Help dialing a GSM Modem for a data calls

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David - 02 Feb 2005 20:51 GMT

Hi,
I have a GSM modem (Wavecom Farge Maestro 20) that I am trying to use
with a Linux server running PPP. I want to be able to "dial-in" to my
Linux server to check its status, etc. The problem I am having though
is with the GSM Modem itself, rather then the computer side of things.

Whilst I can sucesfully use HyperTerminal and issue an ATDT command to
ring my ISP from the GSM Modem, I am unable to make a data call to the
GSM Modem. I have confirmed that the Sim card has the data service
enabled and I am using it's data number to call it. I have also
confirmed this by issuing the command "AT+CRC=1" and when the GSM
modem recieves a data call HyperTerminal displays "+CRING: REL ASYNC".
When I then type "ATA" to answer the call nothing happens and after a
few seconds the message "NO CARRIER" appears and the call ends.

Also, when I tried ringing the GSM Modem for a land-line phone instead
of hearing the usual whistling noises, all I heard was a loud buzzing
noise.

I habe also tried swapping the GSM Modem for a regular mobile phone
with the data service enabled on it and had exactly the same problem,
so it does not appear to a problem with the specific device.

It would seem like I need to issue a "magic" AT command to the modem
to make it answer the data call properlly, but nowhere can I find what
this command might be. As I said earlier, I can quite happily get the
GSM Modem to ring my ISP, but I cannot establish a data call when I
ring the GSM Modem.

What am I missing?

- David Bull

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John Henderson - 02 Feb 2005 23:41 GMT

> Hi,
> I have a GSM modem (Wavecom Farge Maestro 20) that I am trying
[quoted text clipped - 30 lines]
>
> What am I missing?

What's the state of the serial cabling? Is the DCE (modem)
seeing a raised DTR (Data Terminal Ready) signal from the DTE
(terminal/computer)? DTR is implemented on pin 20 of a 25-pin
RS232 connector, and pin 4 on a 9-pin). Dropping DTR is a
standard method for hanging up a call.

"ATA" should answer a data call. An alternative is to activate
auto-answer after a certain number of rings. This is done by
setting the S0 register to a value between 1 and 255. The
usual default value is zero (disable auto-answer). To answer
after 2 rings, the command is "ATS0=2" (where "0" is a zero).

John

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David - 03 Feb 2005 22:38 GMT

> > Hi,
> > I have a GSM modem (Wavecom Farge Maestro 20) that I am trying
[quoted text clipped - 44 lines]
>
> John

I don't know about the cabling - I just have the serial cables that
came with both the GSM modem and the mobile phone plugged into the two
serial ports on the back of my computer.

I can quite happily ring a regular ISP and see "CONNECTED 9600" in
hyperterminal, but I cannot get the GSM Modem to establish a data
connection with the mobile phone or vice-versa.

I had tried playing with the ATS auto answer command as well as
answering the calls manually by typing ATA, but that made no
difference.

- David

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John Henderson - 03 Feb 2005 23:18 GMT

> I don't know about the cabling - I just have the serial cables
> that came with both the GSM modem and the mobile phone plugged
[quoted text clipped - 8 lines]
> as answering the calls manually by typing ATA, but that made
> no difference.

That cabling should be OK. Hopefully, the computer is raising
DTR, as this makes things simpler.

Maybe I'll get some time tonight to try some things with a data
call to my Wavecom Fastrack modem. The last time I tried a
data call it was to a Siemens S55 phone, and that worked first
up with "ATA".

There's more things to try, but I need to do some thinking and
research. The next thing that springs to mind though is
data/fax mode. Is "AT+FCLASS" set to zero (from memory, check
value).

John

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John Henderson - 04 Feb 2005 19:50 GMT

Earlier, I wrote:

> Maybe I'll get some time tonight to try some things with a
> data call to my Wavecom Fastrack modem. The last time I tried
> a data call it was to a Siemens S55 phone, and that worked
> first up with "ATA".

I tried a couple of data calls from a Siemens S55 phone to a
Wavecom Fastrack modem, and the modem answered with "ATA".
Setting up auto-answer with "ATS0=2" also worked fine. I used
the Linux Minicom program as the terminal emulator, and was
able to pass text in each direction on the connections.

