CSU/DSU

A CSU/DSU (Channel Service Unit/Data Service Unit) is something which converts signals, usually connected to network equipment facing telco network.

Technically it is a hardware device about the size of an external modem that converts a digital data frame from the communications technology used on a local area network (LAN) into a frame appropriate to a wide-area network (WAN) and vice versa.

Links

 * http://searchnetworking.techtarget.com/definition/CSU-DSU


 * http://administeria.com/icnd1-4-1-wan-basics/