a new BBS...

Joachim Thiemann joachim.thiemann at gmail.com
Sun Sep 21 10:18:55 CDT 2008


On Sun, Sep 21, 2008 at 3:38 AM, Gordon JC Pearce <gordonjcp at gjcp.net> wrote:
> David Griffith wrote:
>
>> Now as soon as I can figure out how to do POTS modem dialing through a
>> cell phone, I'll call.
>
> Have a look in the AT command spec.  IIRC just sending ATD<number><CRLF>
> should set up a call as an ISDN call.  I'm *fairly* sure you can dial
> ordinary dialup modems with them, but I haven't got one (landline/dialup
> modem pair) to play.

Depends on carrier.  IIRC (someone feel free to correct me), most
modern phones that have serial USB or Bluetooth profiles (most Nokias
and Motorolas do) support CSD (Circuit Switched Data), so serial data
is not voice-band modulated at the phone, but sent to the carrier
digitally, then modulated at their switch.

Unfortunately many carriers have disabled this.  They would much
rather have you use the IP data link you phone is likely to give you.
(Not sure why - these days it would mean more revenue for them. For me
though it would be cheaper: I pay .20c/min for voice, but 5c/kB data
on prepaid.  So at 9600bps CSD is cheaper)  I think it's simply too
much hassle to maintain it for very limited (that is, almost none)
customer demand.

Hooking an analog modem to a (digital) cellphone is very unlikely to
work, the voice codec will mess up the data in very nonlinear ways
that modems were never designed for.  A simple QPSK low-baud signal
might make it, but anything that negotiates for channel conditions
beforehand will have error rates that the codes can't deal with.
300bps could probably work, 1200 maaaaaaybe. 2400 I would find very
surprising.  GSM might get better results than CDMA due to noise
suppression that is mandatory in the latter.

If you still have a TDMA phone and analog service, you're better off,
but those towers are being switched off already.

If you're willing to do some custom protocol hackery, all phones
recognize DTMF :-) and transmit it as side data; however baud rate
(key rate) would be quite slow (due to DTMF specs) and you'd only get
about 3.5 bits/symbol.

So if you want classic computers to communicate, you need to connect
to classic telephony equipment.  I don't see this as a bad thing, just
the passage of time.

Joe.
(Anyone else ever had the ability to whistle 1200 bps fax
initiators??? Probably the least useful thing I learned at an Ottawa
telecom equipment company... no not THAT one... the OTHER one that
went bust :-)



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