# Multi-Layer Problems and Solutions in VANETs: A Review
## 2 Multi-Layer Challenges in VANETs
### Multi-Layer Challenges
#### 2.1.2 MAC Layer  
The MAC Protocol plays a crucial role in fair channel access. The scalability of VANETs also relies heavily on the MAC Protocol, so developing an efficient and reliable MAC protocol is very important. Currently the IEEE 802.11p MAC along with the 1609. Multi-Channel Operations is being used as the MAC standards for vehicular communications under the WAVE technology [4]. Most research in the recent past focus on efficiently utilizing the multi channels and the performance modeling of the EDCA Mechanism to develop an analytical model in order to study the behavior of the MAC Layer. Following things should be kept in mind while designing MAC Protocol for VANETs:  
1. Bandwidth Limitation: The frequent and rapidly changing topology in the VANETs contributes to the increase in the Control Overhead. The Bandwidth limitation should be kept in mind while designing a MAC Protocol in VANETs. 2. Analytical Study of the MAC Layer: It's very important to understand the nature of the MAC Layer in order to design an efficient protocol to fulfill the performance related requirements. A lot of work has been done using the stochastic modeling [8]. The performance modeling and analysis require certain criteria to be fulfilled including saturated/unsaturated conditions, number of access categories, back off counter freezing, internal/external collisions, mobility of the nodes, and computational complexity [9]. Using the Markov Chains, we can compute the transmission and collision probabilities. The analytical model results can be compared with the simulation results. An accurate model will make the behavior of MAC understandable. Currently most of the analytical modeling has been done for the IEEE 802.11p Enhanced Distributed Channel Access (EDCA) mechanism [10; 11; 12]. 3. Dynamic Adjustment of the Contention Window (CW): A node in a VANET should be able to detect the congestion of the network by the received packets sequence and should be able to adjust the CW size accordingly with the help of successful packets received [13]. A number of algorithms have been proposed in literature that are used for the Dynamic Adjustment of CW with better reception rates proved through simulation [14; 15; 16]. This adjustment of CW is closely related to the Congestion Control as a default strategy to avoid congestion in CSMA/CA. 4. Congestion Control: The Decentralized Congestion Control (DCC) can work as a Cross Layer Approach by adjusting the Physical layer parameters by using the MAC layer as a channel sensing mechanism. Several Congestion Control algorithms exist for MANETs but they are not very well suited for VANETs due to the dynamic nature of network. The congestion control solutions provide high throughput (it is important to note here that for urgent safety messages, a fraction of bandwidth should be left, i.e, keep the channel load below a certain threshold). There are two main types of congestion control algorithms: Proactive and Reactive. Although now the third type known as the Hybrid Algorithms are also being researched. The key element in designing congestion control algorithms is fairness which means that the vehicles in the vicinity of each other should participate in controlling the congestion. The congestion control algorithms focus on the transmission power adjustment and the message rate adjustment [7]. In the next section we will discuss these algorithms. 5. Synchronization: Synchronization in VANETs is in a centralized manner. Currently the Universal Time Coordination (UTC) is used for the Synchronization purpose. Distributed synchronization strategies are needed along with the centralized synchronization between nodes in order to coordinate in the case of system wide failure which may result in the unavailability of the Global Clock [17]. 6. Efficient Utilization of the Multi Channels: The Multi-Channel Operation was introduced in IEEE 1609.4 [18]. Currently there are 7 channels, which include the service channel (SCH) and the control channel (CCH).