## 3 Proposed Methodology
### Estimation of Link Delay
#### 3.3.1 Investigating the Factors Affecting Collision Probability to Formulate an Average
As evident from Figure 7, the \(H_{WI}\) at time (t \(-\) 1) is higher (0.1978) than that at time t (0.1105), as the number of wireless links in the network is higher at time (t \(-\) 1) compared to the network at time t. However, the number of wired links is the same in both network instances, so \(H_{WI}\) in both network instances is the same (0.3333). The reason for taking the normalized network per-channel link entropy for predicting collision probability can be explained with respect to the network instances shown in Figure 8. Note that all nodes (A, B, C, and D) in Figure 8 are vehicular nodes. Figure 8: Network instances showing routing paths having different network per-channel link entropy, demonstrating different contention levels. (**a**) Low-entropy network instance having low collision probability, (**b**) High-entropy network instance having high collision probability. Figure 7: Entropy calculation for two network instances. (**a**) Vehicular network instance at time t \(-\) 1, (**b**) Vehicular network instance at time t.  
Note that, for the networks given in Figure 8, each of the nodes A, B, and C needs to send a packet to destination node D. In the low-entropy (0.315) network given in Figure 8a, the routing path from A to D is ABCD; from B to D is BCD; and from C to D is CD, as indicated in the Figure. Note that nodes A, B, and C attempt to send their packets to D around the same time (routing is scheduled around the same time). However, note that A and B should contend with each other, as only one of them can transmit at the same time. Similarly, in the beginning, only one of B and C transmit at the same time. Thus, for the low-entropy network, two nodes contend for the wireless communication channel at the same time. Therefore, the collision probability is lower than when three nodes contend for the same communication channel. On the other hand, for the high-entropy network instance given in Figure 8b, due to the presence of direct links from each and every node to other nodes, the most probable routing path from A to D is AD, from B to D is BD, and from C to D is CD. When all nodes A, B, and C attempt to transmit at the same time, there is a high probability of collision, as there exist wireless links from A to C and A to B such that, in the same communication channel, a packet transmitted from one sender can collide with another. Therefore, in the high-entropy network in Figure 8b, the collision probability is higher, as three nodes contend for the same communication channel. Therefore, for the given high-entropy (1.00) network, there is a very high chance of network contention occurring when packet routing is scheduled at the same time. The above example in Figure 8 graphically proves that, in a well-connected network, the probability of collisions is high due to multiple nodes contending for access to a communication channel. On the other hand, when the per-channel link entropy is low (poorly connected), there can be fewer nodes contending for a communication channel, such that it will have a lower probability of collisions. At one extreme end, when the per-channel link entropy is zero (all nodes are isolated), there will be no routes; this situation is equivalent to an instance with no contention scenario, as there exists no communication channel with which to contend. On the other extreme end, when all nodes are connected to each other (when per-channel network link entropy is one), there is a very high probability of contention. However, per-channel network link entropy is not the only factor that we can identify as affecting collision probability. The other factor that we introduce in this research is called the Pending Transmission Packet Distribution Factor (PTPDF), which measures the packets' degree of distribution among nodes for the pending transmissions in the network. Now, let us compute the average PTPDF of the network (\(\overline{\mathcal{Q}}\)). The PTPDF of node \(i\) (\(\mathcal{Q}_{i}\)) is a dynamic (time-varying) factor that depends on how packets are scheduled and routed.