The Communications Satellite Corp. yesterday signed an agreement with American Telephone & Telegraph Co. that will ensure that one-third of AT&T's new international communications traffic travels by satellites, which face imminent competition from an undersea fiber optic cable AT&T plans to lay.

The agreement between the two companies, encouraged behind the scenes by government officials, was seen by analysts as a preemptive strike by Comsat against the loss of its major customer, AT&T.

Comsat is the majority owner of Intelsat, the global satellite consortium that manages an international satellite system for telephone calls and broadcasts. Two-thirds of the communications traffic traveling over Intelsat is from AT&T, which will soon need to fill the enormous capacity in a transatlantic fiber optic cable it and 28 other communications companies are installing. The cable is scheduled to go in service in 1988.

"Comsat has a lot to lose and for them to work out something with AT&T is strategically important," said Marianne Bye, an analyst with Mabon Nugent. "It's beneficial for both parties; it is protective of Comsat and gives AT&T more flexibility."

The cable will be the eighth transatlantic cable in operation. It is the first using fiber optics, a technology that efficiently transmits voice, video and data signals using light pulses.

Comsat officials said it is negotiating similar deals with MCI Communications Corp. and US Sprint Communications Inc. Comsat makes most of its money by charging U.S. companies for access to the Intelsat system.

Federal rules require that communications companies send 60 percent of their voice and data transmissions using satellites and 40 percent using cables. The rules were put into place in the 1960s to ensure growth for the nascent satellite industry Comsat created.

With the advent of fiber optic cables and advances in satellite technology, the Federal Communications Commission has proposed dropping the rule.

Comsat has said that it stands to lose $313 million in the next 13 years if the FCC drops the requirement immediately.

AT&T and Comsat are asking the FCC to approve their agreement as an alternative to suddenly dropping the 60/40 rule.

"It's obviously a pragmatic approach to getting ready for a competitive environment," said Bruce Crockett, president of Comsat's world systems division, which oversees the Intelsat business. Crockett said the agreement was possible because the company has converted its satellite systems to use the same kind of digital technology as fiber optic cable. The advances allow four simultaneous telephone conversations over one satellite circuit, rather than the one conversation that can take place now over an older circuit.

"It makes us look like, act like, and smell more like a cable than we could in the past," he said. "We're saying we can be competitive."

John E. Berndt, AT&T senior vice president for international services, said the agreement "reaffirms AT&T's long-standing commitment to Intelsat's worldwide satellite system." AT&T has said it is committed to using satellites for some of its traffic because they reach places cables cannot go and because they help ensure a reliable mix of communications alternatives.

Under the agreement, which runs through 1994, Comsat will submit a new tariff by Nov. 16 to the FCC for 10-year leases of satellite circuits by all communications companies.

As part of the agreement, AT&T will increase the number of Comsat circuits it uses from 18,000 to 20,000 by the end of 1987. AT&T will also add an average of 34 percent of all its new communications traffic growth to the Intelsat system at least through 1994. Each circuit ordered between now and 1994 will carry a 10-year contract.

According to Comsat's Crockett, Comsat will hold roughly 47 percent of the international telephone traffic market by 1994, down from 61 percent now. Comsat will get a smaller percentage of overall growth in new international communications traffic, he said, but an increase in the number of satellite circuits, the volume traveling over each circuit, business from other carriers, and other services "will allow our revenues to grow nicely."

While in the short term Comsat could experience slower revenue growth, its "base communications traffic load cannot decline," said Richard McGraw, a company spokesman.

Comsat, which has restructured by selling off several technology products divisions and getting out of a few highly unprofitable diversification ventures, still faces serious questions about long-term growth, said Bye. "This looks relatively attractive compared to {the} worst-case scenario" that Comsat lost more AT&T traffic in the wake of FCC rule changes, she said. "But if the FCC doesn't accept this, you are still walking into a snow storm and Comsat still has more to lose than AT&T."

Peter Pitsch, FCC chief of staff to Chairman Dennis Patrick, yesterday said the deal was "a very encouraging development" and the FCC would review whether to replace current rules with the structure Comsat and AT&T propose.