H.323 SIP to Slow Start

In the following diagram below, a SIP endpoint (such as a UA or a SIP Gateway) initiates a session by sending an INVITE request destined for an H.323 Slow Start endpoint (a GK or GW). Between these entities, the Oracle® Enterprise Session Border Controller is positioned to perform interworking.

The call flow for this type of translation works fundamentally the same way that the translation does for SIP to Fast Start H.323, with the exception of how the media is established. Media is negotiated through the exchange of TCS and OLC messages after the H.323 Connect and SIP 180 Ringing messages have been sent. The first TCS message is sent from the Oracle® Enterprise Session Border Controller to the H.323 endpoint, and it contains information about media capabilities in SDP. The H.323 endpoint accepts and acknowledges this information with a TCS Ack message. Then the H.323 endpoint sends a second TCS, carrying information about the Gateway’s capabilities, that the Oracle® Enterprise Session Border Controller accepts and acknowledges. The H.323 endpoint and the Oracle® Enterprise Session Border Controller then exchange OLC and OLC Ack messages that establish the operating mode and Gateway capability. Finally, the Oracle® Enterprise Session Border Controller completes the 200 OK/ACK sequence on the SIP side, and RTP flows between the two endpoints.