Abbreviation for Data Service Unit.

A DSU, when paired with a channel service unit, allows communication over a digital telephone circuit, most frequently some combination of 64Kbps DS0 channels. One DSU unit will provide some form of interface (e.g. a v.35 serial connection) to telecommunications equipment which is more easily usable than the typical traffic on a high-voltage multiplexed line (e.g. a T1).

Compare to a CSU, which provides the physical interface to a leased line. Almost invariably a DSU is paired with a CSU, and the two are typically in the same modem-sized unit: a DSU/CSU. More recently, the CSU/DSU functionality is being duplicated as one part of a T1 interface card for most phone system and router manufacturers; Current T1 CSU/DSUs from Cisco are generally circuit board modules each the size of a deck of playing cards.

A typical setup would be as follows:

( Phone    )
(  Company ) ------- X -- ( CSU ) -- ( DSU ) ---- ( Router )
(   Switch )    ^    ^  ^                     ^
             T1 Line |  |                     Serial cable
                     |  |                     
            Demarcation |
                  Point |
          ("Smartjack") |
                        Patch cable