ADTRAN DSU IQ User Manual page 160

Appendix D. Glossary
CD
carrier detect. A signal generated by a modem or DSU/CSU. CD indicates the
presence of a carrier signal on a communications link.
channel service unit
CSU. A device used to connect a digital phone line (T1 or Switched 56 line) com-
ing in from the phone company to either a multiplexer, channel bank, or directly
to another device producing a digital signal; for example, a digital PBX, a PC, or
data communications device. A CSU performs certain line-conditioning and
equalization functions, and responds to loopback commands sent from the cen-
tral office. A CSU regenerates digital signals. It monitors them for problems,
and provides a way of testing the digital circuit.
CIR
committed information rate. Less than or equal to the access rate, the CIR is
used by the service provider for rate enforcement when the network is con-
gested. When rates exceed the CIR, frames may be discarded.
clocking
An oscillator-generated signal that provides a timing reference for a transmis-
sion link. A clock provides signals used in a transmission system to control the
timing of certain functions. The clock has two functions, (1) to generate periodic
signals for synchronization and (2) to provide a time base.
CPE
customer premise equipment. All telecommunications terminal equipment
located on the customer premises, including telephone sets, private branch
exchanges (PBXs), data terminals, and customer-owned coin-operated tele-
phones.
C/R bit
In the Q.921 protocols, a bit that identifies a data-link-layer frame as either a
command or a response.
CRC
cyclic redundancy check. A computational means to ensure the accuracy of
frames transmitted between devices in a frame relay network. The mathematical
function is computed, before the frame is transmitted, at the originating device.
Its numerical value is computed based on the content of the frame. This value is
compared with a recomputed value of the function at the destination device.
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