Subject: [Megaco] Multiple SDP "m=" lines

From: "Ron Ho" <[email-masked]>
Date: Thu, 4 May 2006 11:18:31 -0400

Hi All,

I am confused about the SDP "m=" line use in Megaco. Should a session
contain more than one media stream, there should be multiple "m="
lines in the SDP session in the local/remote descriptor. If all the
media streams use different encoding scheme, we could clearly
distinguish one from the other. However, if there exists two streams
with identical encoding scheme. How can we distinguish one from the
other. For example, if we carry left channel and right channel in
different audio streams. How can the other end know which channel is
left and which is right? Should there be clear marking on the channel
to indicate which is which? The matter get more complicated in a IP/IP
border gateway environment. If there is a media streams switching in
the middle of the call, how do we associate the new streams channel
properly. E.g. the left and right channel may be swapped
un-intentionaly by the border gateway. Is there any spec. talked about
this? I would really appreciate if anyone can give me some pointer on
this.

Thanks,
Ron

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---

From: "Kevin Boyle" <[email-masked]>
Date: Thu, 4 May 2006 11:47:57 -0400

Multiple streams must appear in multiple Stream Descriptors.  Therefore,
multiple m= lines are required to be separated by v= lines in order to
distinguish the m= lines as alternatives and not simultaneous options.
If you are trying to set up multiple simultaneous streams, you must have
one Stream Descriptor per stream, each with its own Local and Remote
Descriptor.

I would think the right/left channel information would be encoded in the
stream, so the receiving end will know how to play it out.  If so, then
all the concerns about which side is which and entities in the middle
moving stuff around becomes irrelevant.

Kevin 

-----Original Message-----
From: Ron Ho [mailto:[email-masked]] 
Sent: Thursday, May 04, 2006 11:19 AM
To: megaco
Subject: [Megaco] Multiple SDP "m=" lines

Hi All,

I am confused about the SDP "m=" line use in Megaco. Should a session
contain more than one media stream, there should be multiple "m="
lines in the SDP session in the local/remote descriptor. If all the
media streams use different encoding scheme, we could clearly
distinguish one from the other. However, if there exists two streams
with identical encoding scheme. How can we distinguish one from the
other. For example, if we carry left channel and right channel in
different audio streams. How can the other end know which channel is
left and which is right? Should there be clear marking on the channel to
indicate which is which? The matter get more complicated in a IP/IP
border gateway environment. If there is a media streams switching in the
middle of the call, how do we associate the new streams channel
properly. E.g. the left and right channel may be swapped un-intentionaly
by the border gateway. Is there any spec. talked about this? I would
really appreciate if anyone can give me some pointer on this.

Thanks,
Ron