Modern Modeming (was Re: Geek-only net)

Don THX1138 at dakotacom.net
Wed Aug 9 11:47:14 CDT 2006


JP Hindin wrote:
> 
> Modems and faxes can be pretty spotty over VoIP - it depends largely on
> the carrier you are using and the CODEC involved. Not to mention how the
> 'net is behaving on that day.
> 
> VoIP introduces delay into the line (regardless of whether a human ear
> notices it), which really plays hell with cheaper faxes. I've not tried a
> modem, but I imagine it doesn't help. Naturally the crappier your
> connection, or should Level3 be blowing goats that day (Like it was
> yesterday, grumble), the delay introduced can get up to a 1/4 second.
> The big one is lossy compression CODECs which can alter the tone of the
> blurbles being spat down the line. And since you don't really have any
> control over what CODEC Vonage, or other VoIP providers, are using between
> them and their PSTN gateways... you never really know what's going on.
> Thankfully echo isn't something you're likely to come across, since that
> is generated most often by handsets - and then made noticeable by the
> delay inherant to VoIP... but I suppose if your modem sucks...?
> 
> Fax is not usually considered a "supported" medium over VoIP - T38 is a
> digital fax protocol and is what you're 'supposed' to use when doing this
> sort of thing. Smarter people than I came up with this one.

I'd also tend to be skeptical about working.  I have a digital
PBX at home and know that trying to use an analog modem on any
of those lines is an exercise in retro-technology ("Wanna see
a 56K emulation of a Bell 103??").  You have quantization
effects, delays, phase distortions (killers for the higher
speed modems), etc.

Your ear is amazingly tolerant in the crap it will process
(just *think* about how different you sound on the phone)
but machines tend to be less forgiving.

Note that if your CO has you dangling off a SLIC96, you've
discovered that there *is* a difference.  ;>  :-(



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