## 3. Freestanding Position-Based Routing Protocol
### Location Service
#### 3.4.2 Redundancy Strategy  
As previously mentioned, the aim of the MFWAR selection mechanism is to reduce the loss of discovery packets by considering the hard constraints imposed by the radio channel. Nevertheless, sometimes the discovery packets may not reach the intended primary relay node because of larger than expected path losses or inaccurate position predictions made by the selection algorithm (see Figure 2(a)). Therefore, if the discovery packet sent by the current forwarder node does not reach the primary relay node, then the FPBR algorithm will use a redundancy strategy that considers the use of a backup node, \(BN\). The task of \(BN\) is to rebroadcast the discovery packet towards the primary relay node in order to continue with the discovery packet dissemination (see Figure 2(b)). As such, it is important to mention that \(BN\) does not modify the primary relay nodes table in the discovery packets. Note that the backup node must be located between the current forwarder node (named \(FN_{n}\)) and the primary relay node (named \(FN_{n+1}\) ) in order to improve the reception probability. Besides choosing a backup node, an acknowledgment mechanism is needed to implement the redundancy strategy. However, in the IEEE 802.11a and IEEE 802.11p MAC layer there is no acknowledgment mechanism for broadcast packets. Therefore, a light acknowledgment mechanism is also included in FPBR. This mechanism and the backup node selection procedure are explained next. _Light Acknowledgment Mechanism._ If the primary relay node, \(FN_{n+1}\), receives the discovery packet (see Figure 3(a)), then it will broadcast the discovery packet (with updated primary relay nodes entries) towards a new primary relay node (e.g., \(FN_{n+2}\)). As \(BN\) is located between the current forwarder, \(FN_{n}\), and the primary relay node, \(FN_{n+1}\), \(BN\) should be able to receive \(FN_{n+1}\) broadcast transmission (see Figure 3(b)). This feature is used in FBPR as an implicit acknowledgment procedure. Thus, if a node receives a discovery packet where its ID is listed as that belonging to a \(BN\), then it will wait for \(2\tau\) s. If after this time \(BN\) does not detect the transmission of a broadcast packet from the \(FN_{n+1}\) node listed on the discovery packet, then it will rebroadcast the original discovery packet towards the \(FN_{n+1}\)  
Figure 2: Example of the redundancy strategy implemented in FPBR. (**a**) The discovery packet reaches the \(BN\) node but it does not reach the \(FN_{n+1}\) node; (**b**) The \(BN\) node rebroadcasts the discovery packet towards the primary relay node since the original discovery packet did not reach the \(FN_{n+1}\) node. node. It is important to note that as an implicit acknowledgment approach is being used in FPBR, additional control packets are not required. Therefore, the FPBR acknowledgment mechanism does not introduce additional overhead. _Backup Nodes Selection._ As previously mentioned, the \(BN\) nodes specific task is to rebroadcast the original discovery packet if the \(FN_{n+1}\) node does not perform its corresponding broadcast transmission. In FPBR, \(BN\) is selected in such way that it is located between \(FN_{n}\) and \(FN_{n+1}\). Although choosing a \(BN\) right on the middle between the original transmitter and the chosen primary relay node may be a good starting point, such a simple selection mechanism does not consider the vehicles direction and the consequent path loss increase/decrease. Thus, FPBR implements the MFWAR mechanism to select the backup nodes as well. However, for the selection of the \(BN\) node the nominal radio range is assumed to be equal to the distance between the \(FN_{n}\) and \(FN_{n+1}\). This distance will be referred as \(d(FN_{n},FN_{n+1})\). For the \(BN\) node selection through MFWAR, vehicles are grouped in three sets based on their relative direction to the original forwarder node: \(V_{R_{a}}\), \(V_{R_{c}}\) and \(V_{R_{s}}\). Additionally, three dynamic factors \(\beta_{R_{a}}\,\beta_{R_{c}}\) and \(\beta_{R_{s}}\) are considered.