## 4 Wireless Enabling Technologies
### IEEE 802.11p and Related Standard  
Standardized by the IEEE in 2009 and included in the overall IEEE 802.11 standard in 2012, IEEE 802.11p (and its related standards, such as ITS-G5 in Europe) has been referred to as the standard for vehicular communication until 2017. Since it is ready and tested, it does not require fine synchronization, and it is based on a fully distributed medium access protocol, well known and fairly reliable. In addition, products are available and can be installed on board.  
The 70 MHz band is subdivided in seven channels, each one of 10 MHz: one channel is used as the control channel (CCH), whereas the remaining channels are for service (SCH) [22; 23]. In Table 2, we show the channel allocation in Europe for ITS G5 and in the US for IEEE 802.11p with also the maximum effective isotropic radiated power (EIRP) on the corresponding channels.  
IEEE 802.11p is based on 64 orthogonal frequency division multiplexing (OFDM) subcarriers of which 52 are used for transmission (48 data subcarriers and four pilots). Eight possible combinations of modulation and coding scheme (MCS) are provided; depending on the adopted MCS, the raw data rate varies between 3 and 27 Mb/s. The fast exchange of information is made possible by the IEEE 802.11p medium access control (MAC) level, which enables very efficient communication-group setup without much of the overhead typically needed in nomadic IEEE 802.11a/g Wi-Fi networks,  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**G5 Channel** & \begin{tabular}{c} **IEEE 802.11p** \\ **Channel** \\ \end{tabular} & \begin{tabular}{c} **Center Frequency** \\ **(GHz)** \\ \end{tabular} & \
\begin{tabular}{c} **G5 TX Power Limit** \\ **EIRP (dBm)** \\ \end{tabular} &
\begin{tabular}{c} **IEEE TX Power Limit** \\ **EIRP (dBm)** \\ \end{tabular} \\ \hline G5-CCH & 180 SCH & 5.9 & 33 & 23 \\ G5-SCH2 & 178 CCH & 5.89 & 23 & 44.8 \\ G5-SCH1 & 176 SCH & 5.88 & 33 & 33 \\ G5-SCH3 & 174 SCH & 5.87 & 23 & 33 \\ G5-SCH4 & 172 SCH & 5.86 & 0 & 33 \\ G5-SCH5 & 182 SCH & 5.85 & 0 & 23 \\ G5-SCH6 & 184 SCH & 5.91 & 0 & 40 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Channel allocation for ITS-G5 in Europe and IEEE 802.11p in the US.  
Figure 2: Applications roadmap of penetration of connected vehicles and ETSI priorities.  
simplifying the basic service set (BSS) operations in a truly ad hoc manner for vehicular usage through the so-called outside of context of BSS (OCB) mode, allowing communication between users without requiring authentication/association between them or with an access point [23].  
At the MAC layer, IEEE 802.11p adopts carrier sensing multiple access with collision avoidance (CSMA/CA). On the one hand, it allows a fully distributed and uncoordinated access to the wireless channel, with no need for a resource allocation procedure. On the other hand, however, it implies a not negligible resource waste due to frequent collisions as the channel use increases under heavy traffic conditions, impacting reliability. Regarding VSNs-based applications, the MAC layer does not offer any mechanism for reliable broadcasting, and can suffer from poor performance at a high traffic load, which will be the normal operating condition when the connected vehicles penetrate the market.