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Glossary
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ISU 2x64 Dual Port ISDN Service Unit User Manual 61200051L1-1D
CCITT
Consultative Committee on International Telephony and Telegraphy. A body of
the International Telegraph Union (ITU) which prepares recommendations, com-
monly referred to as international standards, to resolve technical telegraph and
telephone problems.
central office (CO)
In telephony, the facility housing the switching system and related equipment that
provides telephone service for customers in the immediate geographical area.
circuit mode
Type of switching that assigns a call to a specific circuit path. The circuit is not
shared with other calls.
clear channel
A channel in which all the 64 kbps are used for transmission. To achieve this, bit
robbing signalling must be eliminated. (NOTE: Not to be confused with clear chan-
nel protocol.)
common channel interoffice signalling (CCIS)
A signalling system developed for use between stored program switching systems.
All of the signalling information for a group of trunks is transmitted over a dedi-
cated high-speed data link rather than on a trunk. CCIS reduces call setup time
compared to individual trunk signalling.
CPE
Customer premises equipment. A generic term for communications terminal gear
owned by the customer, residing on customer premises.
CSU
Channel service unit. A component of CPE used to terminate a digital circuit, such
as DDS or T1 at the customer site. Performs certain line-conditioning functions,
ensures network compliance per FCC rules, and responds to loopback commands
from the central office. Also ensures proper 1s density in transmitted bit stream
and performs bipolar violation correction. See DSU.
D-channel
The ISDN channel that carrier signalling information to control the call setup, tear-
down, or invocation of supplementary services. The D-channel may also be used
to provide packet mode data service.
DCE
data communications equipment. The portion of a data terminal that provides the
interface to the network.