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

[B Prashanth <[e-mail]>]

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:  [e-mail]
>> Tel:     [phone]
>> Fax:     [phone]
>>
>




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