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effect also occurs for alternating mode, as seen in Figures 10 and 11, but at a much larger value of n. This means the saturation point of the channel is shifted. The continuous scenario uses the available channel resources more efficiently and therefore congestion occurs later. Figure 11. Freshness: the average beacon end-to-end delay for queue size=5 In Figures 12 and 13 the total delay for continuous and alternating mode is shown with a varied number of nodes and queue lengths respectively. Again, it can clearly be seen that the queue length of the nodes has little influence on the delay. Figure 8. Reception Probability for continuous scenario with NPD, varied queue size and number of nodes. Figure 12. Freshness: average beacon end to end delay for continuous scenario with NPD, varied queue size and number of nodes Figure 9. Reception Probability for alternating scenario with NPD, varied queue size and number of nodes Figure 10. Freshness: the average beacon end-to-end delay for queue size=1 Figure 13. Freshness: average beacon end to end delay for alternating scenario with NPD, varied queue size and number of nodes From the simulation experiments can be concluded that in general the continuous scenario performs better than the alternating scenario. However, for a large number of nodes the freshness of the beacons is better in the alternating scenario. 4. MINIMIZING IEEE 1609.4 CHANNEL SWITCHING IMPACT As clearly visible in the simulation experiment results, using the channel switching specified in IEEE 1609.4 has a negative impact on the IEEE 802.11p beaconing performance. The reception probability decreases and the average end-to-end delay increases. When comparing the alternating scenario to the
continuous scenario, it can be concluded that the alternating scenario has negative impact on the beaconing performance. In order to increase the performance of the channel switching, several solutions have been proposed. This section gives an overview of such solutions. It is important to notice that to the best knowledge of the author of this paper, no solutions could be found that focus on minimizing the impact of the IEEE 1609.4 channel switching on the IEEE 802.11p beaconing (i.e., CCH) performance. Some research studies however, see e.g., [3], [9], [18], [17], consider that the standardized IEEE 1609.4 channel switching scheme causes a “bandwidth wastage” problem for the SCH. The argumentation is the following. When a packet transmission is going to take place at the end of the SCH interval (i.e., service frame) and the estimated transmission time for this packet exceeds the residual time of the current service frame, then the IEEE 1609.4 standard recommends that the transmitting node should prevent sending out this packet during the current service frame but instead should send it during the next service frame. In order to decrease the impact of the “bandwidth wastage” problem on the SCH, several alternative schemes have been researched, which will be briefly introduced in the following subsections. These schemes are compared based on the following criteria: (1) focus on minimizing the impact of channel switching on IEEE 802.11p beaconing (i.e., CCH) performance, (2) their complexity & scalability and (3) whether these solutions are standardized or not. 4.1 Immediate and Extended Access Two different channel switching schemes have been proposed in [3] to minimize the SCH “bandwidth wastage” problem. These scheme are the immediate access and the extended access, see Figure 14. These schemes improve the performance of bandwidth-demanding non-safety applications in the SCH at the cost of the CCH. Figure 14. IEEE 1609.4 channel switching schemes, copied from [3]. When the immediate access scheme is used, the node does not have to wait until the CCH interval is over. It can switch to the SCH once the CCH has completed its transmissions and can stay there until the synchronization interval (of 100 ms) is over. The extended access scheme allows transmissions on the SCH without waiting for the CCH to improve the performance of the SCH at the cost of the CCH. These schemes are not focussing on minimizing the impact of channel switching on the IEEE 802.11p beaconing (i.e., CCH) performance. Both schemes are considered to be scalable and not complex, since the nodes’ MAC layer only has to check if there are no more packets awaiting transmission in the CCH to start transmission on the SCH.. 4.2 Fragmentation scheme The Fragmentation scheme [17] is designed to decrease the impact of the “bandwidth wastage” problem on SCH. This is accomplished by using packet fragmentation in order to utilize the residual time at the end of the SCH interval (i.e., service frame). The original packet that needs to be transmitted at the end of the service frame is fragmented in such way that the estimated transmission time of one its fragments is equal to the residual time of the current service frame. In this way there will be no unused time and bandwidth in a service frame. Thus mitigating the bandwidth wastage problem, at the expense of an extra header for the fragmented packet. This scheme is not focussing on minimizing the impact of channel switching on the IEEE 802.11p beaconing (i.e., CCH) performance. Furthermore, it has not been standardized. The complexity of the algorithm depends on the packet fragmentation complexity. 4.3 Best-fit scheme The best-fit scheme, see [17] utilizes the remaining bandwidth by sending packets which fit in the remaining time of the channel’s interval. In particular, the transmitting node checks the estimated transmission time for each packet that needs to be transmitted and is placed at the head of the transmission queue. If the estimated transmission time is less than the residual time in the service frame, it sends out the packet. Otherwise, the scheme checks within the transmission queue whether there are packets whose estimated transmission time is less than the residual time. If there are such packets the scheme selects the packet whose estimated time is closest to the residual time and sends out that packet. This scheme performs better than the fragmentation scheme only if packets with different sizes are present in the queue [9]. This scheme is not focussing on minimizing the impact of channel switching on the IEEE 802.11p beaconing (i.e., CCH) performance. Moreover, this scheme is not standardized and it is hard to implement. First of all nodes need to know the size of all the packets stored in the transmission queue in order to reshuffle the packets. Furthermore, it is difficult for a node to determine the channel transmission speed, because of the frequent changes in the channel congestion. Therefore, it is considered that this scheme is complex. 4.4 Exploitable node-assisted WBSS Broadcasting mechanism In [9] the Exploitable Node-Assisted WBSS Broadcast Mechanism is presented. Due to the CSDM/CA feature used in IEEE 802.11p, nodes cannot send packets if other nodes are sending other packets at the same time and in the same channel. A so called ‘exploitable node’ is a node that is in SCH and is informed that another node is sending data to a Road Side Unit (RSU). The exploitable nodes can broadcast the WBSS to newly coming vehicles, so these new vehicles can join the WBSS faster. When all idle nodes are broadcasting the WBSS simultaneously, most of the packets will collide. To prevent this from happening, a priority control is used. The WBSS coverage of a RSU is divided into a near and a far part, as seen in Figure 15. In Figure 15, node C and D are within the ‘near area’ of the
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