Which SIP message element is mapped to QSIG FACILITY messages being tunneled across a SIP trunk between two Cisco IOS gateways?
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When two Cisco IOS gateways communicate with each other over a Session Initiation Protocol (SIP) trunk, the QSIG (Q Signaling) protocol is used to provide signaling for QSIG-enabled Public Switched Telephone Network (PSTN) networks. QSIG is a signaling protocol that allows various private branch exchanges (PBXs) to communicate with each other, providing advanced call control features.
When a QSIG-enabled PBX wants to send a QSIG message to another QSIG-enabled PBX over a SIP trunk, the QSIG message is encapsulated in a SIP message element. This process is known as "tunneling" QSIG messages over SIP trunks.
Of the SIP message elements listed in the question, only one is used for tunneling QSIG messages over a SIP trunk, and that is the SIP INFO message element.
SIP INFO messages are used to carry non-call-related information during a call. These messages can carry a variety of data, including Dual-Tone Multi-Frequency (DTMF) digits, music on hold, and QSIG messages.
When a QSIG message is encapsulated in a SIP INFO message, the message is sent across the SIP trunk to the other gateway. The receiving gateway then extracts the QSIG message from the SIP INFO message and forwards it to the QSIG-enabled PBX on its side of the network.
In summary, the SIP INFO message element is used to encapsulate QSIG messages when they are tunneled across a SIP trunk between two Cisco IOS gateways.