# A Stable Backup Routing Protocol for Wireless Ad Hoc Networks
## 2 Stable Backup Routing Scheme (SBR)
### Backup Route Construction  
Similarly to [30], backup routes in our SBR are established and updated by overhearing MAC signals during data transmission process. Different from the SLR algorithm proposed in reference [30], the SBR protocol selects backup routes that are more stable than the main route in terms of the path successful rate in order to guarantee the stability.  
For establishing backup routes, both the MAC cache and backup route table need to be constructed for each node in MANET, and the structures of RTS and CTS are also modified, which are described in Figure 4. As we can see from Figure 4a,b, both RTS and CTS messages carry the addresses of sender, receiver and destination. The difference is that \(p^{\text{(Sender, Receiver)}}\) calculated by (2) is appended to CTS, which is used to represent the stability of the main link between sender and receiver. The other fields such as frame control, duration and CRC are set in accordance with the standard IEEE 802.11 RTS/CTS formats. Note that the modified RTS and CTS include 26 and 28 bytes, respectively, whereas the lengths of legacy RTS and CTS are 20 and 14 bytes. The MAC cache in Figure 4c contains a series of entries, which records all the information obtained by overhearing RTS/CTS exchanges that are not for the node itself. Specifically, the item \"Frame Type\" is set as \"RTS\" or \"CTS\" according to the type of the MAC frame received. If RTS is overheard, \(p^{\text{(Sender, Self)}}\) is then calculated by (2) and \(p^{\text{(Sender, Receiver)}}\) should be \"null\" in the corresponding entry. On the other hand, if CTS is intercepted, \(p^{\text{(Sender, Self)}}\) is computed by (2) likewise, and \(p^{\text{(Sender, Receiver)}}\) is achieved or copied from the value in CTS. The existence of MAC cache provides evidence for setting up and updating backup route table. In the backup route table given by Figure 4d, \"Path Type\" means \"IEB\", \"EBR\" or \"DBR\" and \"inefficient node\" is the possibly unreachable node due to link breaking. \"Successful rate\" stands for the stability of the alternate path. All of the entries in the MAC cache have a maximum survival lifetime to prevent being out of date. According to SBR, the qualified RTS or CTS just received overwrites the corresponding entry that already exists in MAC cache and the lifetime of the entry will be reset following such updating. If the renewed RTS/CTS cannot be received within the duration of the lifetime, it will be considered unreachable and the related entry in MAC cache will be deleted.  
We next explain how to obtain alternate routes based on the MAC cache. Establishment process of our three types of backup routes, i.e., IBR, EBR and DBR are illustrated in Figure 5, assuming that there is a path segment B-C-E-F on the main route to a specific destination and a node G that is located outside the main route. To facilitate the understanding without losing generality, we only set up alternate paths for link B-C in this example. For other links, execute them in the same fashion.  
Figure 4: Structures of (**a**) request-to-send (RTS), (**b**) clear-to-send (CTS), (**c**) MAC cache, (**d**) backup route table.