A line interface unit (LIU) is a semiconductor device that implements the T1 or E1 protocol in hardwired silicon. They are used in applications involving interfacing to long haul and short haul twisted pair cable installations. An LIU interfaces two twisted pair lines, one for receive, and another for transmit. They typically allow for software or hardware configuration modes. The software is configured by a serial interface to a processor system, hardware by individual pin settings.

Data traveling on long cable runs is subjected to all manner of noise interference and attenuation. The LIU is designed to recover the original data in the presence of this noise and attenuation, and reconstruct the a clean data stream for retransmission to a local processing system or to transmit it another line. The attenuation is handled by a signal equalizer circuit that adjusts the signal level. The noise is also called data jitter and is recovered using reclocking.

The LIU receiver filters the incoming signals and performs a process called peak detection that is used to recover the input data stream clock in the clock/data recovery block (CDR). This recovered clock is fed back to the filter for sampling. The reference clock used by the CDR is derived from an integrated VCO/PLL. The recovered data stream and associated clock are used to check for the loss of the carrier, and if the carrier is present the data and clock are passed into the jitter attenuator. The jitter attenuator is a FIFO memory buffer that provides for a clock domain crossing to the clean reference clock. At this point, the data can be retransmitted, or passed onto the local processing unit. If the data is passed before the jitter attenuator it is referred to as local loopback, after this block it is referred to as remote loopback. If the data is to be passed on locally to a processor, it is decoded and passed out of the synchronous serial port.

Transmit performs the opposite procedure, encoding the input data stream from the processor or taking the still encoded but reclocked data from the receiver and pulse shaping it before the integrated transmitter line driver outputs it. Both receiver and the transmitter interface to the Lines using isolating transformers.