Layer 2 of the ISDN signaling protocol
is Link Access Procedure on the D channel (LAPD). LAPD is similar to High-Level Data Link Control (HDLC) and
Link Access Procedure, Balanced (LAPB). LAPD is used across the D
channel to ensure that control and signaling information flows and is
received properly. As the expansion of the LAPD
abbreviation-Link Access Procedure on the D channel-indicates, it is
used across the D channel to ensure that control and signaling
information flows and is received properly.
The LAPD flag and control fields are
identical to those of HDLC. The LAPD address field can be either 1 or
2 bytes long. If the extended address bit of the first byte is set,
the address is 1 byte; if it is not set, the address is 2 bytes. The
first address field byte contains the Service Access Point Identifier
(SAPI), which identifies the portal at which LAPD services are
provided to Layer 3. The Command/ Response (C/R) bit indicates whether
the frame contains a command or a response. The Terminal Endpoint
identifier (TEI) field identifies either a single terminal or multiple
terminals. All 1s in the TEI field indicates a broadcast.
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