Problem with return to online data mode (ATO)


Hi all,
I am facing a problem while using ATO to return to online data mode.
When I am attached to GPRS by ATD99**1# and I need to go to command mode I am using "+++"¨. “OK” is returned (and this is ok for me too:). But! After a while “NO CARRIER” response appears and when I’ll try to go back to online data mode with “ATO” I’ll receive “ERROR”.
I cannot understand why.
Do you have any idea why is it so?
Thanks in advance and peace.


Hi Sendog,
When you give ATD99**1# command, the module goes to data mode. The module also sends the PPP frames to the external application so that a PPP session can be established between the module and the external application (like hyperterminal or some other application like windows dialup connection application).In case, you do not provide any data to the module, it will give a NO CARRIER indication and will move out of data mode.

In your case, you have issued ATD99**1# command and have switched the module to AT mode (using +++). In this case, the module is not receiving any response from you (in form of PPP packets). Hence, after some time it gives you a NO CARRIER indication and stops the attempts to make a session. Now if you give an ATO command it would not go to data mode (online mode) as there is no session going on (PPP session).

If you proceed like:

  1. ATD99**1#
  2. +++
  3. ATO (It will work fine as the module is still in online mode).
    But if you proceed like:
  4. ATD99**1#
  5. +++
  6. NO CARRIER (wait for)
  7. ATO( It will not move you back to data mode as the module has stopped the PPP establishement tries).

Hope this solves your problems.

Best Regards,
Open AT Fan.


Hi OpenAT_Fan,

thanks for your reply.
Yes, your observation is logicaly correct. But this “NO CARRIER” problem also occures after the PPP negotiation is completed i.e. module is not expecting any more PPP packets from me (LCP, IPCP is completed). It is all fine with different modem vendors.
The way you said it may works fine, it’s not exactly true:

  1. ATD99**1#
  2. +++
  3. ATO (It will work fine as the module is still in online mode).
    No, AT returns ERROR and after a while NO CARRIER occures.

It is deeper, deeper than I thought.

peace, sendog