GPRS and the APN authentication

Hi to anyone that can help.

i need to get to a TCP/IP server at a known IP address using a GPRS shield with a SIM900.
In the past (with other modems) i have always proceeded as:
set up the context & profile with AT+CGDCONT="IP","internet",,
then get to the APN with ATD99**1#
then others would deal with an authentication phase and link control phase

looking now with the GPRS shield documentation i do not see any mention of logging on to the APN and dealing with the problem of authentication & link controls.

My QUESTION:
where can i find information or source code on how to deal with that?

thanks in advance