CSU/DSU
From Wikipedia, the free encyclopedia
A CSU/DSU (Channel Service Unit/Data Service Unit) is a digital-interface device used to connect a router to a digital circuit such as a T1 or T3 line.
A CSU/DSU operates at the physical layer (layer 1) of the OSI model. It terminates physical data connections, provides signal timing and translates the digital data stream into bipolar signals which are suitable for line transmission. The CSU/DSU also performs some error-reporting and loopback functions.
A CSU/DSU is generally about the size of an external modem, but sometimes are full rack-mount devices. It converts a digital data frame from a local area network (LAN) into a frame appropriate to a wide-area network (WAN) and vice versa. For example, if you have a Web business from your own home and have leased T1 to a phone company, you have a CSU/DSU at your end and the phone company has a CSU/DSU at its end.
The CSU receives and transmits signals from and to the WAN line and provides a barrier for electrical interference from either side of the unit, and can also echo loopback signals from the phone company for testing purposes. The CSU originated at AT&T as an interface to its nonswitched digital data system.
The DSU manages line control, and converts input and output between RS-232C, RS-449, or V.xx frames from the LAN and the time-division multiplexed (TDM) DSX frames on the T1 line. It manages timing errors and signal regeneration. The DSU provides an interface to the Data Terminal Equipment (DTE) using a standard (EIA/CCITT) interface. It also provides testing capabilities.
CSU/DSUs are made as separate products, sometimes part of a T1 WAN card or integrated into a DTE.
