The Christian Social Union (Christlich-Soziale Union) party, a small conservative political party in Germany, centered in Bavaria. The CSU is allied with the CDU (Christian Democratic Union), its much larger partner whose support extends beyond Bavaria to the rest of Germany. The CDU/CSU team is referred to collectively as die Union (the Union).

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)
            Demarcation |
                  Point |
          ("Smartjack") |
                        Patch cable




( Phone    )
(  Company ) ------- X -- ( CSU ) -- ( Telephone equipment )
(   Switch )    ^    ^  ^          
             T1 Line |  |          
                     |  |          
            Demarcation |
                  Point |
          ("Smartjack") |
                        Patch cable