**3.4. CH election**

**Protocols Cluster or CH Scenario Simulator Pros Cons**

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Ns-2, VanetMobiSi m

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Intersections NCTUns Cluster stability is improved

Intersections NCTUns Cluster stability is improved

Reduces the cluster reconfiguration by delaying re-clustering for a certain period of time. To avoid accidental contacts between CHs.

Increases the cluster and CH lifetime.

in intersections.

Cluster overhead and re affiliation are reduced

Considers relative mobility to increase stability.

Suitable for both sparse and dense traffic.

in intersections

Improved the influence of overtaking within the clusters. accurate density estimation within the clusters.

Few neighbour nodes move differently, the method still results in dramatic increase in the variance.

Problem with knowing the final destination a priori as drivers usually do not use navigation system for known routes.

Increased overhead and delay.

It doesn't consider destination of vehicles. Not mention about CH election. Not suitable for intersections

No direction of movement and position is considered for cluster formation. Overhead increased.

The destination of vehicles, speed of vehicles is not taken into account that increases the overhead.

Overhead and delay increased.

Overhead and delay increased.

Random Ns-2

Urban

Highway

Urban

Intersections NCTUns

MOBIC [41]

AMACAD [42]

MCDRIVE [43]

APROVE [44]

ALM [45]

DBC [46]

Maslekar [47]

Maslekar [48]

Variance of relative mobility with each of its neighbour's.

232 Contemporary Issues in Wireless Communications

Relative distance, speed between neighbour's and distance between vehicle and destination.

First vehicle in the direction is elected as CH

Minimum distance and minimum relative velocity between each CH and its CMs.

Variance in relative

Connection graph density, link quality, traffic conditions, node reputation and movement prediction.

Location and direction of vehicles.

The direction which the vehicle will take after crossing intersection. The CH is at the front of cluster.

mobility Box topology

**Table 4.** Comparison between various cluster based routing protocols.

CH selection is important to increase protocol reliability, scalability and delay. In some of CH selection algorithms proposed takes into account the destination of vehicles, including the current location, speed, direction, relative destination and final destination of vehicles as parameter to arrange the clusters. Many researchers have proposed CH election scheme based on ID. Each node is assigned a unique ID, and the node with the lowest ID in its two-hop neighborhood is elected to be the CH. Some algorithms calculate these ID based on the variance of relative mobility of a mobile node with each of its neighbors, where a small value of variance indicates the mobile node is moving relatively less than its neighborhood. Additionally, other approaches consider vehicles having a longer trip are more qualified for being elected as CHs.

A vehicle, which would travel longer time, is assigned higher priority; hence, at the very beginning of starting its travel, the expected travel time of a vehicle is calculated and an‐ nounced using its desired driving speed and the geographic information system once its driver sets the destination. Te stability of the system is improved by electing the vehicles having a longer trip as the CHs. Furthermore, to avoid elected CHs losing connectivity with their neighbors very soon, the eligibility of a vehicle should decrease quickly when its velocity has big difference from the average speed. Thus, a vehicle with large speed deviation is assigned lower priority.

Another type of CH election scheme is based on connectivity level (estimating graph density), link quality (SNR), relative node position and the prediction of this position in the future, and node reputation. The vehicle which is near to that anchor point is elected as CH. Furthermore, some approaches assign generic weight to vehicles based on the position and other set of vehicle parameters like connectivity, mobility, RSS etc. The vehicle with the highest weight is elected as the CH amongst the neighbors. However, since the vehicles are highly dynamic in nature the position of the vehicles change very fast and hence may induce a computational overhead in calculating the weight associated with the vehicles.

In some of the clustering algorithm first vehicle entering into the cluster region is initialized as the CH. It changes from CH to CM due to the discovery of a closer CH, or until the last member of the cluster passes the intersection. However, CH stability is reduced due to distance between vehicle to intersection and due to different directions of vehicles. In some other schemes, the CH selection should resemble like a natural model of location references. CH reelection only occurs when two CHs move within range of one another for a certain contention interval. When a CM moves out of range of its CH, it joins any current CH in its neighborhood, or forms a new cluster. However, in the case in which few neighbor nodes move differently, the method still results in dramatic increase in the variance.
