Data Call using wavecom modems

b.prashant@gmail.com wrote:

> I have some questions regarding wavecom GSM modems. I am right
> now involved in a testing scenario where I have to connect two
> wavecom modems through GSM data link (not GPRS). I have two
> prepaid GSM sim cards for the two modems. I am using hyper
> terminal to communicate with the modem through serial port.
> My testing scenario requires that on one (PC) end I get data
> into one serial port and output it to the other serial port
> (i.e. where the modem is connected). The other end is the
> second modem (second PC) which would receive the data. This is
> the setup.
>
> I have two problems
> 1. I make a data call from one modem to the other. I get the
> rining notification on the other end. But when i answer the
> call it shows that there is no carrier and the call gets
> disconnected

It's almost certain that the reason for the immediate
disconnection is that you're answering a voice call, and not a
data call.

There are several complex issues here.

Firstly, less carriers are offering circuit-switched-data (CSD)
service. Of those which do, some don't allow prepaid SIMs to
initiate CSD calls.

Secondly, are you dialling the receiving modem's voice number or
its data number? If you're dialling its voice number, you've
almost certainly established a voice call. If both SIMs are
with the same carrier, it's just possible (but extremely
unlikely) that they'll let you make a CSD call to a voice
number. If they do allow it, you'll increase your chances of
success by selecting the V.110 ISDN bearer service with

AT+CBST=71

prior to dialling (if your carrier supports it).

In general, GSM voice and data calls have very different
characteristics. The type of call must be determined for both
OTA legs prior to call establishment. The standard mechanism
for establishing a CSD circuit involves two aspects: using the
semi-colon after the destination number to specify CSD on the
calling leg, and dialling a _data_ number to specify CSD on the
receiving leg.

The provision of a data number is usually a separately-priced
option that's not available to users of prepaid SIMs.

> 2. I am not sure how to make the data transfer
> once the call is established ( i.e. on the first PC, i have
> data in COM1 port and the modem is connected to the COM2
> port). Is there a way i can route the data to com2 port once
> the data link is establihed?

On connection, you'll automatically be in data transfer mode.
Characters you type subsequently will appear on the other
device's serial port. This happens in each direction once a
connection is made.

You can then escape to command mode at any time by using the
correct sequence. By default, this is the three-character
string "+++". Because this string could legitimately occur
within data being transmitted, some timing parameters need to
be correct as well. The three "+" characters must be close
together in time, but separated from any other data by a pause
of a second or so (the "guard time"). You'd normally escape to
command mode prior to terminating the call with a hangup
command like "ATH" or "AT+CHUP".

To pass from command mode back to data transfer mode, the
command is

ATO

(that's the letter "O", not the number zero).

John