Chapter 4 Design and Development
4.4. How to poll
Each time T-RSU sends polling packet to the vehicle which is on the top on polling queue and then move it to the end of queue. For example, in Figure 4-2, after T-RSU send polling packet to vehicle A and then move A to the end of queue. The sequence of polling queue becomes C, B, D, A. The T-RSU will delete vehicles in polling queue in two cases:
transaction success or times out. If there is no packet error in three handshake process between T-RSU and vehicles, the transaction process is complete and then T-RSU deletes vehicles from polling queue. Otherwise, T-RSU still keeps vehicle in polling queue even it has been passing transaction zone without being transacted. Thus, we should have some mechanisms to determine whether vehicles in polling queue are still in transaction zone or reservation. Here we use expected leaving time as evaluation criteria. If the expected leaving time of vehicles in polling queue plus a threshold ∆ is earlier than current time, we say that these vehicles have been time out without be transacted and delete them from polling queue.
The threshold ∆ is calculated by lower bound of speed limitation 𝑣𝑚𝑖𝑛 and length of total ETC system which equals to length of reservation zone 𝑙𝑅 plus length of transaction zone
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𝑙𝑇 as following, where ɛis a system parameter to ensure correctness of system in GPS error situation.
∆= ɛ ∙𝑙𝑅𝑣 + 𝑙𝑇
𝑚𝑖𝑛 (13)
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Chapter 5
Experimental Design
In this thesis, we focus on the system based on WAVE DSRC in normal environment even in harsh environment whether it is still reliable. Our ETC system will compare to the ETC system which using single channel contention based transaction scheme for most existing ETC systems. The compared ETC system uses only CCH to process transaction operation which just similar to our ETC system transaction operation on SCH. According to the experiment results, we will show that our ETC system is more appropriate for MLFF architecture and more reliable than single channel contention based ETC system. Then, the results also show the effect of varied length of reservation zone and the reliability in distinct numbers of T-RSU situations.
Sometimes, there is huge number of vehicles passing a toll gate at the same time. ETC systems should be able to service these overwhelming vehicles simultaneously and transact them as many as possible. In simulations, the vehicle arrival process is commonly modeled as Poisson process [19]. For Poisson process of vehicle arrival rate, the vehicle inter-arrival time distribution is an exponential variable. The vehicles arrival rate is defined as an average number of vehicles that appearing at each lane per second in this thesis. Base on exponential distribution, the cumulative probability function is shown as Figure 5-1. 1. The probability
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that less than one vehicle arrival at a lane in one second is 64%. It still has 0.68% probability that more than five vehicles arrive in one second.
Figure 5-1 Cumulative probability distribution function with λ=1
To illustrate the effect of overwhelming vehicles to ETC systems, we define two cases:
normal case and extreme case. In normal case, vehicles arrive on each lane based on exponential distribution with λ = 1. In extreme case, to eliminate the uncertain factors, vehicles have the same speed 120km/hr., it is the maximum speed limitation of highway in Taiwan. We let 5 vehicles arriving at a lane in one second with fixed speed 120km/hr., about 6.67 meters distance between two vehicles at one lane. The extreme case scenario is shown in Figure 5-2.
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Figure 5-2 Extreme case at highway scenario
In real world, there are many kinds of ITS applications based on WAVE DSRC such as road traffic safety applications on CCH or commercial applications on SCH. For road traffic safety applications, vehicles broadcast safety messages including periodical beacon exchange and non-periodical emergence safety message on CCH as mentioned above. These kinds of message transmission on CCH we call them background flow relative to our ETC system.
The background flow on CCH is trigged from several independent applications from upper layer. In the simulation, we assume that background flow is produced at average 25ms with Poisson probability distribution on CCH. Vehicles whether inside or outside reservation zone will all produce background flow. Background flow packets have the same header field as WSA messages. On SCH, not only ETC systems but also other commercial application share bandwidth. To guarantee existing applications working successfully, the ETC system cannot occupy all SCHI for contention free polling. In our simulation, we use only 40% of SCHI as contention free period (CFP) and the reserved bandwidth save for other commercial applications.
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In theory, packet error ratio of WAVE DSRC is only influenced by distance due to signal attenuation and multipath interference. The longer distance between transistor and receiver is, the packet error is higher. Actually, the previous observation of WLAN is not suit for WAVE DSRC due to various network environments which are relatively higher speed and the quick change of network topology. The packet error ratio is impacted by not only distance but also quality of wireless communication device and surrounding environment such as raining or haze. In previous work [20], the filed test in highway scenario shows that the packet error rate of single packet transmission in WAVE DSRC is about 9% at 100 meters. In [21] - [25], weather condition also impact throughput and packet error rate in wireless communication. ETC systems should ensure that the transaction operations can work any time including in bad weather situation. Thus, we define two type of environment in our simulation: general environment and harsh environment [22]. In general environment, there are no raining or other weather conditions that will influence wireless communication.
