CSU/DSUS—Connecting Devices to a Digital Line

Channel service units (CSUs) are required to interface with digital T-1, T-3, fractional T-1 and T-3, switched 56-Kbps and dedicated 56-Kbps lines. T-1 allows 24 voice, data and video transmissions to share one telephone circuit. T-3 circuits carry 672 voice, data and video transmissions (see Chapter 6). They also are used with E-1 and E-3 European service. Often, CSUs are cards within multiplexers and PBXs rather than standalone, external devices.

Although they have different functions, CSU/DSUs are supplied in one integrated piece of equipment. At one time, the DSU and CSU were sold as separate “boxes.” Functionally, the CSU sits between the telephone line and data service unit (DSU). The CSU plugs into the telephone jack. The DSU connects to the customer's equipment, such as the T-1 or E-1 multiplexer.

The CSU is similar in function to an NT1. Maintenance and performance tests can be done from it to determine if a repair problem is in the equipment, the CSU/DSU or the telephone line. The CSU also provides clocking and signal reshaping. The clocking function is responsible for sending out bits in an evenly timed fashion. If the clocking is off, the transmission will not work. In this case, the technician might say, “the line is slipping,” or “the timing is off.” The DSU makes sure the correct positive and negative voltages are present on the signals from the DTE to the CSU.

There are two main types of CSUs: super frame (SF) and extended super frame (ESF). Extended super frame CSUs were developed by AT&T's Bell Labs division in 1985. (Bell Labs is now part of Lucent Technology.) It is superior to SF because T-1 circuits can be monitored while a line is in service. The type of CSU installed on the customer's premise must match the type used in the telephone provider's network. Most network service providers have extended super frame capability.