In this paper the main characteristics and the performance of two systems implemented for two wire digital transmission on subscriber lines are presented. Both systems utilize different time intervals to transmit bursts of bits in the two directions with an information bit rate of 80 kbits/s and a line bit rate in the bursts of 256 kbits/s. One system employing WAL2 as the line code exhibits extreme circuitry simplicity and low power consumption. The other system, which employs CMI as the line code, shows better performance in the presence of line disturbances.