[mdlug] It happened...

[Joseph C. Bender]

Joe Doehler wrote:
> ...
> I have experience using a Siemens S46 and a Motorola V3 tethered to a 
> laptop via USB. These are both GSM phones that act as digital modems. 
> The first thing I found out is that my carrier blocked DUN: I can't 
> dial up to an independent ISP. I believe that this is universal; if 
> someone has a different experience, please share it with us.
> 
	This is because dialing ATDT*99 (or #777 on CMDA) creates a virtual PPP 
connection via termination cards at the MTSO.  Dialing an ISP as a modem 
actually requires that the phone support phone-as-modem mode and that 
the carrier have *other* cards at the MTSO that are MODEM banks that 
actually place the modem call.  A lot of carriers do not implement this, 
it's terribly expensive to them for not much gain.  T-Mobile is the only 
GSM carrier that I'm aware of that does this, and I last used it back in 
2005 or so.  They implemented it largely because they do have a plan 
bolt-on that allows for fax send/receive, and that requires the same 
modem cards at the MTSO.  You're going to get no more than 9600 bps, 
though.  That's the most bandwidth that you can stuff onto a GSM call. 
Oh, and it counts as a regular phonecall, so it'll be burning minutes 
off your plan, too.

(MTSO being mobile telephone switching office, the local cell-switch 
where your calls actually hit the telephone network and terminate from)

> Setting up these phones as digital modems was a nightmare. The key is 
> in setting the proper modem init string. Customer support was of no 
> help, and the provider's web site did not have it anywhere. Another 
> aggravation is that the utilities provided by Motorola reset the init 
> string in the laptop's configuration files to NULL.
> 
	The big issue is the APN string.  This is a AT command (AT+CGDCONT=, 
IIRC) that sets the mobile access point address so your phone knows 
where to throw a GPRS/EDGE connection at.  Googling for "<carrier name> 
APN string" usually provides the required information.

> Both of my phones use GPRS for data transfer. Technically, I should 
> be able to get ~180Kb/s on good days, when the bandwidth is not 
> shared with other users. The most I have ever seen with my V3 is a 
> tad over 30Kb/s (about the speed of a 28.8 analog modem;) with the 
> S46, about half that. SSH and SCP connections routinely fail after 
> about 100KBytes of transfer, but they are getting more reliable with time.
> 
	It depends on how busy the local tower is, as GPRS shares timeslots 
with the voice calls.

	If you're supposed to be getting 180Kb/s, chances are you have a EDGE 
phone anyway.  28.8 tends to suggest that you've either got a really old 
GPRS phone or that the local tower is horrible.  If the phones don't 
support GPRS class 10 or better, you're not going to see the speeds.

	Out of curiosity, what carrier and what area are you trying this in?

> EDGE over GSM is supposed to be faster, but the phone selection 
> narrows. I am not sure how much of a speed advantage EDGE has over 
> GPRS in real life: I am not getting the full promise of GPRS, so why 
> would I with EDGE?
> 
	Latency goes down drastically on EDGE, and I have seen decent 
performance with it.  The modulations/encodations are vastly different, 
you get more bits per GSM timeslot with EDGE.  The tower can also hand 
you more timeslots, or configure them better, theoretically as well.  It 
does largely depend on the phone, yes.  Phonescoop.com will give the 
specs of a given phone.

> I would not recommend this setup for primary Internet access - only 
> for "plan B" if you need it :-)
> 
	Indeed.  If you want cellular wireless as primary, get EVDO on a CDMA 
carrier.  Until the GSM carriers get UMTS or HSDPA rolled out, it's 
going to be slow.  Why Apple made the utterly, utterly stupid decision 
not to make a CDMA version of the iPhone, I do not know).



-- 
Joseph Bender
Bendorius Consulting
P: 248-434-5580
F: 248-434-5581
jcbender at bendorius com