point, or telephone exchange. Running wires between all the instruments was too complicated. In 1878, a small commercial telephone exchange was installed in New Haven, Connecticut, permitting up to 21 customers to reach one another by means of a manually operated central switchboard. The manual switchboard was quickly extended from 21 lines to hundreds of lines. Each line was terminated on the switchboard in a socket (called a jack), and a number of short, flexible circuits (called cords) with a plug on each end were also provided. Inserting the two ends of a cord in the appropriate jacks could thus interconnect two lines.Â
This was manual switching. In 1879, subscribers came to be called by numbers rather than by names. That year, the first telephone exchange was opened in London. Within a decade, the US had 1,40,000 subscribers with 8,000 exchanges. Several switchboards were required and two operators were involved in most calls.Â
The switchboards grew in size. Almond B Strowger, a schoolteacher who also served as an undertaker took up the challenging task of replacing the manual exchange by an automatic one. After five years of hard work, he succeeded in 1889. An automatic switch was developed by 1891, and a step-by-step motion of its parts established the necessary electrical contact.
The Strowger Switch consisted of essentially two parts: an array of 100 terminals, called the bank, that were arranged 10 rows high and 10 columns wide in a cylindrical arc; and a movable switch, called the brush, which was moved up and down the cylinder by one ratchet
mechanism and rotated around the arc by another, so that it could be brought to the position of any of the 100 terminals. The first Strowger exchange was installed in La Porte, Indiana, in 1892. The electromechanical switch ensured rapid and reliable service as well as privacy.
Another development was in 1896, when the rotary dial was introduced. The first commercial automatic exchange was operational by 1897. The change from manual to dialed methods came during the 1920s and 1930s and by 1940, most of the larger cities had automatic exchanges. In 1933, a new switchboard was introduced to permit the use of any position and circuit for trunk connections. Semi-automatic service was introduced between cities in 1911, when Los Angeles and San Diego were linked. Intercontinental dialing was introduced in 1963.
The requirements of the automatic network called for new designs. Local subscribers must be automatically dialed. There were attempts to improve the Strowger system and introduce some element of common control. One is known as the step-by-step director system, first developed to meet the need of the London metropolitan areas. It was tried in Calcutta. The system stored the digits dialed by the subscriber and translated them for efficient use of the network. This would give technical codes for reaching certain exchanges. In 1913 JN Reynolds, an engineer with Western Electric (then the manufacturing division of AT&T), patented a new type of telephone switch that came to be known as the Crossbar Switch. This switch was a grid composed of five horizontal selecting bars and 20 vertical hold bars. With the appropriate movement of the hold and selecting bars, any column could be connected to any row, and up to 10 simultaneous connections could be provided. Televerket, the Swedish government-owned telephone company demonstrated the first Crossbar System, in 1919. The first commercially successful system, however, was the AT&T
No. 1 Crossbar System, first installed in Brooklyn, New York, in 1938.Â
A series of improved versions followed the No. 1 Crossbar System–the most notable being the No. 5 system, which, first deployed in 1948, became the workhorse of the Bell System. By 1978 the No. 5 Crossbar System accounted for the largest number of installed lines throughout the world. Originally designed to serve 27,000 lines, it was later upgraded to handle 35,000 voice circuits. Further revisions of the AT&T crossbar systems continued until 1974, by which time new switching systems had shifted from electromechanical to electronic technology.
Traffic, New Services, and Electronic SwitchingÂ
As telephone traffic continued to grow through the years, it was realized that large numbers of common control circuits would be required to switch this traffic and that switches of larger capacity would have to be created to handle it. Plans to provide new services via the telephone network also created a demand for innovative switch designs. With the advent of the transistor in 1947 and with subsequent advances in memory devices as well as other electronic devices and switches, it became possible to design a telephone switch that was based fundamentally on electronic components rather than on electromechanical switches.
Soon after the World War II, developments in the field of electronics began to attract telecom engineers. Electronic components began to be used with a wired programme control. In order to reduce the number of common control elements, high-speed electronic devices were used. Under one such programme the logic circuits were wired to handle 50,000 calls per hour. Still, changes in wiring were needed if the hardware was to perform a different function. In other world, computer and telephone switching technologies remained apart. They were combined only from 1965.
Is the Switch under Threat of Extinction?
|Answer to that depends upon what you mean by switching.|
The traditional telephone exchanges, as they are today, may not survive for long, at least in the same form. But then, there is hardly any telephone that uses rotary dial today. That does not mean what we use is something else. It is an evolution–maybe with a few technological breakthroughs in between.
Today, everyone talks of IP networks and convergence of voice and data. The traditional telephone networks that were designed for voice-traffic are not optimally suited for the data-heavy traffic. There is a shift, hence, from circuit switching to packet switching.
May be, in near future we will see deployment of much more packet switching switches. But the basic functionality of switching will still remain the same.
There are three technology trends that are noteworthy. One, a shift towards software switching. Even today, a lot of software is built into a traditional telecom switch. But many believe total switching functionality will shift to software and the hardware will be common computing hardware. Two, the evolution of an open standard.
Today, all switches can talk to each other through common signaling protocol, but the switch itself is proprietary. Work has already begun on common switching standards. Both these will lead to a drop in switching price and lower the entry barrier for new players. Three, what is known as optical switching. Using fibre optics to do the basic switching function. This is in the early stages of research, and when developed, will be a real breakthrough.
Tomorrow’s switches may replace today’s switches. But then, today will be replaced by tomorrow.
The electronic exchange is of recent origin. The world’s first experimental Stored Program Control (SPC) electronic exchange was put into operation by the Bell Telephone Laboratories, US, in 1961, with a capacity of 500 subscribers. Between 1960 and 1962, AT&T conducted a field trial of an Electronic Switching System (ESS) that employed a variety of new devices and concepts. Among these innovations was a gas-tube cross point network to perform the actual switching function. In order for a particular switch to close or make a connection, a high voltage was applied to a gas-filled tube, causing the gas to ionize and provide a conductive path between its two terminals. Another innovation introduced in the trial was a read-only memory device known as a flying-spot store, which employed a cathode-ray tube for optically addressing a photographic storage plate that contained the computer instructions for the electronic switch. Yet other innovations were a read/write access memory device known as a barrier grid store, for storing dialed numbers and traffic information, and logic elements constructed of discrete diodes and germanium transistors. Further development resulted in the first commercial electronic exchange commissioned in New Jersey in 1965.
The commercial version of the trial system, placed in service in 1965, became known as the No. 1 ESS. The
No. 1 ESS differed somewhat in architecture from the trial model. It
incorporated a new read-only memory device and a new random-access memory device. These innovations allowed the No. 1 system to serve as many as 65,000 two-way voice circuits, and it permitted hundreds of new features to be handled by the switching equipment. It underwent a number of revisions, including the adoption of semiconductor memory
Till now, all the automatic telephone switches, both electromechanical and electronic, were space-division switches. The first time-division switching system to be deployed in the US was the AT&T-designed No. 4 ESS, placed into service in 1976. This was a toll system capable of serving a maximum of 53,760 two-way trunk circuits. It was soon followed by several other time-division systems for switching local calls. Among these was the AT&T No. 5 ESS, improved versions of which could handle 1,00,000 lines. Time-division switches could also employ space-division switching; an appropriate mixture of time-division and space-division switching was advantageous in various circumstances.