Wayne Holmes

Modeling the impact of deferred transmission in CSMA/CA algorithm of IEEE 802.15.4 for acknowledged and unacknowledged traffic

IEEE has introduced the 802.15.4 standard for a low rate, low power, low complexity and short range Wireless Personal Area Network (WPAN) in 2003. The basic access mechanism used by the standard is the Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). It has been noted that there is a problem of low throughput for short superframe duration due to the collision of the packets at the start of the superframe. Hence in 2006, the standard was revised and the CSMA/CA algorithm was modified such that deferred transmission was assigned with another delay of random backoff periods before assessing the medium. The objective of the present study is to develop an accurate analytical model to represent the actual CSMA/CA algorithm as reported in IEEE 802.15.4/2006. Additionally, the effects of deferments on the performance of CSMA/CA have been analyzed for acknowledged and unacknowledged data traffic, which has not been investigated yet. It has been found that the deferment of packets to a new superframe significantly affects the throughput for short frame durations. The effect is more pronounced for acknowledged data traffic as more collisions occur due to acknowledged packets and as a result, the node wastes more battery power by retransmitting the data. The collision can be avoided by assigning another delay with a random backoff period before assessing the medium. The results reported in this paper have further validated our simulation results for the deferred transmission.

Dielectric Properties of Wood for Improved Internal Imaging

This paper describes the measurement of the dielectric properties of the typical log features of Heartwood, Sapwood and internal Branches. Measurements were made using the waveguide cell technique covering the frequency range of 2.3 to 6.5GHz. This has shown that for the desired contrast between heartwood and Sapwood to be achieved the imaging system must operate above 4GHz. Additional work was undertaken to establish a method to correct these measurements for variations in basic density and Earlywood\Latewood banding, giving rise to a potential moisture content estimation error of 0.63%. This will lead to improved spatial location of features of interest.