# Multi-Layer Problems and Solutions in VANETs: A Review
## 2 Multi-Layer Challenges in VANETs
### Proposed Solutions
#### 2.2.2 MAC Layer
These algorithms sense the channel periodically, measure certain parameters, and perform a comparison with a predefined threshold. It is a closed loop strategy [36] and comes into play only after the congestion is detected in the network. It includes the Power or Rate based Congestion Control [37]. * Hybrid Congestion Control: The Hybrid strategies combine the advantages of both Proactive and Reactive Algorithms. * Scheduling based MAC Protocol: The author in [38] has performed an intensive survey on the use of TDMA protocol in VANETs. There are two approaches for the TDMA protocol: Distributed and Centralized. The Distributed approach includes the Self Organized TDMA which has been extensively researched in the past for its good performance in VANETs. The Centralized approach includes the Cluster Head and the RSU. In this approach, a central node is involved in communication. The central node act as a master and communication is managed by it. Figure 3: Congestion Control Mechanism. The Scheduling based protocols like TDMA offer high throughput, low collision rate, bounded access delay, fairness and efficient utilization of channel compared with Contention based protocols. Furthermore, [PERSON], et al. [38] also discusses the different protocols under each approach with their pros and cons. Nine different TDMA variants exist for a fully distributed VANET and eight different variants for the Cluster based centralized approach. These protocols perform well in different scenarios and their performance depends on several factors including the topology of the network. The STDMA has been presented in several of the recent research papers [5; 39; 40]. The STDMA comes under the category of fully distributed VANET. A scalability comparison of STDMA with the contention based CSMA of IEEE 802.11p is performed in [3]. The results show that STDMA outperforms the CSMA even in unsaturated traffic conditions. Other works include the Stabilization Time Comparison of STDMA with CSMA [41], Hidden Terminal Problem when using CSMA and STDMA [42], and Prediction based STDMA [43]. There is still a large room for research on performance comparison between the TDMA variants for vehicular communication. However, the STDMA has not been standardized as an industrial standard for VANETs. * Hidden and Exposed Node: In order to solve the hidden node problem, some protocols have been proposed like the Multiple Access Collision Avoidance (MACA) and MACA for Wireless (MACAW) [44]. They use the Request to Send (RTS) and Clear to Send (CTS) packets to avoid collision. When a node wishes to send data to another node, it sends an RTS message. The receiver node sends a CTS message in response and communication is established between the two nodes. During communication the other nodes in the vicinity defer to transmit. This solves the hidden node problem. However, MACA suffers from exposed node problem and thus MACAW is used. The MACAW has an additional control packet, called the Data Sending (DS) packet. This packet makes the neighboring (exposed) node aware of time duration for transmission shown in Figure 4 Other protocols to solve this problem include the Floor Acquisition Multiple Access (FAMA) and Busy Tone Multiple Access (BTMA) [44]. While this solution sounds lucrative, it is, however only good for the unicast transmission. In broadcast scenarios, RTS/CTS handshake cannot be used because of frequent collisions. The broadcast nature of IEEE 802.11p CCH for the periodic Beacon transmission, impose a big challenge to VANETs in terms of reliability and latency [33]. * Contention Window size adjustment: One way of improving the performance is through the dynamic adjustment of the contention window. In order to provide a QOS in different application's access categories, a fuzzy logic control based dynamic adjustment of the contention window is presented in [45]. The proposed non-linear control scheme enhances the control of  
Figure 4: Exposed Node Solution in Multiple Access Collision Avoidance for Wireless (MACAW).