Real-World Performance of Clear Channel Assessment in 802.15.4 Wireless Sensor Networks

There is a general belief that in CSMACA-based wireless sensor networks (WSN) a significant number of collisions happen only because of hidden terminals. The situation when nodes start transmissions almost at the same time is considered negligible. Our experiments with motes with CC2420 radio shows that a significant number of collisions can occur even when transmitters can hear each other. The CCA (Clear Channel Assessment) in the CC2420 is calculated on the basis of channel measurement over the last 0.128ms, and for this reason it may fail to detect a busy channel if transmissions started less than 0.128ms before CCA sampling. Our results indicate that during such a collisions only the packets from the node with the weaker signal are lost because of capture effects, in contrast to collisions that happen because of hidden terminals when both packets can be lost. It was also found that close to the noise floor, CCA thresholding allows one to reduce the amount of such collisions significantly.  Improvement for models of 802.15.4-compliant radios is suggested.