Re: [AVT] WGLC on draft-ietf-avt-rtp-g711wb-03.txt

[Hitoshi Ohmuro <[e-mail]>]

Hi,

Sorry for my delayed comment.

I found an alignment problem for implementing the 1-octet payload header.

We are thinking of low-cost implementation of G.711.1 codec and RTP 
stack on a DSP.
Generally, a G.711.1 codec and an RTP stack are implemented as 
independent functions.

An G.711.1 encoder generates even-size audio data, 40, 50, or 60 byte 
data. RTP header always has 4-octet boundary.

If the payload header has a even-size, the RTP stack can combine the RTP 
header, the payload header and the audio data easily, i.e. a DSP has 
only to copy data word by word.

If the payload header has a odd-size, however, a 16-bit DSP has to copy 
the audio data byte by byte using byte swapping like endian conversion. 
This is due to 16-bit DSP memory architecture. This processing causes 
much more overheads than even-size payload header.

I know that the communication protocol should not depend on a specific 
hardware architecture.
However, I know also that most of DSPs designed for speech signal 
processing are optimized for 16-bit data.

I would like to propose that the payload header should be 2-octet and 
the latter 1-octet is treated as a reserved byte which is default to zero.

This reserved byte might waste the bit rate, but we might be able to use 
this byte for an optional purpose in the future, without 
interoperability problems.

Please let me know your comments.

Regards,

Hitoshi Ohmuro


> Hi,
> 
>  
> 
> This is to announce a working group last call on the RTP Payload Format 
> for ITU-T Recommendation G.711.1
> 
> Please send any final comments to the mailing list by May 27^th 2008. 
> 
>  
> 
> If no issues are raised, we will request the IESG consider the document 
> for publication.
> 
>  
> 
>  
> 
>  
> 
> A New Internet-Draft is available from the on-line Internet-Drafts 
> directories.
> 
> This draft is a work item of the Audio/Video Transport Working Group of 
> the IETF.
> 
>  
> 
>  
> 
>             Title           : RTP Payload Format for ITU-T 
> Recommendation G.711.1
> 
>             Author(s)       : A. Sollaud
> 
>             Filename        : draft-ietf-avt-rtp-g711wb-03.txt
> 
>             Pages           : 15
> 
>             Date            : 2008-04-28
> 
>  
> 
> This document specifies a Real-time Transport Protocol (RTP) payload 
> format to be used for the International Telecommunication Union
> 
> (ITU-T) G.711.1 audio codec.  Two media type registrations are also 
> included.
> 
>  
> 
> A URL for this Internet-Draft is:
> 
> http://www.ietf.org/internet-drafts/draft-ietf-avt-rtp-g711wb-03.txt
> 
>  
> 
> Internet-Drafts are also available by anonymous FTP at:
> 
> ftp://ftp.ietf.org/internet-drafts/
> 
>  
> 
> Thanks
> 
> Roni Even
> 
>  
> 
> 
> ------------------------------------------------------------------------
> 
> _______________________________________________
> Audio/Video Transport Working Group
> [e-mail]
> https://www.ietf.org/mailman/listinfo/avt

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