Electronic Switching Is 'In'

Communications News, Sept, 1984

Electronic switching was one of the ten tremendous developments of Communications News' first decade and the promise of electronic switching has been fulfilled during the past decade as the pushbutton telephone, the digital PBX, and digital facsimile and high-speed data transmission over telephone lines have become not only possible but commonplace.

During CN's very first year the first electronic switching office went into commercial service at Succasunna, New Jersey, on May 30, 1965, and "Number One ESS" proved to be a pivotal point in telephone switching history.

That event was the result of a formal Bell Laboratories' effort begun at the end of World War II. The goal was to apply electronics to switching, and bring customers the speed, flexibility, and reliability offered by electronic components. Laboratory domonstration systems developed by the late 1940s relied on electronic tubes for logic, memory, and control functions. These systems greatly encouraged Bell engineers, but also pointed up the need for more economical logic and memory devices. The solid-state revolution, sparked by Bell Labs' invention of the transistor (see story, page 84), provided the breakthrough switching engineers needed. Developments then came rapidly, but the concept had to be tested and retested many times to make sure the new switching systems would meet the service standards Bell subscribers had come to expect. A customer trial from 1960 to 1962 in Morris, Illinois, proved electronic systems could do the job. The system used in Morris contained about 4,000 electronic tubes, and 12,000 transistors.

Two basic types of ESS were developed: the two-wire office used for commercial Bell system service, and the four-wire office used for the government's AUTOVON network. The first ESS in the AUTOVON network began operation in May 1966.

In the early 1960s GTE Automatic Electric was also active on the electronic switching front. A test model of its "Number One EAX" (Electronic Automatic Exchange) was operational in 1962. An initial installation of EAX equipment with a capacity of 600 lines was installed in the Portage, Indiana, exchange of General Telephone Company of Indiana, beginning in 1963, for a field trial.

GTE Automatic's Number One EAX was a common control telephone switching system that met the requirements for a medium to large and office and a combined end and toll office. Switching was accomplished by means of a 2-wire, space-divided, read network, driven by a stored program, with electronic common control.

The first Number One EAX developed by GTE-Automatic Electric was cut into service in July 1972, at St. Petersburg, Florida. A second was cut into service at Erie, Pennsylvania, in January 1973.

Another early electronic switching system was the NX-IE developed by North Electric Company . . . an electronically controlled crossbar switching system.

In early 1969, Northern Electric Laboratories announced the SP-1 electronic switching system, with a field trial taking place in Ottawa, Ontario. The system was cut over in 1970. The SP-1 was feasible in the central-office role for as few as 2000 lines, yet could grow to 20,000 lines or more. The first fully operational office was installed in Alymer, Quebec, in 1971.

SP-1 is an electronically controlled switching system using the "Minibar" miniature crossbar switching matrix. The central control complex consisted of a duplicated electronic data processor, a program store, and a call store. The program store utilized an nondestructive readout memory known as the piggyback twistor, whereas the call store consisted of ferrite sheets. The SP-1 used several discrete wired logic electronic units such as markers and receiver-senders as peripheral equipment. It could be used on a two-wire basis as a local switching system or on a four-wire basis as a toll switching system.

By March 1973, Northern Electric had some 51 orders for the SL-1 and the electronic switching race was on!

Meanwhile, the Bell System continued to expand and improve its electronic switching systems.

The first Number Two ESS, manufactured at the Western Electric Company Hawthorne Works in Chicago, went into operation in late 1970 in Oswego, Illinois. It was developed at Bell Labs for use in medium-sized central offices serving from 1000 to 15,000 lines and it offered a variety of custom calling services available to customers, such as speed calling, call forwarding, three-way calling, and call waiting. It has a ferreed network, permanent-magnet twistor, semipermanent memory, and a ferrite-sheet call store.

Bell's Number Four ESS, developed in 1972, was designed primarily as a successor to electro-mechanical 4A crossbar systems. The 4 ESS was a high-capacity, versatile toll and tanden switching machine for the long-distance telecommunications network which could handle at least 350,000 long-distance calls per hour . . . about three times as many as the electromechanical system. It was the first large machine in the Bell System to switch digital signals directly.