## 2 Related Work  
Position-based routing might be a better choice in VANETs for a number of applications; e.g., collision warnings, alerts messages, advertisements, _etc_. This is precisely because of their ability to handle variation in the position of vehicles [34]. Still, there are issues in position-based routing that need to be addressed. Position-based routing can be improved by incorporating location error probability and its possible solution. Location information can be easily changed by attackers as all the vehicles broadcast their location information to neighboring vehicles. Flooding in multi-hop routing increases the network overhead, therefore, an efficient algorithm that selects a minimum number of next forwarding vehicles is needed [35]. Geographic forwarding protocols select the shortest route that may suffer from a higher packet error rate due to poor link quality. The uneven distribution of vehicles on the roads makes route selection more complex; e.g., the shortest path in terms of geographic distance may experience more frequent path disconnections. The dynamic and rapidly changing topology of vehicular networks can cause frequent communication disconnections among vehicles. The frequent path disconnection is the most important issue in designing routing protocols for VANETs. A few protocols use vehicle density information to select routes, but the inaccuracy of statistical data may cause routes to be incorrectly computed. The link quality depends on the shadowing phenomena caused by obstacles in the path of transmission. Voronoi Diagram based Geographic Distance Routing (V-GEDIR) [31], is an extension of GEDIR. In this paper the authors have presented a loop-free position-based routing algorithm. In the V-GEDIR algorithm, NHVs are selected as vehicles whose Voronoi region either covers or cuts the expected destination zone. This method can be used for both routing and geocasting. An example of the NHV selection process using V-GEDIR is shown in Figure 2.  
In V-GEDIR, node mobility is not considered during the formation of a Voronoi diagram, which limits the applicability of the approach in highly mobile networks such as VANETs. In [32], the authors have suggested a Junction-based Geographic Routing (J-GEDIR) protocol for VANETs. In the protocol, an on-road vehicle passing through a junction point, closest to the destination and making smallest angle with the source vehicle has been selected as next hop vehicle. The greedy forwarding based on distance selects border nodes as NHVs that degrade the performance of the protocol in a high speed vehicular environment due to non-inclusion of link quality in the consideration. In Peripheral node-based Geographic Distance Routing (P-GEDIR) [33], all vehicles present inside the circular strip of width R/2 (R is the transmission range of a vehicle) towards the destination are referred to as peripheral vehicles. Peripheral vehicles are considered as NHVs. In a high speed vehicular environment such as a highway, selecting peripheral nodes as NHVs limits the performance of P-GEDIR due to the high outage probability of peripheral nodes. These protocols are not suitable for dense VANETs which do not include  
Figure 2: NHVs selection using V-GEDIR. link quality into the consideration and their performance starts degrading with the increasing speed of vehicles. An example of NHVs selection process using P-GEDIR is shown in Figure 3.  
A Multi-Metric map aware routing protocol for VANETs has been suggested in [36]. The protocol has used four metrics: distance, density, trajectory and bandwidth, for making routing decisions. In the protocol, fixed and predefined weights are assigned to each of the considered metrics that is one of the most noticeable deficiencies of the protocol considering highly dynamic network environments in VANETs. The Adaptive Connectivity Aware Routing (ACAR) [37] protocol addressed rapidly changing topology and frequent network disconnections by dynamically selecting an optimal route with the best transmission quality.