[AVT] G726 packing specified in draft-ietf-avt-profile-new-12.txt

["Rajesh Kumar" <rkumar@cisco.com>]

Henning and Steve,
The packing of G.726 code words into octets as specified in section 4.5.4
of draft-ietf-avt-profile-new-12.txt  is at odds with ITU I.366.2, which has
been an approved standard since February 1999.

In the ITU  specification, you start packing code words into octets starting
from the most significant rather than the least significant positions in the
octet. Thus, when you start packing, the most significant bit in the
codeword aligns with the most significant bit in the octet. This is more in
harmony with "network byte order" or big-endian order, since it does not
result in splitting a code word as the example in your internet draft does.

Further, for systems that concurrently support RTP and AAL2 modes, the G726
packing in your internet draft  places an unnecessary burden on DSP design,
integration and testing since two inconsistent packing rules need to be
followed for RTP and AAL2.

Since the IETF AVT specifications did not specify ADPCM byte packing rules
for RTP until recently, many vendors had taken a cue from the AAL2
specifications (approved in 1999) and have designed DSPs  that use the
I.366.2 ADPCM code word packing rules for RTP. Using a different, and
obviously less elegant,  rule for RTP without sufficient justification poses
a major backward compatibility issue for those vendors. I call the rule in
your internet draft less elegant since, when using network byte order
transmission, it splits code words gratuitously.

Is there a reason why these rules for packing G726 in RTP could not be
changed?

Thanks!

--------------
Rajesh Kumar
Cisco Systems


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