Caleb Clausen (clausen@connsys.com)
Tue, 12 Dec 2000 19:59:01 -0800
From: "Brian Cruickshank" <cruicb@nortelnetworks.com>
> Glenn, I understand that MS-GSM is currently viewed as the codec of
> choice for VPIM but has this decision been finalized yet? I think a
> good case can be made for adopting the 'audio / basic' G.711 codec as a
> baseline VPIM codec. As well as being a MIME standard, G.711 is the
[snip]
it seems that this discussion has been going on for a long time.
i've grown rather fond of ms-gsm as the default ivm codec. the big
advantage that ms-gsm has over mu-law (i think even ms-gsm
detractors will agree with me on this) is the lower bitrate. however, i
see more useful aspects of low bitrate codecs than just supporting
home users constrained by modems.
consider: lower bitrates means smaller overall voice file sizes,
which leads to less storage used on the email server(s) and on the
ivm gateway itself (if applicable). many mail servers have limits on
the maximum size of a single message. if mu-law is the codec, a
100k limit for a single message translates to 12.5 seconds of voice
storage. (i know of at least one large email server which has 100k
single message limit by default.) with ms-gsm as the codec 100k
means just over a minute for a voice message. (this is neglecting
overhead sources: base64, wave-header, mime-header, and rfc822-
header. such issues apply regardless of the codec.)
even if the connection to the email server is a dsl line, that does
not mean that that bandwidth should be used wastefully. internet
applications need to share bandwidth with other applications, not
hog it all to themselves. the less bandwidth you use, the more is
available for other uses.
if there is a single dsl line serving a small office and the email
server is on the other side of that line, then the ivm device needs to
share the dsl bandwidth with users surfing the web and checking
their (normal) email. there are a couple of scenarios where the
email server could be more than 1 hop from the ivm: outsourced
email, or a branch office with email server at the central office.
multiple transcodings
yes, multiple transcodings are bad with gsm. but one are two seem
to be tolerated with ok resulting voice quality. how likely are
scenarios with more transcodings, really?
forward and comment
if a (gsm-encoded) ivm message is forwarded with additional
messages tacked on the end, it need not imply any transcodings.
one approach is simply to catenate all the gsm data together in a
single data chunk in the .wav file. i was surprised to find that this
works, but it does. another approach has a second data chunk in
the wave file. this will confuse a number of wave file players, tho.
still a third approach has separate wave files in separate body
parts in a multipart message.
standard?
gsm is a standard published by etsi. microsoft's variation
on it is described in an internet draft. (check the vpim website.) so,
i would say that ms-gsm is as much a standard as any of this stuff.
patent
microsoft claims they have a patent on their ordering of gsm fields.
my recent searches of the pto's website for this patent have been
fruitless, so i'm still not sure whether to believe this patent is real or
fud. in any case, several people have disparaged the validity of a
patent that merely re-orders the fields in gsm headers. (ok, so they
dropped a couple of pad fields too. i still say that's trivial.) and,
microsoft has promised to provide royalty-free licenses of any
necessary microsoft intellectual property to implementers of vpim
systems. (presumably ivm as well?)
glenn, where is that updated statement by microsoft you said
would be appearent soon?
a more serious question is that of the potential phillips patent, to
which i cannot speak.
os support
ms-gsm is, nataurally, supported best on windows. i haven't seen it
myself, but i've been assured by some of our tech-support people
that ms-gsm can be played on a mac, provided you want to pay
extra for quicktime pro. i have even seen indications that there are
linux programs which could play ms-gsm encoded wav files. as usual,
with linux, it seems it would require some fiddling.
"The battle
for mental territory
is glory.
End of story."
--KRS-ONE
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