CSU/DSU stands for Channel Service Unit/Data Service Unit. This is the device connected to the incoming data communications line, usually a T1 or Fractional T1 line. This is actually two devices in one. The channel service unit recovers channelized (multiplexed) data, while the data service unit strips off the encoding used to package the data.
Typically, the CSU/DSU needs to be informed of the frame encoding (B8ZS, Bipolar 8 Zero Substitution, or AMI, Alternate Mark Inversion), the frame encapsulation technique (either D4 Superframe, or ESF, Extended Super Frame), the clock source (usually the network), the channel capacity (usually 64kbps), and the number of channels (1 for 64kbps, 2 for 128kbps, 24 for 1.536Mbps).
Adtran makes a very reliable and easy to configure CSU/DSU which they refer to as a TSU, for T1 Service Unit.
The TSU is connected to a router via a V.35 cable.
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