🚀 go-pugleaf

[POSTED TO comp.dcom.modems]
Graham Murray <graham@barnowl.demon.co.uk> wrote:

>Could you possibly explain the power limit restriction to 53k?  As you
>say the high speed is receive only, so the modem will only be
>generating 33k6.  AFAIK the high speed end has to be ISDN connected,
>and so is just sending a digital bit-stream.  I am sure that there
>cannot be any restriction on the data pattern that the ISDN sends,
>even using a voice or 3.1kHz audio bearers otherwise the 'Data over
>voice bearer' that I believe is popular in the USA would not work.

PCM server side modems are capable of transmitting "worst case" bit
patterns (long runs of maximum values) that would cause the output of the
D-A converter at your local telco to exceed the FCC limits on maximum
power (averaged over a few seconds).  The problem can be avoided by not
using those codes (hence reducing the maximum speed given a smaller number
of available codes) and/or by more sophisticated encoding that avoids long
runs of maximum values (and thereby taking less of a speed hit) -- short
runs are OK if the average power stays below the FCC maximum.

ISDN is only transmitting voice encoding, so the problem does not arise in
practice for non-56K use.  (It can of course arise if the ISDN adapter is
being used as a 56K server.)

--
Best regards,
John  mailto:JNavas@NavasGrp.Dublin.CA.US  http://www.aimnet.com/~jnavas/
      28800 Modem FAQ:  http://www.aimnet.com/~jnavas/modem/faq.html