Abbreviation for Channel Service Unit.
A CSU terminates a multiplexed digital leased line and allows communication on the individual component channels of the connection. Most frequently, a CSU allows communication over one or more of the 24 DS0 (64Kbps) channels in a T1 (DS1; 1.544Mbps) or an ISDN PRI circuit (23 64Kbps "B-channels").
A CSU can be used to break out the channels into individual lines so that up to 24 voice-grade phone lines may enter an office/building/residence over a single pair to the telephone company central office.
A CSU also provides loopback testing and line conditioning functions.
Compare to a DSU, which interfaces with data communication equipment, and a DSU/CSU which combines the two. In the OSI reference model, the CSU functions on Layer 1 and a DSU functions on Layer 2.
Two typical configurations involving CSUs are as follows. Note that in the second, the CSU is usually an integral component of the phone system.
( Phone )
( Company ) ------- X -- ( CSU ) -- ( DSU ) ---- ( Router )
( Switch ) ^ ^ ^ ^
T1 Line | | Serial cable
| | (e.g. v.35)
( Phone )
( Company ) ------- X -- ( CSU ) -- ( Telephone equipment )
( Switch ) ^ ^ ^
T1 Line | |