[SIPForum-discussion] Regarding Early Media and Preconditions.

[Stephen James]

The issue that is not addressed is the preconditions. Do preconditions override 
the mode line. That is does
a=curr:qos local none
a=curr:qos remote none
mean that no media, even early media can be communicated backwards until the 
preconditions are resolved?

Note this is on the IMS side towards the handset, so no ISUP interworking is 
involved.

 
Stephen James 
sjames_1958 at yahoo.com
 
We are not princes of the earth, we are the descendants of worms, and any 
nobility must be earned.





________________________________
From: Aniella Juverdeanu <Aniella.Juverdeanu at telus.com>
To: Stephen James <sjames_1958 at yahoo.com>; "discussion at sipforum.org" 
<discussion at sipforum.org>
Sent: Mon, August 13, 2012 6:09:31 PM
Subject: RE: [SIPForum-discussion] Regarding Early Media and Preconditions.


Hi,
 
So MGCF is receiving the Invite with SDP as below – that means G.711, right?
a=sendrecv is just extra information for the default send/receive (when 
a=sendrecv is missing) unless otherwise specified by a=sendonly or a=recvonly.
 
The UAC is just presenting in INVITE SDP its media information for the UAS to 
know where (its media server) to send the packets. The early media will be open 
when UAS will reply with 183 and SDP with information about  the media server 
that plays the announcement (sure PRACK will follow if set between UAS and UAC). 
This SDP could open one direction only from UAS Media end point to UAC media end 
point or just leave it two way.
UPDATE is necessary only if UAS indicated one SDP in 183 and for whatever reason 
that SDP changed, when the new SDP has to be communicated (could be another 183 
as well – depends on vendor).
 
Regards,
Aniella 
 
 
From:discussion-bounces at sipforum.org [mailto:discussion-bounces at sipforum.org] On 
Behalf Of Stephen James
Sent: August 13, 2012 1:17 PM
To: discussion at sipforum.org
Subject: [SIPForum-discussion] Regarding Early Media and Preconditions.
 
The scenario is an MGCF (SIP - ISUP interworking IMS - PSTN). SIP preconditions 
are in use. 


We receive an INVITE with SDP as follows (modified for security)

v=0
o=- 3553598070 3553598070 IN IP4 10.0.0.10
s=-
c=IN IP4 10.0.0.10
t=0 0
m=audio 59068 RTP/AVP 0
a=sendrecv
a=curr:qos local none
a=curr:qos remote none
a=des:qos mandatory local sendrecv
a=des:qos mandatory remote sendrecv

Now, according to 3264, once this offer is sent, the UAC should be ready to 
receive media; however
the preconditions seem to contradict this. And in actuality the bearer is not 
set up and the UAC cannot
receive the early media.

In the MGCF we need to play an announcement due to network conditions. Since the 
SDP is indicating
sendrecv, that should be possible. The UAC does not receive our packets. 

- Should we complete the preconditions sequence prior to playing early media?
  - This is 183, PRACK, 200 OK (Prack), UPDATE, 200 OK (UPDATE)
- Should the UAC not put a=sendrecv in the SDP when in fact it cannot receive 
early media?
  - This would be more explicit in the need to complete the preconditions.
 
Stephen James 
sjames_1958 at yahoo.com
 
We are not princes of the earth, we are the descendants of worms, and any 
nobility must be earned.
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