Couldyou please elaborate your answer a little bit?

Thanks for the answer. But there are still some things I cant understand.

1. How can the modem recognise a AT-command in transparent mode?
  There is no way to distinguish it from any other text-string on its way to the receiver.
  The serial stream could very well contain AT+..…

2. I have tried all sort of AT-commands and none of them are accepted.
 They are simply sent transparently to the receiver the 
 

3 The modem listen to a few AT-commands just after Reset. But after a couple of seconds it enters transparent mode. Please explain the start up procedure.

Sorry to say. There is certainly something I have missed!
/Per

 Per.Svensson 

Thanks for the answer. But there are still some things I cant understand.

1. How can the modem recognise a AT-command in transparent mode?
  There is no way to distinguish it from any other text-string on its way to the receiver.
  The serial stream could very well contain AT+..…

2. I have tried all sort of AT-commands and none of them are accepted.
 They are simply sent transparently to the receiver the 
 

3 The modem listen to a few AT-commands just after Reset. But after a couple of seconds it enters transparent mode. Please explain the start up procedure.

Sorry to say. There is certainly something I have missed!
/Per