## 1 Introduction  
IEEE 802.15.4 [1,2] is a standard designed to address the needs of low power consumption, low data rate and low cost wireless networking applications. It has been applied in areas such as industrial monitoring, personal healthcare and residential automation. As the demand for real-time characteristics increases, quality of service (QoS) support becomes an important issue in these areas. Due to collisions, the mandatory medium access control (MAC) protocol provided by the IEEE 802.15.4 standard (contention-based CSMA/CA scheme) is not tailored to provide QoS support required by real-time applications, especially under high loads. The standard also provides an optional reservation-based Guaranteed Time Slot (GTS) scheme intended to support devices requiring dedicated bandwidth or low latency transmission. However, the active portion of the superframe is divided into only 16 slots, and since a device has to allocate at least a full GTS slot, bandwidth is wasted when slots are used partially. Moreover, only seven GTS allocations are allowed, according to the standard specifications in [1]. Some proposals have been presented in literature to increase bandwidth utilization of the GTS scheme. In [3], authors propose the use of the backoff period defined by the standard, which is smaller than the slot duration, as the allocation unit, in order to reduce the waste of bandwidth. In addition, the slot allocation information, carried by the beacon, is renewed at every superframe, allowing nodes to use different periodic cycles for dedicated transfer. However, the constant renewing of the allocation information coupled with the use of the backoff period (which requires more bits to encode the position and length of the allocation) tend to increase the beacon length significantly. This leads to an increase in the beacon's vulnerability to channel errors, thus reducing the packet delivery ratio (allocated transmissions cannot be performed when the beacon is lost). The i-GAME mechanism proposed in [4] improves bandwidth utilization by allowing several nodes to share the same set of GTS slots. However, the method used by the PAN coordinator to enforce the sharing order of the GTS slots among the nodes remains unanswered. Moreover, this mechanism also requires the requesting nodes to identify traffic specifications and delay requirements of its flows, which tends to increase the complexity of the implementation. The scheme proposed in [5] divides the Contention Free Period (CFP) into 16 equally sized slots, in contrast with the division of the whole superframe as in GTS. This scheme is able to keep the format of the GTS descriptor intact, changing only the way devices interpret the GTS allocation fields. One drawback of this approach is that the slot duration becomes dependent of the CFP length, and therefore current allocations become inconsistent when the length of the CFP needs to change in order to accept new allocations. Moreover, the number of slots provided remains small. The eLPRT protocol described in this paper presents several features designed to enhance the performance in comparison to the GTS scheme of IEEE 802.15.4. Besides improving bandwidth utilization and increasing the number of supported devices, the eLPRT protocol introduces various mechanisms designed to increase the packet delivery ratio in the presence of channel errors. Most of the proposed features are independent of each other and can be adopted separately in the scope of future versions of the IEEE 802.15.4. The eLPRT protocol was implemented and tested in the development context of a motion capture system based on multiple wireless sensor nodes that integrate inertial and magnetic sensors [6,7]. The system presents a modular and versatile architecture where the wireless sensor nodes are placed only inthe user's body segments that need to be monitored.