# Cross-Layer Methods for Ad Hoc Networks--Review and Classification
## 1 Introduction
The OSI model consists of seven layers: physical layer, channel layer, network layer, transport layer, session layer, presentation layer, and application layer. The task of the OSI model is to group similar communication functions into logical layers. The layer can only communicate with the upper and lower adjacent layers. However, with the development of the transport layer TCP protocol and network layer IP protocol, the TCP/IP model became dominant, consisting of five layers: application layer, transport layer, network layer, channel layer, and physical layer [6; 7; 8]. Layer architecture works very well in wired networks with stable  
Figure 1: Network classification. connections and quasi-permanent topology. The layer isolation of different protocols allows for modular independent development of these protocols regardless of other layers. That speeds up the development and performance of the network. This is possible because events at different layers have little impact on each other. In ad hoc networks with dynamic topology and difficult radio communications conditions, events at different layers may influence each other significantly. Therefore, to achieve high performance of an ad hoc network, one should not use an architecture with isolated layers [9; 10; 11]. During the analysis, we constructed the scheme (graph) of the interdependence and influence of OSI layers on each other (Figure 3). Almost all layer influence is indirect, with the help of intermediate events. The only direct influence is transport layer congestion control, which forces the application layer to slow down transmission. In Figure 3, we can, for example, trace the following circular indirect layer influence with the positive feedback loop on the high packet corruption rate. To guarantee a high probability of packet delivery, the packet needs either to be retransmitted multiple times at the channel layer, or it is necessary to increase transmission power at the physical layer. Transmission power increase or multiple packet retransmissions can cause excessive interference to other nodes, which causes a low probability of successful packet delivery. At the network layer, the current route ceases to exist because of the low probability of successful packet delivery, so it is necessary to start the process of new route discovery by broadcasting route request packets, and route request packets cause even more interference. Cross-layer methods strive to improve the performance of ad hoc networks by collaboratively developing protocols using multiple-layer interaction [12; 13; 14]. Compared to the layered OSI approach, the cross-layer approach allows interaction between layers, reading and changing the parameters of some other layers [15; 16; 17; 18]. Cross-layer methods have emerged from the desire to ensure interaction and joint optimization of multiple layers [19]. For example, the physical layer can select data rate, power, and coding to meet application layer requirements. Cross-layer design is also indispensable in increasing battery lifetime (minimizing energy consumption) [20; 21] and in optimizing the end-to-end latency for real-time applications [22; 23], as energy consumption and latency depend on all layers. The theoretical justification of the effectiveness of cross-layer methods is given in one of the first works on network coding [24]. Network coding is the process of using  
Figure 2: Ad hoc network. network parameter settings to meet all the data flow transmission requirements. Reference [24] shows that performing only routing by intermediate nodes results in suboptimal network performance. Cross-layer interactions can be subdivided into two large categories [25]:  
1. **Provision of information**. Adjacent and disconnected layers share information through interfaces or by using a common database. 2. **Layer integration**. Layer separation disappears, and all layers except maybe the application layer are merged into one. There are two main architectures for cross-layer methods (Figure 4): \"MobileMan\" [26] and \"CrossTalk\" [27].