Telecommunications giant Lucent Technologies Inc. in Murray Hill, N.J., has signed a definitive agreement to acquire Ascend Communications Inc. in a stock swap valued at close to $20 billion. The move could signal Lucent's intentions of competing with San Francisco-based Cisco Systems in the data networking market. Ascend, in Alameda, Calif., is the fourth-largest maker of computer networking equipment and is expected to provide Lucent with expertise in Internet-style communications technology. The company has a line of equipment designed to help telecommuters make connections to their headquarters and to assist businesses in establishing computer and telephone networks.