Digital service unit or data service unit. An ancillary device needed to adapt the physical interface on a data terminal equipment (DTE) device---such as a V.35 interface on a port---to a transmission facility---such as leased line or a Frame Relay switch. If the DTE lacks complete digital line interface capability, the DSU can be located with the channel service unit (CSU) on the customer's site and known as a CSU/DSU. See also CSU.