System information

Layer 2
Book Title
17-376
technique ensures that only one terminal can transmit its D message at one time. After successful D
message transmission, the terminal has its priority reduced by being required to detect more
continuous ones before transmitting. Terminals cannot raise their priority until all other devices on
the same line have had an opportunity to send a D message. Telephone connections have higher
priority than all other services, and signaling information has a higher priority than nonsignaling
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 17-3) is very similar to that of HDLC and, 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.
Figure 17-3 LAPD Frame Format
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 17-4, from ITU-T I.451, shows the typical stages
of an ISDN circuit-switched call.
SAPI C/R EA EATEI
Flag FlagFCSControl DataAddress
1 111 Variable
SAPI = Service access point identifier (6 bits)
C/R = Command/response bit
EA = Extended addressing bits
TEI = Terminal endpoint identifier
2