# Distributed Edge Computing to Assist Ultra-Low-Latency VANET Applications
## 4 Latency-Aware Offloading Algorithm for Multilevel MEC VANET System
### Annotation
\begin{table}
\begin{tabular}{c c c} \hline \hline
**No.** & **Parameter** & **Specification/Value** \\ \hline \multicolumn{3}{c}{802.11 radio information} \\ \hline
1 & Data rate & 6.0 Mb/s \\
2 & Channel & 172 \\
3 & Signal strength (dBm) & 23 dBm \\
4 & TSF timestamp & 1508234676 \\
5 & Duration & 172 μs \\
6 & Preamble & 20 μs \\ \hline \multicolumn{3}{c}{IEEE 802.11 Data, Flags} \\ \hline
7 & Type/Subtype & Data (0x0020) \\
8 & Frame Control Field & 0x0800 \\
9 &......00 & Version: 0 \\
10 &......... & Type: Data frame (2) \\
11 & 0000... & Subtype: 0 \\
12 & Flags & 0x00 \\
13 &......00 & DS status: Not leaving DS or network is operating \\  &.........00 & in AD-HOC mode (To DS: 0 From DS: 0) (0x0) \\
14 &......... & More Fragments: This is the last fragment \\
15 &...... & Retry: Frame is not being retransmitted \\
16 &...0... & PWR MGT: STA will stay up \\
17 &...0... & More Data: No data buffered \\
18 &...... & Protected flag: Data is not protected \\
19 &...... & Order flag: Not strictly ordered \\
20 &.000 000 000 000 & Duration: 0 microseconds \\
21 & Receiver address & Broadcast (ffff:ffff:ffff:ffff) \\
22 & Transmitter address & Arada\_05:06:34(00:26:ad:05:06:34) \\
23 & Destination address & Broadcast (ffff:ffff:ffff) \\
24 & Source address & Arada\_05:06:34 (00:26:ad:05:06:34) \\
25 & BSS Id & 00:00:00:00:00:00:00:00:00:00:00 \\
26 &...... 0000 & Fragment number: 0 \\
27 & 0000 000 000... & Sequence number: 0 \\ \hline \multicolumn{3}{c}{Logical-Link Control} \\ \hline
28 & DSAP & SNAP (0 xaa) \\
29 & 1010 101. & SAP: SNAP \\
30 &......0 & IG Bit: Individual \\
31 & SSAP & SNAP (0 xaa) \\
32 & 1010 101. & SAP: SNAP \\
33 &......0 & CR Bit: Command \\
34 & Control field & U, time=UI (0x03) \\
35 & 000. 00.. & Command: Unnumbered Information (0x00) \\
36 &......11 & Frame type: Unnumbered frame (0x3) \\ \hline \multicolumn{3}{c}{Wave Short Message Protocol (IEEE P1609.3)} \\ \hline
37 & Version & 2 \\
38 & PSID & 0x00000020 \\
39 & Channel & 172 \\
40 & Data Rate & 6 \\
41 & Transmit Power & 23 \\
42 & WAVE element id & WSMP (128) \\
43 & WSM Length & 65 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Wave Short Message Protocol (WSMP) parameters. To determine the effectiveness of the VANET with control based on software-defined networking, a series of five experiments were conducted for the architecture shown on Figure 1b. The duration of each experiment was 15 h. During the experiment, 30 Raspberry pi devices were used to imitated vehicle OBUs, and the number of RSUs was 10 devices. The study scenario involved a serial connection of the OBU to the RSU at specified intervals, which was used to simulate the movement of vehicles. In situations where more than three OBUs were connected to one RSU, an additional delay of 5 ms was introduced. For performance evaluation of the proposed structure, the bandwidth and the packet loss are considered to be the performance metrics. During experimental evaluation, we considered measuring average bandwidth for two main cases; the system was deployed with the main cloud only and the system was deployed with the MEC. The results of the experiments are shown in Figure 3.

According to the experimental results, the average packet loss amounted to 0.082%; it is possible to conclude that the controller coped with the requirement of reducing network load, thus confirming the efficacy of the test approach. This can be interpreted by the introduction of MEC servers with the distributed SDN paradigm controlling and managing the traffic. This reduces the traffic congestion and the packet loss. Thus, the overall network load is reduced and the network efficiency in terms of latency and reliability is increased. The effect of distances between RSU stations on the packet delivery time and amount of data offloaded to the core network was checked for the proposed system. Three scenarios were considered for such evaluation; in each scenario three cases were considered. In each scenario, certain value of flow rate of cars was considered.