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D-8 COMMUNICATIONS PROTOCOLS
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TABLE D-2. Some CCITT Analog Standards (Continued)
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Name Title Equivalent
Standard(s)
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V.29 9600 bps modem standardized for use on point-to-point leased
telephone-type circuits
FED-STD 1007
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V.35 Data transmission at 48 kilobits per second using 60-108 kHz group
band circuits
None.
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V.54 Loop test devices for modems RS-449
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Table D-3. Some CCITT ISDN Standards
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Name Subject Equivalent
Standard(s)
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I.430 Defines the basic user-network interface, layer 1 specification None.
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I.431 Defines the primary rate user-network interface, layer 1 specification None.
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I.441 Defines the ISDN user-network interface for the data link layer None.
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I.451 Defines the ISDN user-network interface for layer 3 None.
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Q.921 Defines the data link layer (layer 2) protocol for ISDN LAPD; CCITT I.441
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Q.931 Defines the network layer (layer 3) protocol for ISDN X.25; CCITT I.451
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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.