*3.9.5. Cluster throughput*

*3.9.2. CH stability*

238 Contemporary Issues in Wireless Communications

be obtained.

*3.9.3. Cluster delay*

*3.9.4. Cluster reconfiguration*

The time during which a node is in the state of a CH determines stability. It is the mean time duration of the nodes, remaining its leadership role as CHs. Long CH lifetime implies, few changes and good stability. The information is disseminated by groups enhancing the communication delay, reliability, low data delivery and congestion issues, making the vehicular networks accurate and efficient. CH stability can be affected by different factors such as merging, distance between CHs, exit from the road etc. In VANETs, merging collisions can happen among vehicles moving in the same direction due to acceleration or deceleration, it is more likely to occur among vehicles moving in opposite directions (approaching each other) or between a vehicle and a stationary RSU since they approach each other with a much higher relative velocity as compared to vehicles moving in the same direction. The high mobility of the shifting nodes within the networks causes lots of challenges to face and affects stability.

If vehicles are changing their state very often in intersection scenarios and stay only for short times in the CH state, CH stability is low. In some of intersection based approaches the first vehicle to enter the intersection region in a particular direction is elected as CH to improve stability. Furthermore, some cluster-based routing algorithms, the selection of CH are based on willingness factor which defines the relative stability of a node. CH stability is also based on the threshold distance between the two CHs. Optimal distance between two CHs should

Cluster delay means the time required for sending one message from source to destination (it can be here from CM to the RSU or vice versa). The delay parameter is very crucial for safety applications. The end to end delay can be minimized by selecting proper cluster size, selection of proper MAC protocol to reduce the channel access time, selection of stable CH nearer to the RSU, a selection of proper routing algorithm between CH transmissions. The number of the formed clusters is important to reduce the end to end delay for message transmission.

The frequent cluster reconfiguration generates tremendous communication load, which significantly reduces available bandwidth for message dissemination. Cluster reconfiguration is needed in some cases when the CH leaves the group or numbers of CMs are below the threshold or the distance between two CH is below the threshold. In some approaches, if the distance between two CH nodes is detected less than the particular threshold, the cluster with fewer CMs is dismissed to reduce communication overheads while it's CMs join other clusters. One can expect that a larger dismiss threshold leads to a higher rate of CH changes and higher probability of cluster reconfiguration. The threshold determines the rate of cluster reconfigu‐ ration, and also, depends on the radio transmission range and vehicular densities. Larger transmission provides longer distance for CHs to detect each other, and therefore, more frequent cluster reconfigurations occur. Additionally, some algorithm elects backup CH to avoid cluster reconfigurations. However, most of the protocols are not fit for different traffic Data rate transfer that gives the total number of received packets at the destination out of total transmitted packets. An access collision happens when two or more CHs within two hops of each other attempt to acquire the same available time slot.
