1. Channel Service Unit. Also called a Data Service Unit. A device to terminate a digital channel on a customer's premises. It performs certain line-conditioning and equalization functions, and responds to loopback commands sent from the central office, A CSU sits between the digital line coming in from the central office and devices such as channel banks or data communications devices.
2. Channel Sharing Unit. Line bridging devices that allow several inputs to share one Output. CSUs exist to handle any input/output combination of sync or async terminals, computer ports, or modems and thus these units are variously called modem sharing units, digital bridges, port sharing units, digital sharing devices, modem contention units, multiple access units, control signal activated electronic switches or data-activated electronic switches.