The only factor that will influence packet error rate is quality of WAVE/DSRC devices. In harsh environment, the weather condition, such as snow or haze, is serious so that the packet error rate is increasing. In experiment we assign 5% packet error rate in general environment and 15% packet error rate in harsh environment.
We conduct the simulation using Estinet7.0, the commercial version of NCTUns. The simulation scenario is in a 1km four-lane highway scenario. Vehicles arrive on each lane at average 1 second per vehicle. There are 4 T-RSUs using independent SCH for transaction procedure and 1 R-RSU using CCH for channel reservation setting in the middle of the highway. The transmission range of each vehicle and each RSU is 300 meters and the transmission rate is 3 Mb/s. The more detail parameter is shown as Table 5-1.
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Table 5-1 Simulation parameters
Simulation parameters Value
Length of highway 1 km
Lane width 3.5 m
Number of lanes 4
Number of T-RSUs 4
Length of reservation zone 10 m
Length of transaction zone 8m
Average vehicle speed 90 km/hr.
Deviation of vehicle speed 10 km/hr.
Vehicle speed probability distribution Gaussian
Vehicle arrival rate 1 vehicle/s‧lanes
Vehicle inter-arrival time distribution Exponential
R-Beacon interval 100 ms
Priority level of Reservation message AC_VO
CFP of SCH 40%
Background flow interval 25 ms
Priority level of background flow AC_BE
Packet error rate 5%
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Chapter 6
Experimental Results
There are 4 types of network environment as describing in previous chapter, normal case with general environment, normal case with harsh environment, extreme case with general environment and extreme case with harsh environment, to represent various situations in real world. In this chapter, we will show the experimental results in three parts.
First, we compare our ETC system with the ETC system without reservation scheme which means it use contention based transaction process. Two ETC systems use the same protocol stack, WAVE/DSRC. The major goal is to show the improvement of reliability in the developed ETC system. Second, we focus on the ETC system without reservation scheme and realize why using reservation scheme is more reliable. Finally, the analysis for proposed ETC system with variable length of reservation zone and number of T-RSUs gives some suggestion for developers about how to decide these important parameters of the ETC system.
6.1. With vs. without Reservation
The transaction successful ratio is increasing with length of transaction zone in two ETC systems shown in Figure 6-1. Vehicles stay in transaction zone for longer time due to longer
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length of transaction zone. It has lager probability to be transacted for more times. The transaction successful ratio of ETC system without reservation scheme is lower than ETC system with reservation scheme in all types of network environments. It means the reservation scheme is effective in increasing reliability of ETC systems. In Figure 6-1 (a), the ETC system without reservation scheme needs at least 10 meters transaction zone for 90%
transaction successful ratio in all network environments. In Figure 6-1(b), the transaction successful ration in ETC system with reservation scheme is almost 100% in all network environments even with 6 meters transaction zone.
(a) Without reservation
Length of Transaction Zone (m)
Normal Case with
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(b) With reservation
Figure 6-1 Transaction successful ratio
Figure 6-2 shows distribution of vehicle passing length before be transacted in transaction zone. If vehicles can be transacted early, means passing less length in transaction zone, the probability of expected vehicles transacted times is higher. In Figure 6-2 (a), the ETC system without reservation scheme requires about 7 meters for almost 100% vehicles to be transacted successfully. In Figure 6-2 (b), vehicles in ETC system with reservation scheme only need to pass 2 meters for transacted. It means that vehicles in ETC system with reservation scheme have larger expected transaction times for higher reliability.
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Length of Transaction Zone (m)
Normal Case with
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(a) Without Reservation
(b) With Reservation
Figure 6-2 Distribution passing length of transaction zone
0
Passing Length of Transaction Zone (m)
Normal Case with
Passing Length of Transaction Zone (m)
Normal Case with
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6.2. Analysis for without Reservation
It shows transaction request time distribution in 8 meters length transaction zone ETC system without reservation scheme in Figure 6-3. Most of all vehicles only send transaction request one time due to the limitation of transaction zone length. It causes lower transaction successful ratio and necessary of longer length of transaction zone.
