Question: In an IMS (IP Multimedia Subsystem) with reactive transcoding, what action does an IMS-ALG (Application Layer Gateway) take if the initial SDP (Session Description Protocol) offer is rejected due to lack of support for an offered codec?
Option 1: The IMS-ALG restarts the OMR (Originating/terminating Media Resource] procedure with the answering side after allocating and anchoring a TrGW (Traversal Gateway) with transcoding capabilities.
Option 2: The IMS-ALG forwards the rejection to the originating side without modifying the SDP offer, awaiting further instructions.
Option 3: The IMS-ALG removes only secondary realm instances from the SDP offer before forwarding it again without allocating a transcoder.
Option 4: The IMS-ALG allocates a transcoder but does not anchor a TrGW, then proceeds to restart the OMR procedure.
Explanation: In reactive transcoding scenarios within an IMS, if the initial SDP offer is rejected due to codec incompatibility, the IMS-ALG’s response involves more than just re-forwarding or rejecting the offer. Specifically, it proactively addresses the compatibility issue by allocating and anchoring a TrGW with transcoding capabilities. This step ensures that the media can be translated into a compatible format for the receiving end, thus facilitating successful communication. By restarting the OMR procedure after taking these measures, the IMS-ALG effectively bridges the technical gap between the communicating parties, ensuring seamless media exchange.