Ti-15.4 Stack Collector - Security

Hi,

I am working with TI-15.4 stack to implement a low data rate star topology of sensors with a collector at the center. Going through the collector example, I see there is an option to enable / disable security using.

#define CONFIG_SECURE                true

Would appreciate if someone could clarify the below:

1. What exactly does security do (Does it encrypt the data sent between the sensors and the collectors) ?
2. What layer is security working on? (Does it only encrypt the sensor data being passed between the sensor & collector or does it actually encrypt at the radio layer ?

The reason I am asking this is to possibly turn it off to increase the number of sensor nodes that can be connected to the collector than the default 50, as suggested in this thread .

If security is only encrypting sensor data, then as suggested in this thread, I can turn it off and handle encryption in my application layer.