User`s guide

Glossary G-3
D
data link
ÑA serial data communication transmission path, generally between
two adjacent nodes or devices and without intermediate switching nodes.
data link layerÑIn the OSI model, the network processing entity that estab-
lishes, maintains and releases data link connections between adjacent ele-
ments in the network.
DCE (Data Circuit-Terminating Equipment)ÑEquipment that is either a part
of the network, an access point to the network, a network node, or equipment
at which a network circuit terminates.
dial-upÑDescribing the process of, or the equipment or facilities involved in,
establishing a temporary connection via the switched telephone network.
digitalÑReferring to communications procedures, techniques and equipment
where information is encoded as either a binary
1 or 0.
digital loopbackÑA technique for testing the digital processing circuitry of a
communications device; may be initiated locally or remotely via a telecom-
munications circuit. The device being tested returns a received test message,
the results of which are compared to the original message.
DS0 (Digital Signal Level 0)ÑA 64kbit/s standard digital telecommunications
signal or channel.
DS1 (Digital Signal Level 1)ÑThe 1.544Mbps digital signal carried on a T1
facility.
DS3 (Digital Signal Level 3)ÑThe 44.736Mbps digital signal carried on a T3
facility.
DS4 (Digital Signal Level 4)ÑThe 274.176Mbps digital signal carried on a T4
facility.
DSU (Data Service Unit)ÑAlso called a Digital Service Unit. A device
designed to connect a
DTE to a digital phone line. The DSU transmits and
receives the signal and provides buffering and ßow control. A
DSU and CSU
are often in the same unit, called a
DSU/CSU.
DSU/CSU (Data Service Unit/Channel Service Units)ÑThe pair of communi-
cations devices
(DSU and CSU) that connect an in-house line to an external
digital circuit. At the customer's end of the telephone connection, the
DSU/
CSU
takes data from terminals and computers, encodes it, and transmits it
down the link. At the receive end, another
DSU/CSU equalizes the received
signal, Þlters it, and decodes it for interpretation by the end-user.