The performance of wireless sensor networks (WSNs) with IEEE standard 802.15.4 at different types of devices can be improved with the appropriate utilization of guaranteed time slots (GTSs) in superframe at the media access control (MAC) layer. GTS implementation at the MAC layer enhances quality of service (QoS) by providing guarantee for delivery in time to the data collected at real time by the reduced functional device (RFD) from the surroundings to be transmitted to the fully functional device (FFD). Results prove that trade-off has to be done in implementing GTSs for various network performance parameters like queue size, queuing delay, CSMA medium access delay, CSMA transmission failures, and network output load to improve QoS.