Information Technology Reference
In-Depth Information
Layer 2
Layer 2 of the ISDN signaling protocol is Link Access Procedure on the D channel, also known as
LAPD. LAPD is similar to High-Level Data Link Control (HDLC) and Link Access Procedure, Balanced
(LAPB). As the expansion of the LAPD abbreviation indicates, it is used across the D channel to ensure
that control and signaling information flows and is received properly. The LAPD frame format (see
Figure 16-3) is very similar to that of HDLC; like HDLC, LAPD uses supervisory, information, and
unnumbered frames. The LAPD protocol is formally specified in ITU-T Q.920 and ITU-T Q.921.
Figure16-3 LAPD Frame Format
Field length
in bytes
Variable
1
2
1
1
1
Flag
Address
Control
Data
FCS
Flag
SAPI
C/R
EA
TEI
EA
SAPI = Service access point identifier (6 bits)
C/R
= Command/response bit
EA
= Extended addressing bits
TEI
= Terminal endpoint identifier
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 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. A TEI of all ones indicates a broadcast.
Layer 3
Two Layer 3 specifications are used for ISDN signaling: ITU-T (formerly CCITT) I.450 (also known as
ITU-T Q.930) and ITU-T I.451 (also known as ITU-T Q.931). Together, these protocols support
user-to-user, circuit-switched, and packet-switched connections. A variety of call establishment, call
termination, information, and miscellaneous messages are specified, including SETUP, CONNECT,
RELEASE, USER INFORMATION, CANCEL, STATUS, and DISCONNECT. These messages are
functionally similar to those provided by the X.25 protocol (see Chapter 19, “Troubleshooting X.25
Connections,” for more information). Figure 16-4, from ITU-T I.451, shows the typical stages of an
ISDN circuit-switched call.
