A CSU/DSU (Channel Service Unit/Data Service Unit) is a digital-interface device used to connect a Data Terminal Equipment device or DTE, such as a router, to a digital circuit (for example a T1 or T3 line).
A CSU/DSU operates at the physical layer (layer 1) of the OSI model. CSU/DSUs are also made as separate physical products; CSUs and DSUs. The DSU or both functions may be included as part of an interface card inserted into a DTE. If the CSU/DSU is external, the DTE interface is usually compatible with the V.xx or RS-232C or similar serial interface.
Digital lines require both a channel service unit (CSU) and a data service unit (DSU):
- The CSU provides termination for the digital signal and ensures connection integrity through error correction and line monitoring. Another functions of the CSU are protecting the user equipment and the line from electrical hazards (like lightning, electrical spikes), keeping track of statitics or providing loopback capabilities.
- The DSU converts the data encoded in the digital circuit into synchronous serial data for connection to a DTE device and provides timing to each end.
WIC (Wan Interface Card)
The WIC (WAN Interface Card) may contain an integrated CSU/DSU that can be inserted into a router slot. An example of a WIC is the 1-port 56/64-kbit/s DSU/CSU WIC (WIC-1DSU-56K4) from Cisco.
References
Cisco Networking Academy Program CCNA 3 and 4 Companion Guide. Third Edition. CD-ROM Video: 4.1 Meet the CSU/DSU.
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