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61202192L1-1 Express L768/L1.5 User Manual 111
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.
Frame Relay
A streamlined subset of the X.25 packet switching protocol which has been
used by many corporations for wide area communications for a number of
years.
group 4
A high-speed (56 kbps) facsimile protocol specific to ISDN.
HDSL
Hight-bit-rate Digital Subscriber Line technology is similar to ISDN U-inter-
face, but operates at ahigher bit rate. It provides bi-directional DS1 service
over two copper pairs.
hop count
A routing metric used to measure the distance between a source and a desti-
nation. Particularly used by RIP.
hub
(1) Communications center, (2) Major routing station for connecting channels,
(3) DDS connecting center.
IARP
Inverse Address Resolution Protocol. Used for resolving the protocol address
when the hardware address is known.
IEEE
Institute of Electrical and Electronic Engineers. Professional organization that
defines network standards. IEEE LAN standards are the predominant LAN
standards today and include protocols similar or virtually equivalent to Ether-
net and Token Ring.
IEEE 802.1d
An algorithm used to prevent bridging loops by creating a spanning tree.
IEEE 802.2
An IEEE LAN protocol that specifies an implementation of the LLC sublayer
of the data link layer. It handles errors, framing, flow control, network layer
(Layer 3) service interface, and is used in LANs.