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1-6 Introduction
Layer 3 (network) protocols
Layer 3 (described in CCITT recommendations I.450, Q.920, I.451, and
Q.931) provides the means to use the D-channel for establishing,
maintaining, and completing calls on a logical link created by the link layer.
The type of channel and information carried determines which protocol is
used.
D-channel
On the D-channel, two types of protocols are used:
B-channel
Different types of protocols are used on B-channels. The X.25 network layer
protocol is used only for transmission of packet-switched data. Speech and
circuit-switched data on a B-channel do not require a network layer protocol.
Layer 4 (application) protocols
Layer 4 protocols provide transparent data transfer and application-level
messages.
297-2521-106 Standard 23.01 March 2001
— point-to-point—DMS-250 switches use only point-to-point links
LAPD (a packet-switching protocol based on X.25)—used for low-speed
packet data on basic rate interface D-channels; not used for the PRI
D-channel
Q.931—a call-control protocol used for call control messages, including:
— setup and teardown of calls and features between the network and
PBX
— address (directory number) displays and progress indicators at the
PBX and the network ends
— B-channel control from the network side
— the support of basic error-handling procedures and reinitialization on
recoverable errors
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