Hi Andreas, Comments inline...
Regards Ranjit -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Andreas Byström Sent: Monday, September 12, 2005 4:59 PM To: [email protected] Subject: [Sip-implementors] What should a PSTN gw offer in sdp? Hi all! I have been looking around for hardware to place in a PSTN/SIP gateway. For RTP, it looks like for all hw, you have to specify which codec should be used for a specific "channel" you setup (for the RTP to be "translated" to PSTN and the other way around). Now I wonder what such a GW should offer in SDP. Imagine this case: PSTN subscriber calls SIP subscriber through a PSTN GW. When the gw receives the call on pstn it has to send a INVITE to the SIP network and include sdp. What is most correct of the two following cases: 1) The gw send a the complete list of codecs it supports, say 0,8,18. The UA called can handle the same codecs and therefore answers with 0,8,18. The GW must setup the connection between ip and pstn and, as stated above, must start a "channel" and set the codec to use. It choses the first one (0). According to this setup, should the gw be able to receive ANY of the codecs in the response? Say UA starts sending using 0, everything is ok and then UA decides to send using 8 instead. Is it allowed to do this without sending reInvite/Update? If it does, the gw will not handle voice good at all [Ranjit] here since GW has selected first one (0), it should send and receive media using codec 0. It cannot dynamically switch to another codec. In case it wants to switch to another codec during call, it should indicate to the GW using UPDATE method. 2) gw has a priorization of codecs, This means that it will only offer the codec with the highest prio in the sdp, say 8, so it only includes codec 8 in the sdp offer. This ensures that in the response back there will only be one codec and the gw can start the "channel" without having to worry about UA will start sending other formats without sending reInvite/Update. The risk here is that the UA wotn support the codec that GW has as higest prio and therefore no call will be set up. [Ranjit] Depending on local policy of the GW it can choose which codec to use, either the first one or last one in the order that it sends. So if both UA and GW support the entire list of codecs, then GW could choose the first one or the last one depending on its local policy (priority). Is my assumptinos above correct? If so, which of the two alternatives above do you recommend? Regards, // Andreas _______________________________________________ Sip-implementors mailing list [email protected] http://lists.cs.columbia.edu/mailman/listinfo/sip-implementors _______________________________________________ Sip-implementors mailing list [email protected] http://lists.cs.columbia.edu/mailman/listinfo/sip-implementors
