# Cross-Layer Methods for Ad Hoc Networks--Review and Classification
## 1 Introduction
The \"MobileMan\" architecture [26] preserves the original layer architecture (Figure 4). The main achievement of this architecture is the creation of a central database that collects all network state data that can be used by various layers individually. \"Mobileman\" is the \"provision of information\" kind of cross-layer interaction. The \"CrossTalk\" architecture [27] introduces the notion of global network state and local node state (Figure 4). Global information is collected using the messages from neighboring nodes. An important difference from the \"MobileMan\" architecture is that in \"Crosstalk\", the cross-layer entity controls all layers, while in \"MobileMan\", the layers tune themselves separately using each other's information. \"Crosstalk\" is the \"layer integration\" kind of cross-layer interaction. Figure 3: The interdependence of OSI layers in an ad hoc network. Cross-layer methods, based on the \"MobileMan\" and \"Crosstalk\" architectures, can be presented in terms of iterative optimization of multiple-layer configurations [11] to achieve QoS requirements for the data flows (Figure 5). There are the following difficulties in developing cross-layer methods [11; 28; 29; 30; 31; 32]:  
* **Cross-layer method parameter selection**. One of the fundamental questions of cross-layer methods is the choice of layer parameters on the basis of which decisions about network operation are made. It is very important to consider the physical layer parameters, but it is also important to consider the application layer parameters. For example, the received signal strength indicator (RSSI) is a good indicator of signal power, but this parameter does not take into account noise and interference. The noise level is taken into account in the signal-to-noise ratio (SNR), but interference is also not included. The signal-to-interference + noise ratio (SINR) parameter is much more useful because it simultaneously takes into account the signal strength and noise. With the SNR [33], one can estimate the proximity of the nodes (i.e., the nodes are within each other's radio connection). Queue occupancy information is available at the channel layer. In addition to the above parameters, parameters that provide information about data flows are also used. As shown in [34; 35], another approach is to use composite parameters obtained by combining many other parameters. * **Layer interdependency and the complexity of independent development**. As shown in [9], cross-layer protocol design is nothing more than an OSI violation in which interlinked layers interact. The modification and improvement of a highly interconnected system is difficult. However, achieving modularity is one of the most difficult tasks in developing cross-layer methods. * **Overly complex interaction between layers**. The flow of information between the different layers creates unforeseen relationships between layers, reducing the efficiency of the entire system. Therefore, it is necessary to choose the interaction between layers very carefully. Figure 4: Two main cross-layer architectures: “MobileMan” and “Crosstalk”. Early reviews of cross-layer methods in ad hoc networks reviewed possible approaches to the development rather than existing methods. [PERSON] et al. [9] introduced one of the first classifications for cross-layer method development approaches. The classification consists of four approaches: the creation of new interfaces between layers, the merging of adjacent layers, the development of fixed lower layers without creating new interfaces, and the holistic optimization of all layers. [PERSON] et al. [36] approached the development of cross-layer methods by dividing them into three types: the use of event signaling between layers, the joint optimization of layer subsets, and the holistic optimization of all layers. More recent reviews provide reviews and classifications of methods themselves. References [37; 41] provide a classification of methods according to the layer combinations used in methods.