[wplug] Asterisk Forum?

[Christopher DeMarco]

On Thu, Dec 21, 2006 at 12:42:10PM -0500, Jonathan S. Billings wrote:

> > My brain turned itself inside out when I read this.  If you're
> using > SIP to send a FAX... why don't you use email?
> 
> Probably because a lot of people know how to use a fax, and their
> business

I meant it facetiously :-)

Your explanation was the initial cause of my brain going all toroid,
by starting me thinking about what sending a FAX over SIP actually
looked like.

In case anybody didn't quite catch Jonathan's point, he's quite
clever.

The compression used in VoIP is geared toward preserving ONLY the
parts of the signal that we (humans) care about, to the corresponding
detriment of the ends of the spectrum that we [physically] can't hear.
The compression makes voice "smaller" because it cuts out all the
ultra/infra- sound that we can't hear anyway.

A FAX, OTOH, is a hack on telephony: The phone system is capable of
transmitting (a) resolution (many changes in a small amount of time)
and (b) broad frequency (great magnitude of change), both beyond what
we (humans) can do.  The hack was realizing that we (humans) don't use
the phone lines to their capacity since we can only talk so fast and
don't hear the whole audio frequency spectrum -- so heck, let's let
computers talk over the lines much faster and much higher/lower than
we can.

Compress FAX using voice codecs and you strip out the assumptions
which make FAX possible in the first place, namely letting the
computers talk using the full capacity of the equipment that we
meatbags can't.  In a sense, sending a FAX over SIP is about as smart
as trying to whistle dialup tones.  

Yes, I know Captain Crunch did just that.  That's not my point.


-- 
Christopher DeMarco <cmd at alephant.net>
Alephant Systems (http://alephant.net)
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