The XRT59L921 is an optimized twenty-one channel, E1, line interface unit, fabricated using low power CMOS technology. The device contains twenty-one independent E1 channels. It is primarily targeted towards the SDH multiplexers that accommodate TU12 Tributary Unit Frames. Line cards in these units multiplex 21 E1 channels into higher SDH rates. Devices with 21 E1 interfaces such as the XRT59L921 provide the most efficient method of implementing 63-channel line cards. Each channel performs the driver and receiver functions necessary to convert bipolar signals to logical levels and vice versa.