Access to a CSMA/CD (Carrier Sense Multiple Access with Collision Detection) network is provided via a telephone line by providing a master modem at the head end and a slave modem at the subscriber end of the line. The master modem provides a CSMA/CD interface to the network and controls half duplex communications with the slave modem via the line to avoid collisions of information packets on the line. The information packets are enveloped in frames on the line with error checking information; control information between the modems is contained in the same and/or in separate frames. The modulation method and signal bandwidth can be varied depending on errors to provide optimum communications capacity via any particular line, and a ratio of upstream to downstream information packets can be varied depending on buffer fills at the modems. The master modem can be multiplexed for multiple lines. The modulated signal frequencies are above telephone signal frequencies so that each line can be frequency diplexed for simultaneous telephone communications.