SIP Call Hold

SIP Call Hold feature allows a User to generate a SUS message to put a call on hold or unplug a terminal from a socket and uses a RES message once the terminal is re-connected to a new socket and the T2 (User Initiated) or T6 (Network Initiated) timer has not expired. While the call is suspended, no audio media is passed end-to-end.

SUS Message (Suspend) - A SUS message is generated and sent in either direction to inform the called or calling party is temporarily disconnected. The Suspend (SUS) message indicates a temporary cessation of communication without releasing the call. SUS can only be accepted in an answered state (ICC answered/OGC answered).

RES Message (Resume) - A Resume (RES) message indicates a request to recommence communication. If the RES is not sent within timer T6 or T2 (T2 applies to ITU only), the controlling exchange will initiate a release procedure.

 

RFC

RFC 3398 Integrated Services Digital Network (ISDN) User Part (ISUP) to Session Initiation Protocol (SIP) Mapping: section 9

 

Call Flows

Receive Suspend then Resume from SS7 side

After a call is in the answered state, a SUS message is received from the remote SS7 side. The IMG sends a RE-INVITE message to the remote SIP side with the Connection IP address set to 0.0.0.0 in order to request the gateway to place the call on hold. The call is now on hold and no audio is passed. Before the T2/T6 Timer can expire a RES message is sent with the IP information to reconnect to and the call resumes.

Note: If the RES message is not received before the Timer expires, the  IMG will release the call

cf_sip_callhold_from_ss7.png

 

 

Receive RE-INVITE from SIP. Send CPG of Remote Hold and Remote Hold Release

After call is in an answered state, the  IMG receives a Re-INVITE with the connection address equal to 0.0.0.0. The  IMG then sends a CPG message with the notification indicator set to ‘remote hold’ to put Remote SS7 side on hold. When call is to resume, the remote SIP side sends a Re-INVITE with a valid IP address. The  IMG sends a CPG message with the notification indicator set to ‘remote hold released’ to the remote SS7 ANSI side and the call is released.

cf_sip_callhold_from_sip.png

Receive RE-INVITE from SIP. Send SUSPEND and RESUME to SS7 side

After call is in an answered state, the  IMG receives a Re-INVITE with the connection address equal to 0.0.0.0. The  IMG then sends a SUSPEND message to put Remote SS7 side on hold. When call is to resume, the remote SIP side sends a Re-INVITE with a valid IP address. The  IMG sends a RESUME message to the remote SS7 ANSI side and the call is released.

cf_sip_callhold_from_sip_suspend_resume.png

Suspend then Resume from SS7 Side when SIP-T is Enabled

After a call is in the answered state, a SUS message is received from the remote SS7 side. The  IMG sends the SUS message encapsulated in an INFO message to the remote SIP side with the Connection IP address set to 0.0.0.0 in order to request the gateway to place the call on hold. The call is now on hold and no audio is passed. Before the T2/T6 Timer can expire a RES message which is sent from remote SS7 side is also then encapsulated in an INFO message with the IP information to reconnect to and the call resumes.

Note: The SUS and RES message are encapsulated within the SIP INFO message instead of being sent as a Re-INVITE.

cf_sip_callhold_from_sipt.png

 

Configuration

The SIP Call Hold feature does not need to be configured. The only configuration needed is a user can decide to send either a CPG message or a SUSPEND and RESUME message when a re-invite message is received with c=0.0.0.0. Refer to the SIP ISUP IW topic for information on configuring this.