# Moving Ad Hoc Networks--A Comparative Study
## 3 Vehicular Ad-Hoc Networks (VANET)
The ninth working group of the European DSRC standardization working group CEN/TC278 started the drafting of the DSRC standard in 1994 and passed ENV12253 \"5.8 GHz DSRC physical layer\" in 1997, ENV12795 \"DSRC data link layer\", and the ENN12834 \"DSRC application layer\" standard. In 1997, the TC204 Committee of the Japanese DSRC Standardization Working Group completed the DSRC standard formulation work. In 2001 and 2004, they issued two standards, ARIB STD-T75 and ARIB STD-T88, respectively. In 1998, the US Federal Communications Commission allocated the 5.850-5.925 GHz (75 MHz) frequency band to short-range communications in the field of transportation services [13]. ASTM passed the DSRC standard E2213-02 in 2002 and passed its improved standard E2213-03 in 2003. The standard E2213-03, the IEEE 802.11p and IEEE 1609 working groups began to formulate wireless communication standards in the vehicle environment. In 2006, the IEEE passed the IEEE 1609.1-1609.4 series of standards. In July 2010, the IEEE 802.11p standard was officially released. This standard is the physical layer  
Figure 5: VANET Communication. and MAC layer standard of DSRC. It is an extension of the IEEE 802.11 standard for related applications in ITS [14]. The wireless access methods that may be used in these three domains include IEEE 802.11p (Ad-hoc domain), Cellular Networks (2G/3G/4G for the infrastructure domain), WLAN (802.11 a/g/n/ac for the infrastructure domain or in-vehicle domain), and Bluetooth (in-vehicle domain), etc. [15]. VANET applications may use more than one communication method, such as eCall, which uses GPS positioning and a 2G network. When major sensor signals such as airbag deployment are received by eCall devices, the communication module will be automatically activated and dial local 112. It will report the vehicle GPS coordinates, accident time, license plate number, and other information, and establish voice communication with the local rescue team as soon as possible. A typical VANET communication is shown in Figure 6.  
A typical feature of in-vehicle communication is that, for different applications, the protocols of each layer can vary greatly, and it is often a cross-layer consideration or a simplified layer consideration. The WAVE protocol stack given by IEEE is currently the most authoritative protocol architecture: the physical layer and data link layer are composed of IEEE 802.11p, IEEE 1609.4, and IEEE 802.2; the network layer and the transport layer have two sets of protocols, the traditional TCP/IP protocol and the IEEE 1609.3 protocol, which are specially designed for in-vehicle safety applications; the application layer distinguishes safety applications and non-safety applications, and the SAE protocol is introduced as a message sublayer for safety applications; finally, there is a cross-layer IEEE 1609.2 as a safety protocol. Characteristics of VANETs. Figure 7 shows the Characteristics of VANETs:  
* The topological structure of the nodes in a VANET changes rapidly, and the rapid mobility of vehicles determines the frequent changes of the topological structure in the onboard self-organizing network, which greatly shortens the survival time of the communication link between two vehicle nodes. The usual solution is to extend the lifetime of the link by increasing the transmission power, but the increase in the transmission power and the increase in the communication distance also reduce the throughput of the network. * The rapidly changing topological structure makes it difficult to establish an accurate neighbor node list, and it becomes impractical for each node to obtain and maintain the global topological structure of the entire network. Therefore, the protocol based on the network's topological structure is not suitable for the vehicle-mounted self-organizing network. * High-speed vehicle driving brings rapid channel fading and a severe doppler effect. At the same time, the wireless channel quality is very unstable due to the influence of the road conditions, tall buildings, and trees around the road.