Figure 6-3 Distribution of transaction request times
6.3. Analysis for with Reservation
Table 6-1 shows the transaction successful ratio in variable transaction length ETC system with reservation scheme. The results in general environments show in Table 6-1 (a) and the results in harsh environments show in Table 6-1(b). In normal case in both general and harsh environments, the transaction successful ration is 100% due to small number of vehicles. In extreme case, transaction successful ratio increases slightly with length of transaction zone due to more transacted probability. But when increasing length of
Percentage of Total Request Times (%)
Request Times
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reservation zone, the transaction successful ratio is decreasing.
Table 6-1 Transaction successful ratio with distinct cases in varying environments (a) General environment
General environment (Packet error rate: 5%)
6 8 10 12
Normal Extreme Normal Extreme Normal Extreme Normal Extreme
10 100 100 100 100 100 100 100 100
30 100 99.9153 100 100 100 100 100 100
50 100 99.8305 100 100 100 100 100 100
(b) Harsh environment
Harsh environment (Packet error rate: 15%)
6 8 10 12
Normal Extreme Normal Extreme Normal Extreme Normal Extreme
10 100 99.322 100 99.8305 100 99.5763 100 99.9153
30 100 95.7093 100 99.7458 100 99.4077 100 99.4077
50 100 92.2881 100 99.8305 100 99.9153 100 99.9153
More vehicles in longer reservation zone will contend to access channel on CCHI at the same time. In Figure 6-4 (a), the reservation request times are increasing with larger vehicles density and the more serious effect of surrounding environment. In Figure 6-4 (b), the longer reservation zone can enhance channel reservation successful ratio, but the ratio cannot achieve 100% even with 50 meters reservation zone. The reasons show in Figure 6-4 (c), although vehicles have increasing reservation request times with length of reservation zone, the contention will cause the decreasing of channel reservation successful ratio.
The other drawback of large reservation zone is the longer polling queues show in Figure 6-4(d). T-RSUs need more time to poll all vehicles in polling queue. The polling times
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of each vehicle will decrease due to the longer polling time. It will cause the decrease of transaction successful ratio.
(a) Distribution of reservation request Times
(b) Channel reservation successful ratio
0
Percetange of Total Request Times (%)
Request Times
Percetange of Total Request Times (%)
Length of Reservation Zone (m)
Normal Case with
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(c) Average request times
(d) Average polling queue length
Figure 6-4 Effect of variable length of reservation zone
In Figure 6-5 (a), we show the relationships between transaction successful ratio and
Length of Reservation Zone (m)
Normal Case with
Length of Reservation Zone (m)
Normal Case with
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number of T-RSUs. When using more than 2 T-RSUs, the transaction successful ratio is higher than 99.5%. Using only one T-RSU will cause the significant decrease on transaction successful ratio. The reason is the longer polling queue length shown in Figure 6-5 (b).
(a) Transaction successful ratio
(b) Average polling queue length Figure 6-5 Effect of variable number of T-RSUs
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In Figure 6-6, we show the distribution of vehicle passing length before be transacted in transaction zone with 1 T-RSU and 4 T-RSUs. In 1 T-RSU case, vehicles should passing about 5 meters to guarantee transaction process success. In 4 T-RSUs case, vehicles only require to pass about 3 meters in transaction zone for guaranty of transaction process. It means that the reliability is increasing with number of T-RSUs in ETC system. We can enhance the reliability by using more T-RUSs on independent SCH.
(a) 1 T-RSU case
Passing Length of Transaction Zone (m)
Normal Case with
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(b) 4 T-RSUs case
Figure 6-6 Distribution passing length of transaction zone
0
Passing Length of Transaction Zone (m)
Normal Case with
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Chapter 7 Conclusions
In this thesis, we have developed a WAVE/DSRC based contention-free ETC system.
The transaction area is divided into reservation zone for channel reservation and transaction zone for transaction process. Vehicles use contention access scheme for channel reservation in reservation zone and pay tolls without contention through multiple T-RUSs using several independent SCHs. The simple mathematical analysis shows relationships between length of reservation zone and channel reservation successful probability. In additional, some challenges about channel allocator and polling scheduler have been discussed. We conduct the experiments by Estinet7.0, the commercial version of NCTUns. The experimental design and results show that the developed ETC system is more reliable than other single channel contention based ETC system. Finally, we also have discussed effect of variable system parameter setting, such as length of reservation zone and the number of T-RSUs, and given some suggestions for system developers. In future works, we will design complex channel allocator and polling scheduler for higher reliability and implement the ETC system on real WAVE devices.
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