# A Stable Backup Routing Protocol for Wireless Ad Hoc Networks
E.g., AODV with backup routing (AODV-BR) [19] constructed an alternate path through overhearing RREP packets and a fish bone structure is formed jointly by the primary and alternate routes, which is illustrated in Figure 2. Correspondingly, backup routes in [21] are found by the destination node when the RREQ packets are received. It could be observed that such an approach usually finds all alternate routes within the scope of network topology. However, as nodes move faster, the backup paths generated only in route  
Figure 1: Route discovery in Ad hoc On-demand Distance Vector (AODV) protocol and Dynamic (Mobile Ad Hoc Network (MANET)) On-Demand (DYMO) routing. discovery process may be out of date and become unavailable, which may influence the reliability of data transmission. To be adaptive to topology variations, alternative routes can be created through overhearing datagrams [25; 26; 27; 28; 29]. The authors in [25] proposed an AODV adaptive backup routing named as AODV-ABR without sending many extra control messages. In [26], a novel fast route recovery scheme was presented by overhearing both data and acknowledgements. Listening to data packets to collect the required information may lead to a huge amount of energy consumption because of the data length [30], which is the main shortcoming of such a method. Another strategy for building backup routings is implemented by overhearing MAC signals during the data delivery phase [30]. In [30], a source routing with local recovery (SLR) was presented. Under SLR, an intermediate node listens to request-to-send (RTS), clear-to-send (CTS) and acknowledgement (ACK) messages to update its MAC cache which is established for recording bypass routings. Similar to the schemes of overhearing datagrams, the approach that utilizes MAC signals to set up backup routes may not always discover all the existing candidates. Nevertheless, it has better adaptability to the mobility of nodes. It should be further noted that snooping of MAC headers does not incur any additional energy overhead since a node must try to overhear MAC frames to affirm if there is a data packet being transmitted to it in any case, which is the inherent mechanism of IEEE 802.11 [31]. However, unfortunately, all the existing research results on building alternate paths have not considered the stability of backup routes. In fact, alternate paths should not be selected carelessly because unstable links may cause the rebuilt route to be broken again. In view of the discussion above, this paper aims to improve the stability of alternate paths. In this direction, a stable backup routing (SBR) scheme is presented. SBR constructs backup paths through overhearing signal exchanges at the MAC layer, which is like that in [30]. Different from other existing backup routing protocols, link stability is considered in our scheme, which depends on the bit error rate. In detail, we classify the qualified backup routes into three categories with have different priorities based on hop counts. According to both the priority and link stability, nodes choose suitable alternate paths when link breaking occurs. As described above, our proposed algorithm is perhaps most comparable to the SLR in [30], which also uses RTS/CTS signals to build backup routes. Compared with SLR, the SBR algorithm proposed in this paper uses the power information of the physical layer to set up more stable backup routes, which further improves the network performance. Simulation results verify our analysis. The rest of article is organized as follows. Section II gives the algorithm and realization process of SBR. Simulation and analysis are introduced in Section III. Finally, we conclude in Section IV.