## 3 The eLPRT Protocol  
The eLPRT is an enhanced version of the LPRT protocol described in [9], and therefore it inherits most of the characteristics of the latter. The eLPRT introduces new mechanisms to improve the performance and uses protocol message formats similar to the ones used by the IEEE 802.15.4. In the original LPRT protocol, the allocation information announced in the beacon frame is valid only for the respective superframe, while in the eLPRT protocol and GTS schemes, the allocation information is  
Figure 2: Example of the superframe structure of IEEE 802.15.4.  
valid for successive superframes until it is changed. In order to preserve clarity to the reader, the following description of eLPRT also includes previous LPRT features, and focuses on the differences to the GTS scheme. The eLPRT MAC protocol presents a superframe structure similar to the one defined by the IEEE 802.15.4, but introduces several new features designed to increase the number of supported devices, improve bandwidth utilization and increase the robustness against channel errors. Each of these features enhances one aspect of the performance in comparison with the GTS scheme, and most of them can be adopted independently from each other. A basic feature of the eLPRT and LPRT protocols is the division of the superframe into a much larger number of slots (500, in the current implementation) than the IEEE 802.15.4 (16 slots). This feature increases significantly the granularity of slot allocation in the CFP, avoiding the waste of bandwidth and, consequently, contributing to increase the throughput efficiency and the number of supported nodes. The price of this increased granularity is reflected in terms of tighter synchronization bounds that need to be respected by both coordinator and node devices. The increased granularity also increases the number of bits required to encode the allocation length and start slot. Another feature of the eLPRT protocol is the provision of a larger set of options for the superframe period, to closely match the packet generation interval imposed by the application. Similarly to the GTS scheme, in order to allocate/deallocate slots in the CFP, the sensor node sends an Allocation Request command containing the Allocation Length, the Allocation Direction and the Allocation Type (allocation or deallocation). While in the GTS scheme the sensor node receives an ACK frame as response, in the eLPRT and LPRT protocols the node receives an Allocation Response command containing an Allocation Identifier (AID), which is assigned dynamically to the node during allocation and released in deallocation. The AID is shorter than the Device Short Address (16 bits) that is used by the IEEE 802.15.4. In the current implementation, the length of the AID was set to 6 bits, which is enough to support the traffic of up to 64 nodes in the CFP and is significantly greater than the seven nodes allowed by the GTS scheme. The eLPRT allocation descriptor is composed by 6 bits for the AID, 9 bits for the Start Slot and 9 bits for the Allocation Length. Thanks to the shorter AID, the length of the eLPRT allocation descriptor is the same of the corresponding GTS descriptor of the IEEE 802.15.4 (3 bytes), even though the reference to a slot requires more bits in the eLPRT protocol (9 bits) than in the GTS scheme (4 bits). Another feature of the eLPRT and LPRT protocols is the inclusion of an ACK bitmap field in the beacon, which provides the acknowledgment of uplink transmissions made in the CFP of the previous superframe. This feature eliminates the protocol overhead associated with the reception of individual ACK frames for each uplink data packet. The correspondence between each uplink transmission and the bits of the ACK bitmap is made using the AID of the corresponding node. For downlink transmissions, the conventional ACK frame is used. In the GTS scheme of the IEEE 802.15.4, all allocations take effect from the moment they are announced in the beacon.