How to measure LQI in Digi Mesg

I am using one coordinator , 1 router and 3 end device. How to measure the LQI in the Digi mesh for individual device. Weather its calculating based on the signal strength of the zigbee device?
i want to know about there LQI request and frame format description of the LQI request and response.

I will be grateful for any help you can provide.