## 3 Road-Based Multi-Metric Forwarder Evaluation Scheme
### Information Collection Stage
#### 3.1.4 Accessible Bandwidth Estimation  
In this subsection, the employment of Accessible Bandwidth Estimation (ABE), which is an important parameter metrics for achieving quality video streaming has been discussed. The VDR,VTJ and the H\({}_{\rm m}\)R\({}_{\rm r}\) are important metrics, but cannot determine the level of availability of bandwidth of the link between two vehicles. The ABE has been employed in some vehicular communication research works, for example, in [11; 23]. The ABE is based on the wireless standard for vehicular communication, that is, IEEE 802.11p/WAVE protocol. Considering the fact that a video data has a high data rate, the consideration of the available bandwidth is very essential to achieve a better video quality and delivery. Therefore, the ABE has been considered as one of the metrics for video stream forwarding decision to assist in choosing optimal NFV. However, in some previous research works, the packet loss has been considered based on packet collision that occurs in the entire network scenario. This concept is not adequate since in ABE only the bandwidth of a link between two vehicles is feasible. Therefore, the ABE is estimated at each single link, that is, the link between the PFV and C-NFVs. The major phases of the ABE are synchronization of channel idle period and, back-off and collision phase. In the ABE, individual vehicles compute it idle duration via sensing of the channel medium. The wireless link idle duration of the transmitting vehicle (ID\({}_{\rm{tv}}\)) and the receiving vehicle (ID\({}_{\rm{rv}}\)) are estimated in ABE. Further, the back-off and collision phase are considered when a sender vehicle transmits a video packet and the medium of the receiving vehicle is not idle. The operation process involves that each vehicle senses its channel medium and compute its idle time percentage. The (ID\({}_{\rm{tv}}\)) and (ID\({}_{\rm{rv}}\)) of a wireless link in ABE are computed based on link capacity C. The concept is that for communication to occur there is a need for both the transmitter and the receiver vehicle to be at an idle state, which is not realistic considering the high density of urban scenario. Due to this reason, it is difficult to synchronize the idle duration of both transmitting vehicle and the receiving vehicle. There is possibility of video packet loss due to video packet collision. Therefore, the ABE estimate the hello messages collision probability (hm\({}_{\rm{prob}}\)). The probability of video packet (vp) collision of a given number of (N) vehicles with average speed (Sp) in an urban scenario is formulated in Equation (11). \[{\rm{prob}}({\rm{vp}},\,{\rm{N}},\,{\rm{Sp}})={\rm{f}}({\rm{vp}},\,{\rm{N}},\, {\rm{Sp}})\times{\rm{hm}}_{\rm{prob}}({\rm{vp}},\,{\rm{N}},\,{\rm{Sp}}) \tag{11}\]  
The f(vp, N, Sp) is derived based on Lagrange polynomial interpolation as in [PERSON] et al. [23] by utilizing the video packet losses and the hello message losses obtained from the simulation. The overhead generated due to the binary exponential back-off of the collision phase has been derived based on [24]. Equation (12) represents the back-off estimation mechanism (K), the duration difference between the transmission of two video frames is represented as D\({}_{\rm{diff}}\). The Distributed coordination Function Inter-frame Space (DIFS) (IEEE 802.11 standard, 2003) is a static range of time for vehicle node to access the channel medium when it is free for duration not longer than DIFS. The \(\overline{\rm{back-off}}\) is the average number of decremented back-off slot for a single frame. \[{\rm{K}}=\frac{{\rm{DIFS}}+\overline{\rm{back-off}}}{{\rm{D}}_{\rm{diff}}} \tag{12}\]  
The two different phases, which are essential for estimating the ABE between transmitting and receiver vehicle, are integrated. In this study, the transmitting vehicle is considered as PFV, while the receiving vehicle is the NFV. The transmitting vehicle computes the ABE of each of the C-NFVs' wireless link based on [23] as in Equation (13).