AT&T; will offer a new high-speed data circuit.

The new circuit will transmit data over optical fibers 30 times as fast as its current top-of-the-line model, AT&T; said. The company has applied to the FCC to begin offering the service Jan. 3. It said the service originally would be limited to cities on the East and West coasts and parts of the Southwest but will expand by the end of 1988 to more than 120 metropolitan areas. The service, called Accunet T45, can carry 45 million bits of data per second.