[wplug] Asterisk Forum?

[Jonathan S. Billings]

> In response to "Ryan R. Schuster" <ryan at myerstechservices.com>:
> It's been a few years since I've worked with *, but (at the time) the
> basic answer was, "you can't fax over VoIP"  The reason is that all
> VoIP transmission protocols use lossy compression.  You can't hear it,
> as it's too subtle, but the loss is enough that fax data becomes
> corrupt.  Theoretically, there are ways to work around this, but I'm
> not aware of any actual implementation.

Voice calls are digitally encoded using algorithms that preserve the
audible spectrum of voice, to a limited extent.  However, FAX calls use a
wider spectrum of of frequencies that are simply not preserved.  If you
think about it, sending a Fax is turning analog data (a picture) into a
digital sequence of pixels, which is then converted to another analog
format, sounds.  Those sounds traverse the phone lines (you can safely
pretend that they stay as an analog signal because the phone company is a
black box) to be converted back to a digital form, to either be printed
(analog) or saved as a digital file.

If you are using SIP to send a fax, you are doing another analog->digital
and then digital->analog conversion, and you're using a protocol not
designed to handle the analog medium required.  It shouldn't be a surprise
it doesn't work.

You'd probably be better off using a service such as eFax for faxes, and
not try to force SIP to do something it was never designed to do.


-- 
Jonathan Billings <billings at negate.org>