Appendix D. Glossary T1 ESF CSU ACE
60 © 2004 ADTRAN, Inc 61204025L1-1B
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 origi-
nating 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. See also FCS.
CS
See CTS.
CSU
channel service unit. A device used to connect a digital phone line (T1 or Switched 56 line) coming 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 central office. A CSU regener-
ates digital signals. It monitors them for problems, and provides a way of testing the digital circuit.
CTS
clear to send. A signal on the DTE interface indicating that the DCE is clear to send data.
dB
decibel. A unit of measure of signal strength, usually the relation between a transmitted signal and a standard
signal source.
DCE
data communications equipment. Device that provides all the functions required for connection to telephone
company lines and for converting signals between telephone lines and DTE. Also see DTE.
DDS
digital data service. A private line digital service, for transmitting data end-to-end at speeds of 2.4, 4.8, 9.6, and
56 kbps and in some cases 19.2, 38.4, or 64 kbps. The systems can use central hub offices for obtaining test
access, bridging legs of multipoint circuits, and cross connecting equipment. DDS is offered on an inter-LATA
basis by AT&T and on an intra-LATA basis by the Bell operating companies.
DE
discard eligibility. A user-set bit indicating that a frame may be discarded in preference to other frames if con-
gestion occurs, to maintain the committed quality of service within the network. Frames with the DE bit set are
considered excess data.
DLCI
data link connection identifier. A unique number assigned to a PVC end point in a frame relay network. Identi-
fies a particular PVC endpoint within a user’s access channel in a frame relay network and has local signifi-
cance only to that channel.
DSU
data service unit. A device designed to transmit and receive digital data on digital transmission facilities.
DSU loopback
A telco initiated test which loops the DSU back to the telco and is used to test the DDS circuit as well as the
DSU/CSU.
