I wonder if you could hold up your smart phone to a landline handset like an acoustic coupler to send a fax?
Ask HN: Why can't I send faxes from my phone?
41–50 of 168 posts
Re: Ask HN: Why can't I send faxes from my phone?
#42This used to be possible over GSM, at least in the UK, and so I assume most places that use(d) GSM. I did it from various generations of the Nokia Communicator. The phone would initiate a special kind of phone call (from memory, the same AT string used to start an ISDN connection) and then send digital data over the link, not audio tones directly. This would emerge onto the analogue phone network as fax tones. I supp…
This was called circuit-switched data (CSD). The handset would emulate a serial modem to the attached computer and the mobile network would know how to emit the correct "noises" to the far end of the connection.
It was brilliant for getting into systems I managed via a dial-in modem before Internet access was broadly available. In its twilight years, carriers that had a tariff to support it would charge something like $1.35/minute to discourage its use until the tariff could be updated.
Re: Ask HN: Why can't I send faxes from my phone?
#43I give up and tell them I can't send a fax where I'm from. When they ask where I'm from, I tell them I'm from the year 2020.
Re: Ask HN: Why can't I send faxes from my phone?
#44How about a service that can grab a document out of your Dropbox or other cloud account, then fax it for you? $10 for 50 pages could result in hella margins, if it's done right. (Everything done digitally, of course.)
The account could even be managed from a phone app.
Re: Ask HN: Why can't I send faxes from my phone?
#45The fax requires a 56KHz channel which has not been used for mobile telephony since the 1980s (when AMPS and GSM were developed. Before then it was just a radio link through an operator). That’s why you can’t use a high speed modem either. Some gsm phones had the ability to transmit a fax but they actually used a different protocol to talk to a fax gateway. I never saw one of these in the wild. I haven’t been in that…
>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 engi…
> 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.)
Re: Ask HN: Why can't I send faxes from my phone?
#46The fax requires a 56KHz channel which has not been used for mobile telephony since the 1980s (when AMPS and GSM were developed. Before then it was just a radio link through an operator). That’s why you can’t use a high speed modem either. Some gsm phones had the ability to transmit a fax but they actually used a different protocol to talk to a fax gateway. I never saw one of these in the wild. I haven’t been in that…
>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 engi…
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.)
Re: Ask HN: Why can't I send faxes from my phone?
#47Earlier quoted context omitted.
>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 engi…
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.
Re: Ask HN: Why can't I send faxes from my phone?
#48You could when your tablet was a Newton.
Re: Ask HN: Why can't I send faxes from my phone?
#49Faxes rely on modems. modems rely on precise timing and aren't able to cope with any slippage.
There's actually a protocol, T.38, which is used on modern VoIP based systems to allow faxes to be transmitted. At each end it actually decodes the fax data and sends it as ordinary data packets. Implementations play some tricks on the fax modem at each end, in order to prevent it dropping out of sync if the data packets are delayed. You could communicate with one of these from phone, but you'd need a T.38 gateway a the other end.
More practical is to cut out the middleman and use twilio's fax gateway at $0.01/page (+ sip trunking cost, which should be similarly trivial): https://www.twilio.com/fax/pricing
Re: Ask HN: Why can't I send faxes from my phone?
#50Earlier quoted context omitted.
Considering a fax is a print, I'm not sure what the printout comment is regarding. The presence and non-exclusion of email/IP gateways show how backwards the law surrounding this is.
The law is not based on reality. If I sent you a fax, and your fax machine (of whatever type) printed it out, that copy is legally as good as the original. If I scan, then email, and you print it out, it is not legally as good as the original. It doesn't matter if it makes sense, it is what it is.
I wouldn’t be quite so hard on the law here.
The law is based on precedent.
Fax was a new technology. For the first time a signed document could be sent over the wire in minutes, instead of having a courier go via train and take days. This is 100 years before computers were common.
When computers became common fax was already well established so there was no good reason to add a second exception.