ISDN uses ITU-T Q.931 to establish and tear down calls. Call control and signaling information
is carried over the D channel. Figure 26.6 shows the Q.931 procedures.
ISDN call setup and teardown
Setup
Call Proceeding
Alerting
Setup
Disconnect Disconnect
Release
Alerting
>Ring<
Connect Connect
Connect Acknowledge Connect Acknowledge
Release Complete
A B
Call Proceeding
>Pickup<
Release
Release Complete
The process for ISDN call setup and teardown is as follows:
1. First, a SETUP message is sent from device A. The SETUP contains information necessary
to make the call.
2. Next, the switch sends a CALL PROCEEDING back to device A.
3. An ALERTING message is sent back when device B is contacted. You might hear the phone
ring at this point.
4. CONNECT and CONNECT ACKNOWLEDGE messages are sent to indicate that the call
has been accepted.
5. Call teardown starts when one of the users hangs up. Here, device A hangs up, and a
DISCONNECT message is sent to device B. The switch now disconnects B and sends a
RELEASE to A. A RELEASE COMPLETE message confirms the process.
Using the debug isdn q931 command, you get the following output.
ISDN BR0: TX -> SETUP pd = 8 callref = 0x05 _
Bearer Capability i = 0x8890 Channel ID i = 0x83
Keypad Facility i = '8358662'
ISDN BR0: RX <- CALL_PROC pd = 8 callref = 0x85 _
Channel ID i = 0x89 Locking Shift to Codeset 5
Codeset 5 IE 0x2A i = 0x809402, '`=', 0x8307, _
'8358662', 0x8E0B, ' TELTONE 2 '
ISDN BR0: RX <- CONNECT pd = 8 callref = 0x85
ISDN BR0: TX -> CONNECT_ACK pd = 8 callref = 0x05
ISDN BR0: TX -> DISCONNECT pd = 8 callref = 0x05 _
Cause i = 0x8090 - Normal call clearing
ISDN BR0: RX <- RELEASE pd = 8 callref = 0x85
ISDN BR0: TX -> RELEASE_COMP pd = 8 callref = 0x05
Table 26.8 describes the output from the Q.931 command.