1. Configuration Guide
  2. SIP Signaling Services
  3. SIP SIP-I Interworking
  4. Interworking for Call Hold and Resume Cases

Interworking for Call Hold and Resume Cases

IMS nodes can send UPDATE or Re-INVITE messages to the SBC during a call to put the session on hold or to resume it. In turn, the SBC encapsulates these messages, into INVITE, re-INVITE, or UPDATE messages as call progress messages (CPG) within MIME content and sends them for interworking out the SIP-I side of the call.

A caller can put a call on hold at any time after the alerting phase has begun using an UPDATE. A callee can only put a call on hold or resumed after it has answered the call. Both caller and callee can initiate hold/resume using an UPDATE or Re-INVITE. As a result, a call can only be put on hold in the SIP to SIPI direction by the caller before the call is answered. After the call is answered, either side can hold and resume the call. Furthermore, after the call is in progress, either side can effectively put the other side on hold. These actions are signaled using the a attribute in the SDP.

To put a session on hold, an IMS node sends an UPDATE or re-INVITE message with an "inactive" or a "sendonly" SDP attribute. Upon receipt of the hold request, the SBC encapsulates a CPG message containing the Generic notification indicator parameter with notification indicator set to "remote hold" while forwarding the UPDATE or re-INVITE towards the SIP-I side.

The SBC populates the following Event Information and Generic Notification Indicator subfields for hold cases in the CPG:

  • Event Information:

    • Event Indicator—PROGRESS (0000010)

    • Event Presentation—No indication (0)
  • Generic Notification Indicator:

    • Notification Indicator—REMOTE HOLD (1111001)

To resume a session, an IMS node sends an UPDATE or re-INVITE message with a "recvonly" or "sendrecv" SDP attribute, depending on the current state of the session. Upon receipt of the resume request, the SBC encapsulates a CPG message containing the Generic notification indicator parameter with notification indicator set to "remove retrieval" while forwarding UPDATE / re-INVITE towards SIP-I side.

The SBC populates the following subfields of Event Information for remote resume cases:

  • Event Information:

    • Event Indicator—PROGRESS (0000010)

    • Event Presentation—No indication (0)
  • Generic Notification Indicator:

    • Notification Indicator—REMOTE RETRIEVAL (1111010)