## 3 System Model
\begin{table}
\begin{tabular}{c c} \hline \hline
**Symbol** & **Definition** \\ \hline \(R\) & the radio range for transmission between vehicles \\ \(T_{i}^{\rm C}\) & the consumed time for the \(i\)-th control sub-phase \\ \(T_{i}^{\rm d}\) & the consumed time for the \(i\)-th data transmission sub-phase \\ \(T_{\rm e2e}\) & the overall content transmission time \\ \(T_{\rm data}\) & the useful data delivery time \\ \(N_{\rm P}\) & the number of transmission phases \\ \(V\) & the amount of data to be delivered \\ \(C\) & the link bit rate of physical layer \\ \(L_{i,j}\) & the holding time for the \(j\)-th link of the \(i\)-th phase \\ \(s_{ij}(t)\) & the RSS of the \(j\)-th link in the \(i\)-th phase at time \(t\) \\ \(s_{\rm th}\) & the minimum RSS threshold for maintaining the radio connection \\ \(H_{i}\) & the number of hops for the \(i\)-th transmission phase \\ \(T_{L_{i,j}}\) & the delay for the \(j\)-th link in the \(i\)-th transmission phase \\ \(T_{\rm L}\) & the backoff timer delay for a general link \\ \(w_{k}\) & the weight for the decision on the \(k\)-th candidate SOV \\ \(d_{k}^{0}\) & the initial inter-vehicle distance between SUV and the \(k\)-th candidate SOV \\ \(t_{\rm min},t_{\rm max}\) & the minimum and maximum backoff timer values \\ \(t_{\Delta}\) & The maximum backoff timer difference \\ \(D\) & a general representation of the inter-vehicle distance \\ \(D_{\rm max}\) & the maximum radio range (defined as \(R\) in this paper) \\ \(L_{\rm max}\) & the maximum link lifetime defined for the backoff timer calculation \\ \(\epsilon\) & the weighting ratio between link lifetime and transmission distance \\ \(\rho_{i}\) & the traffic density for the \(i\)-th road lane (vehicles per meter) \\ \(v_{i}\) & the expected speed for the \(i\)-th road lane (meters per second) \\ \(\lambda_{i}\) & the average number of one-hop neighboring vehicles for the \(i\)-th lane \\ \(l_{k}\) & the \(k\)-th road lane with increasing speed \\ \(X_{n}\) & the lane state for the \(n\)-th BV, which can be \(l_{1}\) or \(l_{2}\) for a two-lane scenario \\ \(Y_{n}\) & the backoff timer delay between two consecutive BVs \\ \(\rm E[T^{c}]\) & the expected delay for a generic routing phase out of the \(N_{\rm P}\) \\ \(\rm E[H]\) & the average number of hops \\ \(\rm E[T_{L}]\) & the expected timer delay in one V2V link \\ \(d_{\rm SD}\) & the end-to-end distance between the SUV and the selected SOV \\ \(\rm E[D_{L}]\) & the average distance between two consecutive BVs for each link \\ \(T^{(1,1)}\) & the backoff timer delay conditional on \(X_{k}=l_{1}\) and \(X_{k+1}=l_{1}\) \\ \(T^{(1,2)}\) & the backoff timer delay conditional on \(X_{k}=l_{1}\) and \(X_{k+1}=l_{2}\) \\ \(D^{(1,1)}\) & the transmission distance conditional on \(X_{k}=l_{1}\) and \(X_{k+1}=l_{1}\) \\ \(T_{k}^{(1,1)}\) & the backoff timer delay for the \(k\)-th candidate vehicle given \(X_{k}=X_{k+1}=l_{1}\) \\ \(t^{(1,1)}\) & the expectation of random variable \(T^{(1,1)}\) \\ \(\
u_{\Delta}\) & the difference between the speed levels of the two lanes \\ \(T^{(1)}\) & the minimum backoff timer delay given the sender vehicle locating in lane \(l_{1}\) \\ \(P\) & the lanes state transition probability matrix \\ \(p_{ij}\) & the transition probability of message delivery from vehicle on \(l_{i}\) to vehicle on \(l_{j}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Symbols definitions. SOV, SOurce Vehicle; SUV, SUbscribing Vehicle; BV, Backbone Vehicle. content transmission time, which is composed of transfer time for control signals and target contents. We have:  
\[T_{\text{e2e}}=\sum_{i=1}^{N_{\text{p}}}\left(T_{i}^{\text{c}}+T_{i}^{\text{d}} \right)=\sum_{i=1}^{N_{\text{p}}}T_{i}^{\text{c}}+T_{\text{data}}\,, \tag{1}\]  
where \(T_{\text{data}}\) denotes the useful data delivery time and \(N_{\text{p}}\) is the number of transmission phases. It should be noted that \(T_{\text{data}}\) can be obtained from the target content volume and achieved link rate over the wireless channel.