# Cross-Layer Methods for Ad Hoc Networks--Review and Classification
## 1 Introduction  
In this work, we present the classification of cross-layer methods in ad hoc networks and consider the methods themselves. An ad hoc network is a network of mobile radio nodes with no infrastructure: without gateways, access points, base stations, etc. (Figures 1 and 2). A classic, well-established approach to network protocols is to split protocols into OSI model layers. This is a very effective approach for wired networks with reliable communication channels where the performance of protocols of different layers does not depend on the state of other layers. In the case of a mobile ad hoc network with dynamic topology (with radio channels exposed to noise, interference, no line of sight, etc. ), the insulation of layers leads to sub-optimal network operation. The use of cross-layer methods in the development of protocols of ad hoc networks allows achieving optimal performance for ad hoc networks. Here and further, cross-layer methods are considered methods with no layer isolation, intensive exchange of information between layers, and joint layer management. Ad hoc networks have the following properties [1]:  
1. **Self-organization. ** Nodes are organized into networks and manage the network independently and in a distributed manner without centralized intervention. Each node acts as both the user and router [1]. 2. **Peer network. ** Nodes of ad hoc networks are equal and perform the same functions [1]. 3. **Limited resources. ** Ad hoc networks usually consist of portable devices with low computing power, memory, and battery charge [1]. 4. **Network robustness. ** The failure of any network node almost never affects the performance of the network as a whole, unlike the failure of the central router of classical networks [2; 3]. 5. **Dynamic topology. ** The network nodes move relative to each other. As a result, node connections and routes between nodes appear and disappear [2; 3]. * **Limited constantly changing capacity of radio lines**. Radio lines are susceptible to interference from neighboring nodes, noise from outside, multipath fading, and the Doppler effect [4]. * **Shared channel resource**. The lack of dedicated channel resources (the contention of nodes over channel resources) leads to interference and packet collisions, which reduces network bandwidth [4]. * **Vulnerability to attacks**. In classical networks, all data from a single subnet passes through one powerful firewall, and an attack from outside the network is directed only to the firewall. In the ad hoc network, each node is equally vulnerable to attacks from outside [5]. * **Large amount of service data for routing**. It is necessary because of ad hoc network dynamic topology to send more (than static networks) service information about the lines state, etc., to find network routes [4]. * **Frequent network fragmentation**. Network mobility can cause the ad hoc network to be divided into isolated subnets [4]. * **Complexity of providing QoS**. The poorly predictable instability of radio lines, interference of nodes, frequent route breaks, and delay in finding a new route all make the task of providing QoS in ad hoc networks very difficult [4]. Ad hoc networks are often used in the following areas [1]:  
1. **Military use**. It is difficult to deploy a cellular network in a warzone, but an ad hoc network can be deployed almost instantly. 2. **Rescue and search operations**. In the event of a disaster, the local infrastructure is destroyed, and in the case of search operations in sparsely populated areas, the infrastructure initially does not exist. The rapid deployment of an ad hoc network allows immediate rescue and search operations. 3. **Distributed file sharing**. During a meeting, such as a conference, one can quickly deploy a network to share files and presentations. The International Organization for Standardization (ISO) created sub-cluster 16 (SC16) in 1977 to develop architecture for standard protocols. In 1979, the OSI [6] was created.