## 3 The eLPRT Protocol
In the event of a GTS reallocation, the slots previously allocated to a node can be reallocated to another one. Therefore, whenever a node misses the beacon at the beginning of a superframe, it must not transmit in the allocated slots of that superframe, since its allocation could have changed. The eLPRT protocol introduces a new reallocation mechanism that enables the use of slots allocated for transmission in a superframe when the corresponding beacon is lost. Thismechanism works through the inclusion of a reallocation counter (RC) field in the beacon that normally takes the value zero (0). Whenever a reallocation is necessary, the RC field is changed to the integer value \(N_{RC}\) (15 in the current implementation). During consecutive beacons, the value of the RC field is decremented, until it reaches zero again. While \(N_{RC}>0\), all changes in allocations are announced, through the inclusion of the corresponding updated allocation descriptors in the beacon; however, the new allocations only take effect when the value of the RC field returns to zero. This mechanism ensures that a node can use its allocated slots even when it misses up to \(N_{RC}\) consecutive beacons. An additional feature of the eLPRT and LPRT protocols is the provision of a contention-free retransmission mechanism. This mechanism provides automatic allocations, on demand, of supplementary slots in a superframe for retransmission of data packets whose original transmission failed in the previous superframe. These supplementary slots form an extra reserved period in the superframe referred as Retransmission Period (RP). As shown in Figure 3, the RP is placed after the Contention Access Period (CAP) and before the period reserved for the original transmissions, which is named Normal Transmission Period (NTP). The eLPRT allocation descriptor for retransmissions requires only 2 bytes, because the allocation length does not need to be announced, since it is the same of the original transmission. The goal of the retransmission mechanism is to increase the reliability in case of channel errors while avoiding collisions. With this mechanism, extra bandwidth is automatically allocated to isochronous traffic connections, in detriment of asynchronous traffic transmitted in the CAP, in order to better fulfill QoS requirements for scheduled allocations. The eLPRT protocol also introduces a simple frequency hopping mechanism to deal with interference, especially from IEEE 802.11 networks. The operating frequency changes from superframe to superframe, according to Equation (1), where \(i_{s}\) represents the index of the superframe and \(n_{j}\) is the channel jump adopted. This jump has to be an odd number to ensure that the network jumps through all 16 available channels from the 2.4 GHz frequency band. Since an IEEE 802.11 channel occupies  
Figure 3: Superframe structure of the eLPRT protocol. the equivalent to four IEEE 802.15.4 channels, the value of \(n_{j}\) was set to 5 in the current implementation. This ensures that the network does not suffer the interference of an IEEE 802.11 channel during two consecutive superframes. In combination with the retransmission mechanism, the frequency hopping mechanism enables the exploration of frequency diversity to recover from transmission errors:  
\[Ch(i_{s})=\ 11+\left(n_{j}\times Ch(i_{s}-1)\right)mod\ 16 \tag{1}\]