One thing that's different from your setup is that I don't have
a data number. Telstra Australia allows V.110 data calls to
voice numbers to be handled as data calls, so I used a V.110
ISDN digital connection instead of the default V.32 analog
(which results in a voice call in my situation).

This shouldn't make any difference to the call answering
behaviour, But it may be worth your trying it to see if your
carrier supports it. The main advantage is the much quicker
call setup time - about 8 seconds instead of about 30. To make
a 9600 bps V.110 call, set the bearer service on the
originating GSM device with "AT+CBST=71".

Check your existing value first with "AT+CBST?". It (the first
argument) is probably set to zero (autobaud) or 7 (9600 bps
V.32).

John

Reply to this Message

David - 07 Feb 2005 11:15 GMT

> Earlier, I wrote:
>
[quoted text clipped - 27 lines]
>
> John

Hi,

>Is "AT+FCLASS" set to zero

Yes it was.

>"AT&D0" ("0" is a zero) tells the modem to
>completely ignore the DTR line status.

My friend, who I am posting these messages on bahalf of, told me that
this command "didn't work". I'm not sure whether he means the modem
didn't understand the command or whether it made no difference.

We also tried calling the Modem's voice number but as you can plug a
handset into it to do voice calls it didn't work. If you call the
modem's data number from a PSTN line then you hear can hear "buzzing"
which is presumably the data, but when you call the voice number you
hear nothing as we don't have a handset to connect to the modem but
the modem doesnt hang the call up after a few seconds.

This modem was needed for a server that was being deployed last friday
so it could be accessed remotely. However as we could not get it to
work in time (it didn't help that we only had a few days notice!),
another solution has been used. We may have another look at getting
the GSM modem to wok again in the future though.

Thanks very much for you help.

- David

Reply to this Message

John Henderson - 07 Feb 2005 20:13 GMT

> This modem was needed for a server that was being deployed
> last friday so it could be accessed remotely. However as we
[quoted text clipped - 4 lines]
>
> Thanks very much for you help.

You're welcome, but I'm sorry we didn't get it working in time.

These things are difficult to diagnose at long distance. More
so by print than by voice, and with added complexity (and scope
for hidden assumptions) when there's an intermediary involved.

These data-capable GSM devices usually work "straight out of the
box", with maybe 1 or 2 things to tweak.

I'm still unsure whether to think the "ATA" might have answered
the call or not. As I alluded in a previous post, I'd expect
30 seconds or so to elapse from answering to a "CONNECT"
response on a V.32 circuit. Within that time, negotiation
starts almost immediately, and the call could terminate fairly
quickly if something's amiss. I'd expect this to happen, for
example, if the call was made from a PSTN handset (not a modem)
to a GSM data number. That would still be a data call (because
you dialled the data number), but it wouldn't be a data-capable
call, and would be recognized as such some time after
answering.

Equally, negotiation would fail if one of the modems had some
idiosyncratic setting that wasn't supported by the answering
modem or the network. Despite the call being answered, you'd
see "NO CARRIER" rather than "CONNECT".

So the initialization string used on the calling modem would be
useful information to have. The possibilities are still open
in my mind - from flawed testing methodology to unusual
settings or environment.

John

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John Henderson - 04 Feb 2005 03:20 GMT

> I had tried playing with the ATS auto answer command as well
> as answering the calls manually by typing ATA, but that made
> no difference.

For testing purposes, it's worth taking DTR out of the equation
completely. "AT&D0" ("0" is a zero) tells the modem to
completely ignore the DTR line status.

John

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Kevbert - 18 Jun 2005 21:04 GMT

David Wrote:

> Hi,
> I have a GSM modem (Wavecom Farge Maestro 20) that I am trying to use
[quoted text clipped - 28 lines]
>
> - David Bull

Hi,
Data calls can be very tempremental. Try calling one GSM data numbe
with another mobile. The most important thing is that both modems ar
running at the same rate, for 9600 use
at+cbst=7,0,1
at both ends. The other end should answer with ata but if the mode
is it data mode as opposed to command mode you may need to enter
+++at
at the receiving end. You'll find that any speeds slower than thi
will drop the line.
Regards
Kevbert

--
Kevbert

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