A CSU/DSU (channel service unit/data service unit) is a device, similar to a modem, that connects a LAN to a wide-area network, such as a frame relay service, by converting the frame of one network into a type suitable for the other.
Once sold or leased as stand-alone devices, today some CSU/DSUs come integrated with routers.
Managed CSU/DSUs are designed explicitly for monitoring and diagnosing local loop problems. They can echo back "loopback" messages from a carrier to help diagnose and isolate problems.
