Performance Study of Testing Node 01

Firstly, we found that the diagram for delay represents itself in a bell-shaped curve, which can be divided into three main parts: two lines and one parabola. The former are due to vehicle group 01 moving from the point far enough from the left edge of vehicle group 02. Therefore, vehicles have content media with counterparts only within their own group and also provide a stable performance within 1 minute. After that, the two groups start to overlap gradually, which can be considered as more vehicles merging into each vehicle group. Figure 7-14 illustrates the case.

Figure 7-14 Illustration of Vehicle Merge

From the view at the centre of effective communication coverage, vehicles there will experience a greater traffic feed externally, i.e. more vehicles come inside. In our scenario, only group 02 can fill with a 2 Km long effective communication coverage. Thus, the peak delay we saw happened in the 80 vehicles per group situation. Such a phenomenon might look simple, but has an important meaning, which implies two different VANET performances in congested and loosened traffic situations. Concerning the first case, where vehicles are almost continuously fed into certain coverage, the delay would be relatively low but would maintain at a stable level, which helps vehicles in this range precisely evaluate the situation more and also conducts AC adjustment accordingly, such as packet size or generation frequency, for the purpose of enhancing some AC communication quality. On the other hand, the latter instance subsequently causes large-scale fluctuation and jitter. It is necessary to be understood that, since CCH works in the broadcast mode, it is hardly possible to obtain information about a communication environment during that period, but would count in the SCH period, whose establishment, however, completely depends on the successful communication of WSA at AC 0, i.e. a deteriorative process. On the other side, it can be observed that, through the increase of vehicle numbers, the delay of all ACs increase at approximately the same pace and also maintain the spacing with each other just like the

and the relatively independent behavior of each AC in VANET.

Besides the delay in the dynamic environment, Figure 7-14 discloses a more critical issue in practical application, which is a hidden node problem that is rarely mentioned in WLAN nowadays, due to the successful CTS/RTS mechanism. It has never been brought out or found in previous scenarios because they are generally set in either a neglectable dimension or a logical subnet, in order to better study other factors impacting on VANET. However, in scenario 5, we found that there are always two throughput peaks happening at approximately 97s and 137s for each AC at testing node 01. After comparing to previous scenarios, I conclude that such a phenomenon is due to a hidden node issue, as there is no CTS/RTS mechanism in the broadcast mode. Apparently, vehicles in previous scenarios contend media with each other either in a logical subnet or an extreme dimension, in order to better study other changeable factors, thus never experiencing the hidden node problem.

However, in practical application, we need to divide collisions into two types: media contention collision and hidden node collision, which are combined in order to degrade VANET performance. In scenario-4, the packet received per second keeps increasing unless the loss rate reaches nearly 100%, although adding a vehicle number results in a higher collision rate. Concerning scenario-5, every node joining the effective communication coverage of testing node 01 will arouse a higher rate of both kinds of collision, which at some stage will overwhelm the impact of adding ACs. Simply put, even though more packets have been transmitted every second, the collision rate increases much faster and results in fewer packets being successfully received. Figure 7-15 provides the procedure of a vehicle group merge from the view of testing node 01.

Figure 7-15 Vehicle Group Merge Procedure

In fact, the mechanism concerning dimension is very complex, although the above diagram looks simple. I will try my best to explain it by using AC 03 as an example. In our scenario, it shows one peak happening at stage 03. Firstly, from the view of testing node 01, we divide its effective communication coverage into two parts (01 and 02), whereby there is no hidden node collision within the same part due to pre-defined 1 Km coverage. When two vehicle groups start to merge with each other from stage 01 to 02, the overall packet transmission rate in part 01 increases, while the rate in part 02 remains the same, according to the conclusion from scenario-4 and the nature of a non-saturated network. Thus, at these stages, even with the number of hidden node collisions, testing node 01 can still receive more packets within a given time. Simply put, vehicles in part 01 outnumber its counterparts of 02. However, when vehicle group 02 pierces the middle line of group 01, where testing node 01 is located, the vehicle number in part 02 will increase, while the number in 01 does not change. At that time, a collision due to hidden node increases is due to more vehicles being fed into part 02, which act as antagonists against their counterparts in part 01. However, at the initial stage after piercing, a higher packet transmission rate may still be able to offset the impact of a greater overall collision rate; however, it loses its domination at a certain point in time.