*3.3.2. Clustering in network layer*

**Protocol CH Election**

Maximum number of messages received from cluster relays is the CH.

230 Contemporary Issues in Wireless Communications

Vehicle nearer to middle of the cluster is the CH.

No reception of a message longer than a particular time units from a CH, then it elects itself as CH.

CH is based on waiting period of Hello messages to neighbors.

No CH

Lane weight, average distance, maximum number of neighbors, and average distance level.

Random after relaying one packet to ordinary node.

HCA [25]

Zaydoun [26]

Xi Zhang [27]

CBMAC [28]

> RCM [29]

TCMAC [30]

CFIVC [31]

**Cluster Formation**

Maximal distance between a CH and CM is two hops.

> RSS > threshold.

Undecided state to CM based on reception of one CH messages.

Geographical area. Vehicles are assigned to different channel pools.

According to speed.

**Table 3.** Comparison between various cluster based MAC protocols.

Not Specified TDMA City

TDMA in CMs-CHs, CSMA/C A CHs-CHs

**MAC Scenario Simulator Pros Cons**

End to End delay reduced.

Support both safety and nonsafety applications.

Reduces datacongestion and supports safety messages.

Minimizes the hidden terminal problem.

Reduced contention and throughput is increased.

Channel utilization, scalability, avoids hidden terminal problem, decreases collisions and packet drops.

> Avoids data collisions.

Overhead and packet loss is increased. Do not consider the direction of movement.

> Not suitable for high traffic. High overhead.

High overhead and complex algorithm. Require two transceivers

Does not select a stable CH during initial CH election.

Low bandwidth utilization in sparse scenarios.

Cannot be used for safety applications, it is delay intolerant.

It neglects any condition that might affect the maximum speed achievable by the vehicle nodes.

SUMO

C++ with graphical interface.

> Simone 2000

I-V Communic ation Based on Traffic Modeling.

TDMA City OMNeT++,

Highway

TDMA Highway A. law el al

Not simulated

Not simulated

TDMA City

Not specified TDMA Highway Ns-3

CDMA, MCSCD MA

In network layer clustering, a virtual network infrastructure must be created through the clustering of vehicles sharing similar characteristics in order to provide scalability. Routing protocols for VANETs mostly based on periodical broadcast messages to reveal their positions and traffic information to neighbors. Nevertheless, deterioration of routing performance is anticipated in urban areas due to high density of vehicles in the network. Basically, excessive broadcast messages as well as broadcast overhead may increase, resulting packet losses (due to collision) and significant routing performance deterioration. Information transfer or dissemination needs multi hop communications. When exchanging information between vehicles, there are network issues that must be addressed, including the hidden terminal problem, high density, high node mobility, and data rate limitations.

In multi-hop data forwarding method, the key problem is selecting the relay/CH for data routing. Most of the relay/CH selection method presented is more suitable for highway scenarios. In a city environment, the widely adopted method is the store-carry-forward scheme. Reactive protocols find routes on-demand. If a node wants to communicate with a node to which it has no route, the routing protocol will try to establish the shortest route between them. Here there is significant delay in determining the route. Proactive (table-driven) protocol, which is based on the exchange of control packets and it is continuously updating the reachability information in the routing table, so routes are immediately available when requested but there is high overhead in maintaining updated periodic routing tables and also maintains the routes that are not going to be used. Hybrid protocol is combination of proactive and reactive protocol. It is also known as cluster based routing. It is a convenient way for developing efficient routing scheme in VANETs. In Table 4 we compare between various cluster based routing protocols proposed in VANETs.



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