I'm a little bit confuse on how the transceiver perform energy scanning to select the best channel.
Does the term 'energy' refer to signal generated from other wireless devices such as WI-FI, Bluetooth and others. If yes, then how do we quantify and measure those noises because signal from WI-FI and other wireless technologies as it is already modulated with different modulation scheme.
The term energy really means received signal power at the receiver. However, the IEEE standard does not require that the measurements are physically accurate. Rather, it should be a rough indicator, called received signal strength indicator (RSSI).
So my answer is quite specific, but from the receivers I know (CC2420 and AT86RF230), the signal energy is determined after the antenna input is digitized and filtered. To find the value, the input magnitude of the signal is averaged over 128us. This means that a physical quantity independent of the modulation scheme is measured, but for very short in-band interference like Bluetooth or 802.11, the measurements may be way too low because of the averaging, and mis-detections can happen.
In tinyOS (OS for WSN motes) you can write code to measure RSSI value, for CC2420 radio it usually ranges from 0 to -100 dBm... Based on RSSI value we can select best channel for communication..
You can checkout the following link for RSSI demo performed in tinyOS
http://tinyos.stanford.edu/tinyos-wiki/index.php/Rssi_Demo
The closer the RSSI to 0 dBm, stronger the signal strength.
dBm values are reported relative to 1mW.