AT&T DEFINITY 2500 series Network Manual page 280

D-8
COMMUNICATIONS PROTOCOLS
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Name
V.29 9600 bps modem standardized for use on point-to-point leased
telephone-type circuits
V.35 Data transmission at 48 kilobits per second using 60-108 kHz group
band circuits
V.54 Loop test devices for modems
Name
I.430 Defines the basic user-network interface, layer 1 specification
I.431 Defines the primary rate user-network interface, layer 1 specification
I.441 Defines the ISDN user-network interface for the data link layer
I.451 Defines the ISDN user-network interface for layer 3
Q.921 Defines the data link layer (layer 2) protocol for ISDN
Q.931 Defines the network layer (layer 3) protocol for ISDN
Two other CCITT protocols play important roles in the ISDN specification:
LAPB (link access protocol — balanced) — A link-layer protocol adopted by the CCITT as part of the
X.25 packet-switched network standard. It is a subset of HDLC.
LAPD (link access protocol D-channel) — A link-layer protocol adopted by the CCITT for D-channel
communications. At the network layer, X.25 is employed for packet switching, and I.451 is employed
for control signaling.
The ISDN protocol scheme is given in Figure D-2. The figure shows that D-channel and B-channel
communications are packaged in different protocol suites. In addition, when signaling, data packets, or
telemetry are transmitted over the D-channel, each uses a different subset of protocols. Likewise, on the
B-channel, CCITT plans for circuit-switched, leased-line, and packet-switched communication allow for
differing protocol suites.
TABLE D-2. Some CCITT Analog Standards (Continued)
Title
Table D-3. Some CCITT ISDN Standards
Subject
Equivalent
Standard(s)
FED-STD 1007
None.
RS-449
Equivalent
Standard(s)
None.
None.
None.
None.
LAPD; CCITT I.441
X.25; CCITT I.451