The RFID system relies on exchange of power and information between the reader and the tag. The reader provides power and communicates with the tag over the RF link then the tag communicates back to the reader with its data by backscattering the signal from the reader. If any aspect of the communication fails, the RFID link fails. Historically, powering the tag through the RF signal (forward link) has been the challenge. With recent improvement on tag sensitivity (ability to harvest power), the reverse link (backscattered signal detection) is starting to cause failures. This paper will describe the physics behind overall RFID link and describe how to determine where link failure can occur from hardware specifications.