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Is IEEE 802.11p V2X Obsolete Before it is Even Deployed? 37 Table 1 Cumulative size of CAM high and low frequency packets High frequency Low frequency CAM 330 bit 347 bit Security header 93 byte BTP header 4 byte GeoNetworking (SHB) 36 byte LLC 8 byte MAC 28 byte Total 211 byte 213 byte The generation of CAMs is started when all vehicles have entered the road and reached their target speed to guarantee a constant channel load. Each ITS-S adds a random offset from 0 to 500 ms from this time until it generates the first CAM to minimize the risk of collisions. In a real environment the CA basic services of different vehicles would also be asynchronous. We chose this setup as it best resembles the environment in which CACC applications will typically operate: a highway condition with high vehicle speeds and few obstructions to wireless signals. In this scenario, each ITS-S will generate a CAM approximately every 111 ms. Only the mandatory information is included into the generated messages, yielding the smallest possible CAMs. Table 1 gives an overview of the PDU size as a CAM is passed down through the network stack. Packets are sent via the AC_VI queue. The DCC mechanism was configured according to the values specified in [10] and shown in Table 2. For the Active state, however, we constrained the packet interval to 0.1 s rather than the 0.5 s default packet interval. This is consistent with the specification as the Active state for the CCH only enforces TPC [10]. This implies that applications are free to override the default values defined for other parameters—like the packet interval—as long as they do not exceed the minimum and maximum values. We chose 0.1 s for the packet interval longer intervals quickly render timing critical applications like CACC impossible. The simulation was run with 10, 20, 30, 40, 50, 60 and 90 vehicles participating in the VANET. Different statistics were recorded, the channel load, DCC state, packet collisions as well as the minimum, mean and maximum age of information from neighbor vehicles. Each run lasts 300 s, the results are presented in the following section. Table 2 DCC parameter settings for AC_VI used in the simulation; from left to right: states RELAXED, Active, RESTRICTIVE Channel load Parameter
38 J. Hiltscher et al. 5 Results In this section we present statistics of the channel load and CA update rate recorded in the simulation runs. The channel load gives an impression of how much the communication system is stressed by the application. Furthermore, it allows for assessing the effectiveness of the DCC mechanism. Table 3 shows the mean of the channel load values recorded by the individual vehicles as well as the maximum value. Channel load is defined as the amount of time the wireless channel was sensed busy in reference to a measurement interval. The measurement interval is one second with a granularity of 10 µs as proposed in [10]. The runs with 10 and 20 vehicles only generate a light load which does not activate the DCC mechanism. The maximum information age, also presented in Table 3, equals the CAM generation interval. The values reported are the worst ones encountered for all vehicles in the run. The run with 30 vehicles shows a different picture; while the mean information age is only slightly above the CAM generation interval, the maximum age is significantly longer. It is also evident that the maximum measured channel load is above 15% and the recorded DCC state shows that one of the vehicles enters Active state. Examining other recorded statistics reveals that several vehicles in this scenario experience packet collisions, thereby leading to CAMs of those vehicles being lost. The chance of collisions obviously rises as the number of vehicles increases but is also dependent on the random CAM generation offset and MAC backup mechanism. This outlier clearly highlights the shortcomings of the IEEE 802.11p medium access mechanism. The remaining runs show the DCC mechanism in action. The doubled data rate decreases the channel load below the 15% threshold, causing DCC to constantly fluctuate between RELAXED and Active state for the runs with 40, 50 and 60 vehicles. Figure 2 shows a sequence of the channel load as measured by three different vehicles in the 40 vehicles scenario. The run with 40 vehicles shows that the DCC mechanism effectively reduces packet collision likelihood due to the increased data rate in Active state; maximum and mean information age is close to Table 3 Measured channel load and update rate statistics Vehicle Channel load Age (s) (%) Count Max Mean Max Mean StdDev 10 4.7 4.6 0.1109 0.1108 1.6 10 −5 20 9.3 9.2 0.1108 0.1108 7.3 10 −6 30 18.1 13.7 0.6652 0.1136 0.02 40 16.8 11.8 0.1119 0.1108 4.8 10 −5 50 19.1 13.4 1.44 0.1147 0.04 60 21.4 15.1 2.437 0.1141 0.045 90 30.2 21.6 4.4308 0.1134 0.04
Page 1 and 2: Lecture Notes in Mobility Tim Schul
Page 3 and 4: More information about this series
Page 5 and 6: Editors Tim Schulze VDI/VDE Innovat
Page 7 and 8: vi Preface cross-sectorial roadmap
Page 9 and 10: Steering Committee Mike Babala, TRW
Page 11 and 12: xii Contents Robust Facial Landmark
Page 13 and 14: Part I Networked Vehicles & Navigat
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