## 5 Simulations
### Simulation Results for Different File Sizes  
Here we compare three CCD metric values under three different file sizes, that is, 10 M, 6 M and 2 M bytes, for SocialCode, SC without DP, RCM + CodeOn (RCM for short) and FRAME + CodeOn (FRAME for short). It is obvious that the larger file has smaller decoding percentage and longer file downloading delay to decode enough SLNC-coded pieces back to the original file, as compared to those of the smaller ones, which are shown in Figures 10 and 11. For example, average file downloading delays of SC for 10 MB, 6 MB and 2 MB files are about 412, 253 and 125 s, respectively. Further, no matter the file size is, average decoding percentages of SC without DP, RCM + CodeOn and FRAME + CodeOn  
Figure 14: Average credit vs. the number of nodes (**a**) 3468 nodes; (**b**) 867 nodes; (**c**) 216 nodes. are smaller than those of SocialCode, as shown in Figure 9. Further, their average file downloading delays are longer than those of SocialCode, which means the average file downloading delays of RCM + CodeOn > FRAME + CodeOn > SC without DP > SocialCode, as shown in Figure 10. As mentioned above, the vehicular node adopting both SCs first exchanges its content bitmap with all nearby nodes and then calculates the correct number of distinct SLNC-coded content pieces, the backoff time and the exact number of re-encoded pieces by the proposed CalUtility procedure and Equations (1)-(3). Hence, SocialCode and its variety achieve better performance than RCM + CodeOn and FRAME + CodeOn do on both metrics. Moreover, by assigning dynamic forwarding priorities with Equation (4) and backoff delays, that is, \(\Delta t_{int}(i)\), with Equation (5), SocialCode could prevent a particular intermediate node forwarding its pieces for a long time, which in turn uses up its credits and stops it to forward any content piece at last. Consequently, SocialCode has more excellent decoding percentages and file downloading delays than SC without DP does. Oppositely, because both RCM + CodeOn and FRAME + CodeOn let the destination node pay its credits to the intermediate node for forwarding the SLNC-coded content pieces, some destination nodes may exhaust their credits soon, which terminates their cooperative content distribution processes. In this situation, they cannot broadcast and receive content pieces to and from nearby nodes when they are outside the signal range of the RSU. Hence, RCM + CodeOn and FRAME + CodeOn suffer from poorer decoding percentages and file downloading delays than SocialCode and SC without DP. Table 5 lists normalized ratios for average file downloading delays of SocialCode, SC without DP and FRAME + CodeOn over that of RCM + CodeOn under three file sizes. It shows that SocialCode achieves much lower normalized ratio for the file downloading delay than SC without DP, RCM + CodeOn and FRAME + CodeOn do when the file size is smaller. With respect to three different file sizes, average credits of three types of nodes, that is, the destination, intermediate and overhearing ones, for SocialCode, SC without DP, RCM + CodeOn and FRAME + CodeOn are illustrated in Figure 11a-c, respectively. The destination node with RCM + CodeOn needs compensating all intermediate nodes one unit of its credit for broadcasts a content piece to them. Thus, RCM + CodeOn consumes the credits of the destination node quicker than FRAME + CodeOn does, which is shown in Figure 11. Oppositely, FRAME + CodeOn has a slower rate to drain the credits of the destination node than RCM, because it will reward the winning vehicular node some definite credits by its sweepstake component. According to guidelines in Table 3, as the destination node with SocialCode and its variety forwards an innovative SLNC-coded piece to nearby destination or overhearing ones, it will earn credit \(\alpha\) from them, which occurs frequently in the beginning of the CCD procedure.