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I am using an embedded microprocessor to communicate with a SIM900 GPRS modem. The sim900 has an embedded tcp/ip stack but no embedded PPP stack, and my embedded environment has neither of these stacks.

With other devices I have followed these steps:

  1. at+cgdcont="IP","internet",,
  2. atd*99***1#
  3. called a PPP function that took take care of the login to the APN (and presumably the authentication & LCP etc).
  4. called a TCP function that connected to the remote server.

But now I have to do the PPP part myself. so I am at a loss locating all the necessary information.... can anyone point me in the right direction?

when I use hyperterm so as to see what happens with ATD*99***1# it connects and returns a lot of PPP frames which i suppose i have to write a module to deal with. But I did notice, on a number of sites, that the login to the APN is not mentioned at all, so is there a way of bypassing the login?

When I follow the example in the SIM900 docs, I get an error indicating (quite rightly) that the PDP is deactivated...

BenMorel
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moshejay
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  • You could try activating the PDP context manually with CGACT=1,1, also switching it manually to data with AT+CGDATA="PPP",1. I think these should automatically be done when you enter ATD*99***1. But possibly this relies on a PPP stack being present. – user1725145 Mar 02 '13 at 16:36
  • Yes...as soon as enter ATD*99***1# the presumption is that i have my own PPP & TCP stacks. I did do some research on this the past 2 days: So - If, on the other hand, i use the on-board TCP functionality..that will takes care of the PPP, the TCP, AND the APN login. So i suppose i have answered my own question! – moshejay Mar 04 '13 at 06:47

2 Answers2

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I think my original question was really 2 questions...

1) does one need to write an APN logon module when using the onboard TCP functions of the GPRS module? - the answer here is NO. The module takes care of that....see cmd AT+cstt="apn","user","pwd".

2) where can one find opensource code for a PPP stack?...that remains unanswered.

moshejay
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The 1.4.1 release and some older versions of LWIP have an implementation of PPP in it. Not certain if it can easily be used standalone though. You might still be able to decouple it from LWIP and use it.

ElvishPriestley
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