what is a hardware inside the zigbee for generating RSSI signal?

I did one experiment in that there were two zigbees. zigbee1 and zigbee2. zigbee1 was receiver while zigbee2 was transmitter. i was monitoring the RSSI value on zigbee1 (i.e receiver) on pin no. 6 i.e pwm signal. for first time i kept both zigbees 1 feet apart from each other and monitored RSSI signal then i increased the distance between both zigbees by 1 meter still there were no change in RSSI signal. again i increased the distance now the distance is 4 meter still RSSI signal was same there were no little change. when the distance was greater than 6 meter there was little change in RSSI signal. why this is so?
why the RSSI signal is not deviating for distance between two zigbees is 1feet ,2feet, 3feet…and so on?

That is because you were flooding the front end of the product with a signal that was Too loud. The RSSI is only going to give you a valid level from the receive sensitivity to -60dBm. Anything louder it is not capable of reporting.

Then what should I do to get valid RSSI level
Please tell me in detail
Thank you

First off, at one and two foot distances, you are really not going to get valid RSSI levels from any RF enabled product. The output power from most devices is simply too much.

Additionally unless you are directly cabled or in a very quiet RF wise environment, you are never going to get the real RSSI level once you are within the devices ability to report it. In order to get accurate readings, you most likely would need to connect a spectrum analyzer between the radio and the antenna. Even then, you are still going to hear the background noise level as well.