DSU's

In telecom, a data service unit (DSU) is a device used for interfacing data terminal equipment (DTE) to the PSTN (Public Switched Telephone Network). It is also a type of short-haul, synchronous-data line driver, usually installed at a user location, that connects user synchronous equipment over a 4-wire circuit at a preset transmission rate to a serving CO (Central Office). This service can be for a point-to-point or multipoint operation in a digital network.
A DSU is a two or more port device; one port is call the WAN port and each other is called a DTE port. The purpose of the DSU is to transfer serial data synchronously between the WAN port and the DTE ports. If more than one DTE port is used, the DSU assigns the DTE data according to time slots (channels) on the WAN side.

On the WAN side, the DSU interfaces with a digital carrier such as DDS (Digital Data Service), DS1 (T1) or DS3 (T3). To accomplish that it generates the proper bit rate, and generates and consumes the proper framing. The WAN port also generates and consumes line coding as necessary. On the DTE side, the DSU provides control lines, timing lines and appropriate physical and electrical interface. To maintain the synchronous relationship between the ports, the DSU manages timing by slaving ports to the bit rate of another or to its internal clock. Typically, the DTE port provides timing to the data terminal equipment while the WAN port dictates the rate.

Constraints on the WAN side may require a minimum "one's density" of the WAN data. In those cases, the DSU may be called on to throttle the rate of the DTE traffic and to insert "1" bits in one bit position of each time slot. The bit "stuffed" in that way is usually #7, but sometimes bit #2 is stuffed instead.

DSUs usually include some maintenance capabilities. At minimum, they can loop data back at either the WAN or DTE ports, or at both. When only one port is looped back, the data received at that port is simultaneously sent back toward the port and passed in normal fashion to the other port. Most DSUs also allow various data patterns to be generated and monitored to measure error rate of the communication link.