Re: [AVT] Frame Interleaving in AMR profile->RFC 3267

[B Prashanth <bprashanth@neomagic.com>]

Hi Magnus!

   Thanks for replying in detail. It has cleared lot of my doubts.

Regards,
Prashanth

Magnus Westerlund wrote:

> Hi,
>
> When it comes to meeting the limitations the sender side has the 
> freedom to select any combination of ILL and number of frames that 
> satisfies the restrictions. Another restriction then I that may be 
> used is "maxptime" which puts a limit on the number of frames per packet.
>
> In your example with I=30 the sender has the choice to go from ILL=0 
> (No interleaving, with 30 frames in a single RTP packet, up to ILL=14 
> and only two frames per packet. It is assumed that the sender will use 
> the combination most suitable for the current network performance and 
> congestion state.
>
> There is no need for the payload format to put in more limitations. 
> The RTP payload format has great flexibility and for any receiver 
> implementing the interleaving mode can decode these payloads 
> independent  of which ratio between ILL and N that are used. The I 
> restriction value is there to put a limit on buffering and 
> interleaving induced delay. However using many AMR frames per packet 
> is generally not a good idea. If one would use the 130 frames per 
> packet that you calculate, I hope you are aware that this represent 
> 2.6 seconds. That can be a whole sentence lost if this single packet 
> is lost. So filling voice packet to the limit is not suitable in 
> environments where losses occur. However if one has retransmission or 
> uses a reliable channel more frames per packets may give satisfactory 
> performance. My opinion is that more than 5-10 frames per packet is to 
> much. And if one is going as high as 10 frames per packet one should 
> definitely consider using interleaving with fairly decent length.
>
>
> [Chairs hat on]
>
> RFC 3267 is a payload format NOT a profile. A RTP profile is something 
> else and must not be confused with payload formats. I have seen this 
> mistake being done also by others. The current available profiles are 
> RFC 1890 (AVP) or rather draft-ietf-avt-profile-new-13. Two new 
> profiles are with the IESG, SRTP (SAVP) draft-ietf-avt-srtp-09 and 
> Extended RTCP feedback (AVPF) draft-ietf-avt-rtcp-feedback-07.
>
> [chairs hat off]
>
> Best Regards
>
>
> Magnus
>
>
> B Prashanth wrote:
>
> > Dear All
> > The frame interleaving in the AMR profile (RFC 3267) states that
> >
> > " The sender of the payload MUST only apply interleaving if the
> >    receiver has signalled its use through out-of-band means.  Since
> >    interleaving will increase buffering requirements at the receiver,
> >    the receiver uses MIME parameter "interleaving=I" to set the maximum
> >    number of frame-blocks allowed in an interleaving group to I.
> >
> >    When performing interleaving the sender MUST use a proper number of
> >    frame-blocks per payload (N) and ILL so that the resulting size of an
> >    interleave group is less or equal to I, i.e., N*(L+1)<=I. "
> >
> > ILL is the interleaving Length i.e. number of RTP packets across 
> > which interleaving happens.
> > NOTE: The value of ILL starts from 0. Hence, one needs to do ILL +1 
> > in the above formula.
> >
> > Nmax, the maximum value N (number of frame-blocks per payload) can 
> > take, is calculated by the formula ->
> > Nmax = (RTP MTU SIZE)/(The frame size of a frame block)
> >
> > Assuming,
> >
> > RTP MTU SIZE = 1500 bytes, and
> > The frame size of a AMR frame block = 13 octets for AMR frame type=0
> >
> > The above assumption gives:
> > Nmax = 1500/13 = 115 packets.
> >
> > The information obtained from out-of-band means, say, SDP is 
> > "interleaving=30", which means that an interleaving group can have a 
> > maximum of 30 frames per group such that:
> > (ILL+1) * N less than or equal to I          where    N varies from 
> > 0<N<Nmax.
> >                       ILL is the interleaving Length i.e. number of 
> > RTP packets across which interleaving happens.
> >
> > Now, in the case taken by me above, Nmax > I   (i.e. 115>30). So, the 
> > condition (ILL+1)*N less than or equal to I with N=Nmax doesn't hold 
> > good.
> >
> > So, how does one select the value of N and ILL such that it is less 
> > than I, in the above case?
> >
> > Right now, I can think of only one option of handling this:
> > My option:  Assume the minimum value of ILL, say 1. A value of 2 
> > makes sense because one needs at least 2 RTP packets to do 
> > interleaving. Based on this value calculate N
> > N = (30/2)-1
> > N = 14
> > With this value of N we satisfy both the conditions, namely,
> > (ILL+1) * N less than or equal to I  -> (2+1)
> > and
> > N varies from 0<N<Nmax.
> >
> > I wanted to ask the others on this list whether this solution is the 
> > only solution? Or there are other ways of handling this?
> > Shouldn't the values of N and ILL be very apparent rather than AMR 
> > profile designer making such assumptions about it?
> >
> > --
> > Prashanth B.P.
> > Sr. Design Engr.
> > NeoMagic Design Center.
> > Polyplex Building, First Floor
> > B-37 Sector 1
> > Noida INDIA 201301
> > E-Mail:  bprashanth@neomagic.com
> > Tel:     +91-120-2536208
> > Fax:     +91-120-2536207



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