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
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
group 4
A
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