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In wireless personal area networks such as wireless body area sensor networks, stations or devices have different bandwidth requirements and thus create heterogeneous traffic. For such networks, the IEEE 802.15.4 MAC can be used in the beacon-enabled mode which supports guaranteed time slot (GTS) allocation for time-critical data transmissions. This paper presents a general discrete-time Markov chain model for the IEEE 802.15.4-based networks taking into account the slotted CSMA/CA and GTS transmission phenomena together in heterogeneous traffic scenario and non-saturated condition. For this purpose, the standard GTS allocation scheme is modified. For each non-identical device, the Markov model is solved and the average service time and the service utilization factor are analyzed in the non-saturation mode. The utilization factor is higher for the higher data rate devices. The GTS transmission scheme also results in higher utilization for the devices. The analysis is validated by simulations using Network Simulator (NS) version 2.33.