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There is sometimes a need to use a cell phone like an old-fashioned phone line with modems on both ends (see links -- I'm probably not the first one to half-bake this).
This software algorithm would push data over GSM's voice optimized Residual Pulse Excited LPC algorithm, and hopefully get somewhere
near 13 KB/Sec. The modem could be implemented in hardware (dedicated low-bandwidth GSM data device), over bluetooth, or using the sound card with something resembling an acoustic coupler.
The advantage would be a point-to-point data connection over any sort of GSM line (including pre-paid). This might be useful for remote data gathering, encryption, etc.
GSM Codec in detail
http://books.google...cited%20LPC&f=false
[cowtamer, Dec 27 2010]
Discussion of Crypto and LPC
http://books.google...cited%20lpc&f=false
[cowtamer, Dec 27 2010]
Good discussion of Speech Compression
http://www.cs.colum...4-speech-coding.pdf
(Powerpoint PDF file -- see last slide) [cowtamer, Dec 27 2010]
GSM Scrambler soft codec idea
GSM_20scrambler_20soft_20codec
Such a system (and similar systems) would need a way to reliably push data over GSM [cowtamer, Dec 27 2010]
Others looking for this
http://www.misterso...el-nbsp-459511.html
[cowtamer, Dec 27 2010]
Perhaps baked?
http://whitepapers....t.aspx?docid=155509
[cowtamer, Dec 27 2010]
Surfing the web on an 1964 Livermore Data Systems Model A Acoustic Coupler Modem
http://hackaday.com...odem-surfs-the-web/
[Spacecoyote, Dec 28 2010]
Echelon
http://en.wikipedia...als_intelligence%29
Trust no-one [8th of 7, Dec 28 2010]
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Ah - but it's circuit switched (n'est pas?). The data rate may be improved for the connection, but it will be operating on reserved bandwidth that isn't shared by other users. |
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Worst case scenario you could use a modified acoustic coupler (or couple through the headset adapter) with binary frequency shift keying. Its ancient, but it would work. [link] |
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HSCSD was around a decade ago and could cram 38 kBaud (19.2 kBaud/slot) into two GSM slots with ease. GPRS with Enhanced Gaussian Shif Keying (q.v.) does even better, which leaves your 13kBaud pretentions trailing in the dust. And that's without using sophisticated time-domain compression algorithms. |
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Sorry, [cowtamer], [marked-for-deletion], Baked. Nothing personal. |
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8th of 7, I am aware of the existence of GPRS/EDGE. The question is, can you use it without the phone company's blessing? There are many places in the world where you can't just buy any SIM card and use it for data -- I would like a system that can send data over _any_ audio connection, including GSM's heavy voice compression. To address Spacecoyote: as far as I know, you can't just use an acoustic coupler with a cell phone and expect it to work... |
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// use it without the phone company's blessing? // |
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// _any_ audio connection, including GSM's // |
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With your own CODEC, the service provider can't (and doesn't bother to) analyse the switched-circuit packet stream. You're constrained to the bandwith of your TDMA slot, but apart from that it's up to you. You don't have to flag the circuit as data; the bearer will just pass the bitstream blindly. |
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The whole "data" vs. "voice" thing is part of the call setup protocol and the MI/MT circuit negotiation. It's a convention, not an absolute. If you try to MI a data circuit to a voice receptor, the GSM system will deflect it, but if you MI as voice but pass data the stsrem switches it through without checking. It means a bit of firmware patching in both ends of the circuit, but it's not rocket science. |
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"They" (NSA, GCHQ, Echelon) don't want you to know this, by the way, as it makes secure calls all too easy to implement. |
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//To address Spacecoyote: as far as I know, you can't just use an acoustic coupler with a cell phone and expect it to work// |
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Besides the shape of the receiver potentially posing a problem (that can be easily fixed by modifying the coupler) I think one could expect reasonable data fidelity with BFSK as it only requires 2 different frequencies (exactly which frequencies can be changed) on each side for a total of 4 frequencies, also it only operates at 300 baud (or less) so the sounds shouldn't blend together. Its basically full duplex morse code, and as such is slow, but reliable. |
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Additionally, the Circuit Switched Data article on Wikipedia seems to imply that before CSD, people used regular modems attached to cell phones. Which would actually mean this is baked. |
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[marked-for-discretion] baked but obscure. |
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You don't need an accoustic coupler. Many cellphones have an earphone socket giving direct acces; the problem being that you're limited to about 3kHz audio bandwidth (with an LF filter too). |
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Yes, you can pass 300 baud using CCiTT tones, but surely you could do better things with your time, like knapping flints, consididering the possibilities that an agrarian rather than a hunter-gatherer lifestyle might be practicable, domesticating the flea, or even making marks on cave walls with a charred stick. |
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<LOUD VOICE AS IF ADDRESSING SLIGHTLY DEAF AND RETARDED CHILD> |
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IT'S JUST SOFTWARE. ONE GSM SLOT IN A FRAME IS A 19.2 kBaud VIRTUAL CIRCUIT. THE SERVICE PROVIDER DOESN'T CHECK. |
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Presumably, knowledge of the use of fire and the application of edged tools are not novelties to you, or do you still eat your meat raw and sleep up a tree ? |
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<Wanders off, muttering about re-inventing the wheel> |
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I'm presuming that since the OP is too cheap to spring for a "sanctioned" data connection (and too lazy to set up some kind of ad hoc optical wireless network or some such or write software), they aren't in any particular hurry to do whatever it is they want to do, they just want it to work, reliably, cheaply, and without a whole lot of effort. |
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