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Comment by throwlaplace | Hacker News Reader

>The fax requires a 56KHz channel

hmm so you're basically saying that fax machines only listen to data coming across on a 56KHz carrier wave? i guess that makes sense since the same device (a fax machine) can function both as a telephone and fax (and you'd need to distinguish in hardware which mode you were operating in upon "picking up the phone").

but what i don't understand is the telephony (i'm not a network engineer): when i make a phone call from either a fax or a phone isn't there, at this point in time, a hop through a digital connection? so doesn't some interchange between me and the person i'm calling ultimately decide what frequency carrier the receiver hears?

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Yes, once your call gets to the central office it's moved onto a digital connection -- essentially an ISDN line.

Notably, the entire telephone network is standardized on G.711 as a codec, which is raw PCM audio at 64 kbps, from 300–3400 Hz sampled at 8,000 samples/sec. (There's some regional differences that determine whether the network uses G.711u or G.711a, but that's not really important here.)

Also relevant: The digital telephone network is exclusively a switched (not packet-based) network, so you're guaranteed near-zero latency. This is particularly important for fax modems, which don't tolerate any jitter.

The end result is a connection that might not sound the best (due to the narrow bandwidth), but has call quality characteristics that are standardized and guaranteed across the network.

(At some point in the future the telephone network will upgrade codecs to G.722 and AMR-WB, which will give higher voice quality. This is the "HD Voice" feature you see on VoLTE phones. But it'll be a while before that's universal.)

>The digital telephone network is exclusively a switched (not packet-based) network, so you're guaranteed near-zero latency.

Seems like this fact could be exploited more often in internet applications. High bandwidth channel for static assets, low latency channel for e.g. player movement data?

One of the professors when I was in a university was from ex-national telco R&D, and he was very vocal about those “real” bandwidth/latency/privacy assured telecom techs, “far superior” than “toys” like TCP/IP.

And that was for the sake of education. He’d hold his lab meetings in Skype, not over a multipoint ISDN video calls on circuit switched wireless ATM. Only when the show is on, speaking to us undergrads through the mic, he declined to admit that IP is more adopted.

Circuit switched data looks pretty on paper, that’s true, but at the end of the day no one pays for or bother with it.

Low jitter is a luxury for the document web, but if approximately all human social interaction is going to be through digital video for the foreseeable future, we might start to care...

It is already used with QoS [1]

[1] https://en.m.wikipedia.org/wiki/Quality_of_service

Which is traffic prioritization on the same channel. Having an extra link for high-priority traffic would be...exotic...but workable, sure.

It isn't a 56kHz channel. It is a 56k bps channel encoded as complex tones onto a 4kHz voice connection. The telephone system wiring does not have the bandwidth to carry a signal that is 2-3x the range of human hearing.

> so doesn't some interchange between me and the person i'm calling ultimately decide what frequency carrier the receiver hears?

Probably it's an interchange between your POTS handset and your local phone service provider's head-end, that determines the "media type" of the call; and then, in the case of fax, that call is forwarded (including media type) to the other land-line's service provider's head-end, where the service provider then negotiates+accepts the media type, rings the fax on the line until off-hook, and then sets the opened line to operate at that frequency.

I'm guessing that the thing picking up the carrier on the sender's head-end is the same thing that parses DTMF tones off the line; and therefore, not a component that exists in the signal chain of cellular service providers.

(A similar thing happens in cellular voice signalling—the negotiation of a voice codec—but it happens out-of-band, not in-band, AFAIK—on both ends (like a web browser talking to a web server) unless one end is POTS, in which case the cellular voice provider jumps to the POTS network with a DAC+bank of line couplers. In theory, this means that a cell phone might be able to send a fax to a fax machine, if there was a special voice codec option in GSM/CDMA/VoLTE/etc. for "lossless 56KHz PCM", that would be decoded at the POTS gateway into a raw line signal.)

Copper wire has a lot of bandwidth, but traditional phone lines had low-pass filter at 4KHz, so anything above is cut off. Between switching stations signal is modulated and multiplexed, but on the receiving end it is again restored to the original analog signal. Therefore fax-machines couldn't possibly expect anything on 56KHz for sure, it would never get to them. Also, faxes make that well-known noise when establishing connection, and if it were around 56KHz, we wouldn't be able to hear it - anything above 15KHz is very hard to hear for adults. So the OP probably meant 56kbps as a transmission rate, which again is not true because faxes existed much before 56K was possible and many worked on as low rates as 2400bps.

I seem to recall some occasions in the early 90's where there was exactly this problem and fax was not guaranteed to work with some land line due to the digital nature.

In the early days of running "analog" telephony over digital circuits, the edges of the audible hearing range would get clipped and compression would muddy the sharpness of the tones used. Humans couldn't tell much of a difference but devices cared a great deal. Nowadays you can run a fax over specific codecs designed to not do this (G.729, I think? Maybe G.711, it's been a few years).

We would also pull off a clever trick with the reverse. If the modem sensed a digital carrier and your ISP was digitally-connected to your telephone company, the connection could be put into a sort of digital-hybrid mode where the downlink of the connection would be 56kbps (the upstream would still be 28.8kbps maximum). This is where US Robotics "X2" modem names came from.

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