Communications: AT&T plans ahead: The telecommunications giant will enter the commercial marketplace, drawing on its traditional technical strengths

After being in unchallenged control of 80 percent of all local telephone services in the United States for decades, the American Telephone & Telegraph Co. is preparing to lay aside that dominant role and enter such formerly forbidden marketing waters as data processing, computer communications, and telephone and computer-terminal equipment. All are areas from which it once was barred by Federal regulations. The radical shift, which will put the company in competition with International Business Machines Corp., Xerox Corp., Digital Equipment Corp., and other formidable rivals is based on the agreement last January between AT&T and the US Department of Justice. AT&T will emerge a much leaner company but with the potential for more profits, according to some analysts. The company´s technology thrust, they say, will continue in five major areas: microelectronics, digital systems, software, photonics, and human-factors engineering.