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Glossary
central office (CO)
In telephony, the phone company switching facility or center, usually a Class
5 end office, at which subscribers local loops terminate. Handles a specific
geographic area, identified by the first three digits of the local telephone num-
ber. Usually the facilities of the local BOC.
CSMA/CD
Carrier Sense Multiple Access Collision Detect. A channel access mechanism
where devices check the channel for a carrier before transmitting. If no carrier
is sensed for the specified period of time, the device can transmit. If two de-
vices transmit at once, a collision occurs and is detected by all colliding devic-
es. This collision subsequently delays their retransmissions for a random
length of time. CSMA/CD is used by Ethernet and IEEE 802.3.
D-channel
The ISDN channel that carriers signalling information to control the call setup,
teardown, or invocation of supplementary services. The D-Channel may also
be used to provide packet mode data service.
DDS
Dataphone Digital Service. AT&T private line service for transmitting data
over a digital system. The digital transmission system transmits electrical sig-
nals directly, instead of translating the signals into tones of varied frequencies
as with traditional analog transmission systems. Digital techniques provide
more efficient use of transmission facilities, resulting in lower error rates and
costs than analog systems.
digital hub
Designated office where DDS channels are interconnected and where synchro-
nous network timing, testing access, and additional service features are pro-
vided.
Ethernet
A local area network used for connecting computers, printers, workstations, a
terminals, servers, etc., within the same building or campus. Ethernet operates
over twisted wire and coaxial cable at speeds up to 10 Mbps. Ethernet speci-
fies a CSMA/CD.
four-wire circuits
Telephone lines using two wires for transmitting and two wires for receiving,
offering much higher quality than a 2-wire circuit. All long distance circuits
are 4-wire. Almost all local phone lines and analog phones are 2-wire.
group 4
A high-speed (56 kbps) facsimile protocol specific to ISDN.
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Express L128/L128T User Manual
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