Controlling 183 Messages using PEM

The SBC supports configuration that causes it to issue 183 Session Progress messages with or without an ISUP ACM toward the SIP-I side of an interworked call.

When the value of iwf-for-183 parameter in sip-isup-profile configuration is set to pem-controlled, the SBC interworks 183 Session Progress responses received from IMS node as follows:

• If the received 183 Session Progress message contains P-Early-Media (PEM) header with value set to sendonly or sendrecv, the SBC behaves as if iwf-for-183 is enabled.
• If the received 183 Session Progress message does not contain a PEM, the SBC behavior as if the iwf-for-183 parameter is disabled.
• If the received 183 Session Progress message contains a P-Early-Media header with a value other than sendonly or sendrecv, the SBC behavior is the same as if the iwf-for-183 parameter is disabled.

The call flow depicts the SBC configured to pem-controlled, receiving three 183 Session Progress messages, and forwarding messaging to the SIP-I side based on the contents of each 183:

1. Contains a PEM header with a value of sendonly, resulting in a 183 Session progress message that includes an ISUP CPG with inbound info available.
2. Contains a PEM header with a value that of inactive, resulting in a 183 Session progress message with no CPG.
3. Does not contain a PEM header, resulting in a 183 Session progress message with no CPG.