American Telephone & Telegaph Company has expanded its telecommunications business and says it will do for data communications what the switchboard did for telephone service.
AT&T, the nation's largest nonindustrial corporation, has formed a new subsidiary, called American Bell Inc. Its first product will allow different types of computers and computer terminals to communicate with each other.
It will be called the Advanced Information Systems/Net 1 and provide data storage, transmission, and programming capabilities. Arch McGill, AT&T's vice-president of business marketing, says the new system will be the ''cornerstone'' of American Bell's business.
Under Net 1, as the telephone executives call it, customers will connect their terminals to one of several service points American Bell has established around the country. If an insurance company, for example, wanted to send information to another office that had a different type of computer system, it could contact one of these service points. American Bell, then, would translate the sender's computer message into a language that could be read by the receiver and send it on.
AT&T, under the terms of two decisions handed down by the Federal Communications Commission in 1980 and 1981, had to provide so-called ''enhanced services,'' through a fully separated subsidiary by Jan. 1, 1983. Enhanced services are those where customers' information is changed, processed, or stored. Last week, the FCC approved the telephone company's capitalization plan for its new subsidiary, which will be totally owned by AT&T. Charles L. Brown, AT&T's chairman, says he does not think it would be incompatible with AT&T previously announced plans to divest itself of its operating companies.
Linell McCurry, a vice-president with Butcher & Singer, a Philadelphia-based brokerage house, says the important thing about American Bell's new system was the fact that it would be Ma Bell's first venture into an unregulated business. As such, she states, ''a lot of people will be watching it to determine how successful they are.''