## 2 Related Work
Such redundancy is not observed in backbone-like clustering schemes. In these, no node can be part of two clusters at the same time. Precisely, chain-branch-leaf [14] is  
Figure 1: OLSR mesh creation through multipoint relays (MPRs). (**a**) The 1-hop (red) and 2-hop (blue) neighborhood of a node (green). (**b**) Resulting MPR set of a node (red). (**c**) Mesh created (red) with all MPR nodes in the VANET. a clustering scheme designed for VANETs, which specifies the roles the CBL nodes can endorse in order to create a virtual communication infrastructure. Each cluster member (leaf node) elects its cluster head (branch node), and the latter serves as the gateway nodes to connect the clusters. CBL nodes rely on two metrics to select branch nodes: connection time and chain time, both computed from mobility information such as position, speed, and steering angle. The nodes that remain close together and whose relative speed and steering angle difference are low are favored, which leads to covering each direction of travel with an independent backbone, as shown in Figure 2. The resulting structure of CBL connects each cluster in a linear way, forming what is called a \"chain\". Simulations have shown that managing vehicle-to-vehicle communications with CBL results in stable clusters and chains in medium- and high-density highway scenarios [14]. Its functioning only depends on the periodic sharing of node state through HELLO messages, which encouraged the authors to implement it into the OLSR protocol (CBL-OLSR). Previous simulations comparing its performance to that of other popular protocols in the literature, namely, OLSR, ad hoc on-demand distance vector (AODV) [19] and the geographic routing protocol (GRP) [20], showed its superiority in terms of end-to-end packet delay and routing traffic overhead [14]. However, the evaluations did not consider the case where a mesh-like clustering scheme such as MPR would operate only using road traffic direction. In low-mobility MANETs (up to 10 m/s), the authors of [21] showed that an OLSR-based clustering approach, even with overlapping clusters, can help reduce end-to-end delay and routing traffic overhead. The authors did not conduct any comparison with a backbone-like clustering scheme. Other papers focus on cluster stability metrics (such as cluster duration or number of cluster head changes). In [22], the authors propose a clustering algorithm with two cluster heads: one main cluster head is active; the second is passive and takes over the main cluster head when the connection is lost. This mechanism also integrates cluster merging. Assessed via simulation for a medium- to high-speed VANET, it was shown to reduce the number of clusters in the network and increase their lifetime as well as routing traffic overhead. However, the results do not state its performance regarding end-to-end delay and packet delivery ratio for application traffic, and the authors did not conduct any comparison with a mesh-like clustering solution. Other authors [23] used the angle between vehicle velocity vectors and the speed limit on the road in order to cluster nodes. The protocol was evaluated against other clustering protocols in the literature regarding cluster lifetime and number of cluster head changes. Their angle-based clustering algorithm (ACA) approach is shown to perform better in highway scenarios regarding both stability metrics. However, they did not analyze its performance regarding the end-to-end delay of application traffic and packet delivery ratio. In [24], the authors reduced the total number of MPR nodes. Network nodes called \"cooperative nodes\" execute a modified version of the OLSR. By exploiting adjacent nodes' MPR information to select already-elected MPRs, routing traffic overhead is reduced by 15% in static scenarios and from 8 to 14% in mobile scenarios. However, the authors did not  
Figure 2: CBL chain structure.