Hacker News Reader

Comment by joecool1029 | Hacker News Reader

Theoretically, maybe. If you got a good 'HD voice' (higher bitrate EVS codec) call, it's possible you could negotiate and send a slower fax signal. No way on a full-rate or half-rate GSM codec or amr-nb though, just doesn't have the bandwidth, chops too much.

Even back in the early 2000's when I was using free nights and weekends with my T68i to make calls to a free dialup provider in new england (9600baud yea), my call would still drop every hour or so just due to signal hiccups.

(T-Mobile never enabled HS-CSD for 28.8kbps using more timeslots and it's unlikely my current rate plan will allow for CSD calls, but it doesn't mean I won't try, I still have an old sim card able to be used in 2G only phones, including that very same t68i which I tested last summer, still works.)

View on HN

I thought VoIP codecs all chopped the frequencies that seems like it would be very detrimental to a data signal. Is the "HD" quality high enough for that, or was the analog data signal not as full frequency as I'm thinking they would be?

Most internet-based VoIP services (read: full telephone providers, not just chat providers) use the same exact G.711 codecs used in the digital portions of traditional telephone lines for normal calls.

In theory you can run anything over this that could run over a long distance PSTN link.

In practice most modem technologies do not handle jitter and packet loss well at all, and quirks which a human would not even notice will trip up a data call. It still works, I have clients running credit card machines on VoIP lines all day, but the longer the call and the higher the bandwidth the more likely something is to go wrong. A short 9600bps data exchange from a card machine is a lot easier than a 30 page 33.6k fax which is a lot easier than a 56k modem on a PC.

There's a special method called T.38 that allows VoIP providers with compatible endpoints to convert digital fax calls back to a data stream instead of audio which makes it a lot more tolerant of delay and jitter as well as slightly more tolerant of loss. Something similar could hypothetically be done for data modems but I haven't seen it.

v.150.1

You want 722 really, one of the choices for "HD voice", it mangles it much less than even 711

No, unlike G.711, G.722 is lossily compressed using psychoacoustic concepts, and modems wouldn’t know what to do with the extra acoustic frequencies anyway.

G.722 might sound better to humans, but G.711 is definitely better for modems since it’s effectively just uncompressed PCM (disregarding a bit of dynamic range compression which modems are generally fine with).

It is compressed, yes, but at higher bitrates it is actually usable for modems/faxes, you're limited to low baud rates anyway due to jitter and sample intervals. But really the benefit is 16k sampling rate instead of 8, I'm not saying it's great but it's the best we've got.

It would be possible to mask this with a relatively simple FXS device to hide all of this and pretend it's a modem while packetising the actual data, but I guess the demand is almost zero so why bother.

I've been looking for a solution for many years to retain dialin services without having racks full of modems and trunks that are rapidly going out of fashion and at some point will not be available except via IP (and now we have the same problem at both ends), but they just don't exist.

> really the benefit is 16k sampling rate instead of 8

Regular modems can't make use of that, since the actual phone lines they were designed for (whether analog or digital via G.711) didn't support more than 300-3400 kHz anyway, so anything beyond 8 kHz sampling (corresponding to a Nyquist frequency of 4 kHz) is wasted on them.

Maybe you could go crazy with a custom modem that knows how to exploit the additional high-frequency components of G.722 while dealing with the lossy/psychoacoustic compression across all bands, but I doubt it would yield any improvement over G.711:

> It is compressed, yes, but at higher bitrates it is actually usable for modems/faxes

No, they're both exactly 64 kbps (after compression/"compression"), so you wouldn't be able to fit any more signal in it from an information theoretical point of view.

Have we forgotten about T.38[1]? It is designed just to let analog fax machines/modems send documents over IP.

1: https://en.wikipedia.org/wiki/T.38

But only fax, and it's basically a software version of what I suggested above. It isn't actually a modem of any kind, it's basically the equivalent of sending a fax via sip messages. The closest we have is iaxmodem which does actually work for low baud faxes, but it's no DSP emulation for higher speeds, even if the line was good enough.

G.711 is what's used on the actual PSTN, so there's not even a hypothetical benefit from G.722 unless you're going direct over IP to another host supporting 722, and if you're doing that then you're probably better off with T.38 or just dropping the modem entirely for serial-over-IP.

Lots of people say faxing over VoIP just won't work, but at least for faxes with only a few pages and having a decent internet connection I've had excellent results.

I had a project using hylafax with about a dozen USB fax modems hooked up to a SIP gateway, connecting through a VPN back to the office where the PBX was and then out to our SIP trunk provider on G.711. It sent a lot of 2-10 page faxes. The system sent over a million faxes over its life, and excluding one receiver that tended to constantly have problems no matter what had an >95% success rate.

It really mostly comes down to jitter. On a low-jitter, low-packet-loss path and/or using large receive buffers (which adds delay, but that doesn't matter at all for faxes) fax is very possible.

So I looked into it (too late for me to edit), if you have a government account it might be possible to add CSDOPTION to your account for $9.99/mo. It's highly unlikely though, I found the old and current price schedules and this was removed from it.

Old gov contract expired 2022: https://www.gsaadvantage.gov/ref_text/GS35F0503M/0VX37Z.3RNG... (search for CSD, it's there)

New gov contract in effect (no CSD): https://www.gsaadvantage.gov/ref_text/47QTCA22D008N/0YTCAI.3...

EDIT: I tried to CSD dialup using my K850i which can handle my new and older SIM cards, no good. Simple Choice plans are way too new for this (the old plan I had in the early-mid 2000's was the Get More one)

Ive gotten 2400 and occasionally 9600 over audio voip. Ymmv.

Reply on news.ycombinator.com