[SIPForum-discussion] [Sip-implementors] ptime/Maxptime support in SDP

[Manpreet Singh]

I was reading some threads on this last year and there is says this way
there is no way to find out the priority of the codecs to be used. So if one
does implement this how would the priority be decided? So in the example
below, which has higher preference, payload 18 or 4?
 
Is this defined somewhere formally?
 
thanks

  _____  

From: Bala Neelakantan [mailto:neel at quintum.com] 
Sent: Wednesday, May 16, 2007 10:34 PM
To: 'Manpreet Singh'; sip-implementors at cs.columbia.edu
Subject: RE: [Sip-implementors] ptime/Maxptime support in SDP



For that, you have to have multiple m lines, with each having just one
codec, and the ptime can be specified for that. 

 

In your example, you can try separate m= line for each codec in the Offer.

 

m=audio 22836 RTP/AVP 4
c=IN IP4 216.168.161.230 
a=rtpmap:18 G729/8000 
a=fmtp:18 annexb=yes;

a=ptime:20 
a=maxptime:40 

m=audio 22838 RTP/AVP 18

a=rtpmap:4 G723/8000 
a=fmtp:4 bitrate=6.3;annexa=yes;

a=ptime:30 
a=maxpime:60 
 



Thanks,

Neel

 

-----Original Message-----
From: Manpreet Singh [mailto:msingh at ibasis.net] 
Sent: Wednesday, May 16, 2007 3:28 PM
To: balasubramanian_neelakantan at quintum.com;
sip-implementors at cs.columbia.edu
Subject: RE: [Sip-implementors] ptime/Maxptime support in SDP

 

Well each codec is defined as the media attribute so for each media
attribute, one can define packetization to be used. Different codec support
different packetizations so they have to be defined explicitly and we cant
use a session level ptime for all media attribiutes.

So how does one specify packetization in the SDP for each codec used is
really my question. Lets assume a UA wants to use G723 with 30 ms
packetization and G729 with 40 ms packetization.

-----Original Message----- 
From: Bala Neelakantan [mailto:neel at quintum.com <mailto:neel at quintum.com> ] 
Sent: Wednesday, May 16, 2007 4:15 PM 
To: 'Manpreet Singh'; sip-implementors at cs.columbia.edu 
Subject: RE: [Sip-implementors] ptime/Maxptime support in SDP 

Ptime is media attribute.  It can't be specified per codec. 

>From 2327. 

   a=ptime:<packet time> 
       This gives the length of time in milliseconds represented by the 
       media in a packet. This is probably only meaningful for audio 
       data.  It should not be necessary to know ptime to decode RTP or 
       vat audio, and it is intended as a recommendation for the 
       encoding/packetisation of audio.  It is a media attribute, and is 
       not dependent on charset. 

 

You can see some notes on usage of ptime in rfc3264. 

>From RFC 4040. 

   Optional parameters: ptime, maxptime 

             "ptime" gives the length of time in milliseconds 
             represented by the media in a packet, as described in RFC 
             2327 [6]. 

             "maxptime" represents the maximum amount of media, which 
             can be encapsulated in each packet, expressed as time in 
             milliseconds, as described in RFC 3267 [9]. 

 

Thanks, 
Neel 

 

> -----Original Message----- 
> From: sip-implementors-bounces at cs.columbia.edu 
> [mailto:sip-implementors- <mailto:sip-implementors->
bounces at cs.columbia.edu] On Behalf Of 
> Manpreet Singh 
> Sent: Wednesday, May 16, 2007 2:22 PM 
> To: sip-implementors at cs.columbia.edu 
> Subject: [Sip-implementors] ptime/Maxptime support in SDP 
> 
> Whats the right implementation for ptime/maxptime in the SDP. I read 
> few recommendations but what is the correct way of doing it? 
> 
> m=audio 22836 RTP/AVP 4 18 
> c=IN IP4 216.168.161.230 
> a=rtpmap:18 G729/8000 
> a=fmtp:18 annexb=yes;ptime=20 
> a=maxptime:40 
> a=rtpmap:4 G723/8000 
> a=fmtp:4 bitrate=6.3;annexa=yes;ptime=30 a=maxpime:60 
> 
> OR 
> 
> m=audio 22836 RTP/AVP 4 18 
> c=IN IP4 216.168.161.230 
> a=rtpmap:18 G729/8000 
> a=fmtp:18 annexb=yes; 
> a=ptime:20 
> a=maxptime:40 
> a=rtpmap:4 G723/8000 
> a=fmtp:4 bitrate=6.3;annexa=yes 
> a=ptime:30 
> a=maxpime:60 
> 
> is this how we can define ptime and maxptime per media attribute? Any 
> place this has been defined? 
> 
> Thanks 
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