Markov Model of Modified Unslotted CSMA/CA for Wireless Sensor Networks

IEEE 802.15.4 protocol is intended to accomplish the characteristics of low power and low rate Wireless Sensor Network (WSN). The IEEE 802.15.4 employs carrier sense multiple access with collision avoidance (CSMA/CA) algorithm to access the network. It regulates the channel accessing by two modes: slotted and unslotted CSMA/CA. This paper focuses on the unslotted CSMA/CA where the node tends to wait for a very restricted backoff exponent (BE) value. Therefore the collision probability that two nodes or more select a similar backoff period value is high. The nodes cannot perform clear channel assessment (CCA) until backoff procedure is completed, so the average delay of packet transmission is high. In this paper, a modified unslotted CSMA/CA is proposed to enhance the unslotted CSMA/CA performance based on new backoff procedure. This procedure divides the backoff delay into Temporary Backoff (TB) and Next Temporary Backoff (NTB). The Markov model of the modified unslotted CSMA/CA is presented to assess a single hop IEEE 802.15.4 behavior. This Markov model is applied to derive expressions for average delay, reliability and energy consumption metrics. OPNET simulation tool is used to compare the modified unslotted CSMA to the standard unslotted CSMA. It is also used to validate the Markov model proposed. The results demonstrate that the modified CSMA scheme improves the reliability, while reducing the average delay significantly. The metrics derived from the proposed model is very close to the metrics measured from the network simulated by OPNET.