IEEE 802.15.4 protocol is designed for low-rate Wireless Personal Area Networks with very low cost and power consumption. IEEE 802.15.4 provides Guaranteed Time Slots (GTSs) which allow a device to operate on the channel within a portion of the superframe that is dedicated exclusively to that device. The GTSs always appear at the end of the active superframe starting at a slot boundary immediately following the Contention Access Period (CAP). The Personal Area Network (PAN) coordinator may

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... ate up to seven of these GTSs and a GTS can occupy more than one slot period. This paper has investigated the impact of varying the number of slots allocated for GTS on the IEEE 802.15.4 protocol performance on the radio level. We have observed the relationship between the interference in the radio channel and the number of slots allocated for GTS. Additionally, the measurements have been implemented when we are having a wireless channel that exhibits path loss variation versus one that it does not. The results have revealed that the IEEE 802.15.4 protocol is significantly affected by the variation of the GTS and the wireless environment. Better performance is achieved when the number of slots allocated for GTS equals three where the number of packet failed to be received due to the interference was at its lowest value. We have carried out our experiments by using Omnet++ and Castalia simulation packages.