[MLB-WIRELESS] ISDN at local call costs

[Ben Ryan]

> On Fri, Jan 18, 2002 at 09:30:41AM +1100, Jonathan Oxer wrote:
>> Comparing a 64kbps ISDN channel to a V90 (theoretically 56kbps) modem
>> connection is not as easy as it sounds, because a V90 connection is
>> assymetrical with double the incoming data rate to the outgoing data
>> rate, is based on theoretical optimum conditions and is *after*
>> compression, while the ISDN's 64kbps is *before* compression. In plain

> I don't think this is correct - 56kbps is the raw speed without compression.


True, and actually, neither of those quoted line speeds are 'compressed' speeds.
They're raw data rates, and in the case of ISDN, varies with telco/hardware
 as to whether a B channel is 56kbps or 64kbps. US stuff is 56kbps usually -
 That's why a T1 is 1.544mbps but here a E1 is 2.048mbps.
You can't quote a compressed speed at any real value because it depends on the
data being compressed.
Like the Tape mfg's like Sony... their AIT drives are sold at a capacity which
is way overinflated... they are assuming a compression ratio of 2.6:1.
That's how you end up with 35Gb of REAL storage, but a quoted capacity of 90Gb.
Misleading :)

They DO use different encoding techniques to pack more into a given symbol rate,
but that's not compression. For analog modems, V42bis and MNP5 provide a max of
4:1 compression, but the penalty is increased latency.
One can run a 64k B channel and use a compression algorithm such as stac to
compress the PPP payload. MPPC is another, predictor is another. They're just
PKZip on the fly, if that makes sense.


hey, wireless is supposed to let us forget all about this black art comms stuff
:)

cheers

ben


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