- Call Setup - Network Side

When an ISDN call is set up and the ISDN network wants to put the call on hold, it will transmit a SUSPEND message to the CSP. The CSP will send a PPL Event Indication (0x43) of SUSPEND to the host. The host will send a PPL Event Request (0x44) of SUSPEND ACK or SUSPEND REJECT to the CSP which will generate the required response to the ISDN network.

If the SUSPEND request is successful, the CSP will start timer T307 (call hold timer, default value of 3 min.). If this timer expires, the CSP will release the call with cause 102, recovery on timer expiry.

When the ISDN network wishes to resume the call, it will transmit a RESUME message to the CSP. The CSP will send a PPL Event Indication of RESUME to the host. The host will send a PPL Event Request of RESUME ACK or RESUME REJECT to the CSP which will generate the required response to the ISDN network. If the call has been successfully resumed, the CSP will stop the call hold timer, T307.

Call Flows

The following are call flows for the various scenarios:

Figure 9 SUSPEND Request Successful

Figure 10 SUSPEND Request Rejected

Figure 11 RESUME Request Successful

Figure 12 RESUME Request Rejected

If the host does not respond to the ISDN RESUME message within T318 value (default 4 sec.), the ISDN network will release the call with cause of 102, recovery on timer expiry.

API Values

The following PPL Events for the PPL Event Request message (0x44) and PPL Event Indication message (0x43) support this feature.

There is also a PPL Timer.

PPL Component is 0x0005 (ISDN L3P Call Control)

PPL Event Requests

0x0015 - SUSPEND Ack

0x0016 - SUSPEND Reject

0x0018 - RESUME Ack

0x0019 - RESUME Reject

PPL Event Indications

0x0020 - SUSPEND

0x0021 - RESUME

PPL Component L3 Call reference (0x0008)

PPL Timers

Timer ID Description Default Value (seconds)
0x0007 Q.931 T307 180 (3 minutes)