ADTRAN Express 6100 Manual page 152

Glossary
paths for carrying voice, data, video, or other information simulta-
neously. It bears a remarkable similarity to today's T1 link, except
that T1 carries 24 voice channels. In ISDN, 23B+D gives twenty-three
channels and one D channel for out-of-band signalling. However, in
T1, signalling is handled in-band. See ISDN.
two-wire circuit
A transmission circuit composed of two wires, signal and ground,
used to both send and receive information. In contrast, a 4-wire cir-
cuit consists of two pairs. One pair is used to send and one pair is used
to receive. All trunk circuits (long distance) are 4-wire. A 4-wire cir-
cuit delivers better reception, but also costs more. All local loop cir-
cuits (those coming from a Class 5 central office to the subscriber's
phone system) are 2-wire, unless a 4-wire circuit is requested.
U-interface
A twisted pair subscriber loop that connects the NT1 reference point
to the ISDN network, as defined in the I.411 Recommendation. This
interface provides Basic Rate Access with an operating frequency of
160 kbps and an information rate of 144 kbps. Under U.S. regulations,
this also marks the line of demarcation between customer-owned
equipment and the public network.
ZIP
Zone Information Protocol. The AppleTalk session-layer protocol
used to maintain and discover the internet-wide mapping of network
number ranges to zone names.
Zombie Routes
Routes that have been identified by the router to be deleted. They re-
main in the router's route table for a specified amount of time with an
infinite metric so that all other routers will learn of this router's inten-
tion to delete them.
Glossary-12 Express 6100/6120 User Manual 61200176L1-1