## 2 Vehicular Networks
### DSRC-Based Networks  
Standard organizations have allocated different frequency bands to different regions for efficient VANETs communication. This frequency spectrum allocation provides multi-channel capabilities with minimal collision and congestion during transmission. In the US, the Federal Communication Commission (FCC) allocated 75 MHz bandwidth for DSRC in the frequency band of 5.9 GHz [15]. Similarly, in Europe, the European Telecommunications Standards Institute (ETSI) provides vehicular communication for V2V and V2I with an allotted range of 50 MHz bandwidth in the frequency band of 5.855 to 5.905 GHz.  
On the other hand, China has been provided by the Ministry of Industry and Information Technology (MIIT) with a dedicated 20 MHz and bandwidth range in 5.905-5.925 GHz. In Korea, the Ministry of Science and Information and Communications Technology(MSIT) allocates a frequency band of 10 MHz for V2I communication with a bandwidth of 5.895-5.905 MHz for CCH and SCH. For Japan, the Association of Radio Industries and Businesses (ARIB) is the organization that allocated 80 MHz as the frequency for DSRC, with bandwidths of 5.770-5.850 GHz and 755.5-764.5 MHz [16].  
The WAVE architecture has seven channel divisions, each with 10 MHz, and 5 MHz is kept aside for future use as backup [17]. One of these channel divisions is reserved for safety applications through control channels (CCH), and the rest are for safety and non-safety applications through service channels (SCHs). The transceivers sense these fixed channels for multi-channel access at the same time interval without collision. Orthogonal frequency division multiplexing (OFDM) is used in the WAVE standard for interference avoidance during transmission [18], with data rates of 6-27 Mbps. The ITS-G5 standard uses 2 amplitude-shift keying (2 ASK) or 2 phase-shift keying (2 PSK), with data rates of 6-12 Mbps; quadrature phase-shift keying (QPSK) supports data rates of 1-4 Mbps. These are some of the modulation techniques used for vehicular communications. Vehicle-to-everything (V2X) communication with LTE and 5G has been adopted along with QPSK [19], which provides a high data rate for better transmission efficiency compared with OFDMA and 2 ASK. Table 1 summarizes these standards.  
In IEEE 802.11 [20], the distributed coordination function (DCF) deals with medium access based on CSMA with collision avoidance (CSMA/CA). Here, the device first listens to the network channel before transmitting for collision avoidance. In IEEE 802.11, the main focus is on RTS/CTS/ACK mode of packet exchange to access the medium. A network allocation vector (NAV) is set according to the transmission duration indicated by RTS. However, CSMA/CA is not suitable for real-time scenarios due to its inherent channel access delay. The vehicular networks incorporates dedicated short-range communication (DSRC) for enriching the driver's comfort and safety. Wireless access in vehicular environment (WAVE) [21] defines the IEEE 802.11p for MAC layer implementation in VANETs. The DSRC documents the physical (PHY) and medium access control (MAC) layers of the WAVE stack [22]. To achieve quality of service (QoS), the WAVE stack of IEEE 802.11p for a MAC protocol incorporates an enhanced distributed channel access (EDCA) mechanism [23]. The messages are divided into four types based on access priority, as access category AC[0]\(\sim\)AC[3] with separate contention windows and frames set for each category.