Hey all! Thank you for your interests on this project. However, I built this super-simple library back in the mid-2016, when there is no other good library for interfacing with SIM900 GSM Module.
Now, we have TinyGSM Library which is better in every aspects. Go try it!
I'm deprecating this project, and will keep it as a reference only.
An arduino library for utilizing MQTT Client using SIM900 GPRS GSM Module