# Cross-Layer Methods for Ad Hoc Networks--Review and Classification
## 6 Future Directions  
There is a lack of cross-layer methods concentrated on applications in which applications perceive network state, communicate with each other (inside one node), and generate data flows at the necessary rate or with a rate as close to the network congestion occurrence as possible. The transport layer is responsible for congestion control, but congestion control is usually not QoS sensitive; multiple applications will be given equal access to available bandwidth. Therefore, application congestion control is capable of achieving QoS requirements better than other layers or, at least, acknowledging that QoS requirements cannot be met. Cross-layer methods concentrated on routing protocols with route selection based on physical layer antenna characteristics (beamforming capabilities, number of antenna sectors etc.) are still not completely developed. There were quite a few routing protocols created that consider antenna properties, but in an abstract way, never using data about antenna patterns (dependence of antenna gain from elevation and azimuth angles) to the authors' knowledge. [PERSON] et al. [114] consider ad hoc networks with random beamforming. Nodes steer beams in random directions, eventually scanning all angular space and, with some probability, obtain reply packets. Routes are constructed based on line existence probability. [PERSON] et al. [115] propose a routing algorithm for ad hoc networks with sector antennas. Nodes in that network can transmit or receive simultaneously in multiple sectors without intersector interference. Sectors are effectively considered as additional lines compared to the one omnidirectional antenna. [PERSON] et al. [116] use q-learning and ant-colony algorithm optimization to jointly select beam and transmission power. The routing protocol of [PERSON] et al. uses the ratio of node beams to node the neighbors' number. Nodes with bigger ratios are preferred during route construction as their lines have less interference. [PERSON] et al. [117] propose the route request scheme dealing with the problem of the impossibility of receiving route replies from all sides when using directional antennas. One can estimate interference of packet transmission using knowledge about antenna patterns and transmission power. Choosing routes with directive antennas transmitting packets with different antenna patterns at each hop can minimize network interference and packet collisions and increase network capacity. In particular, multicast routes can benefit from antenna transmission pattern selection because multicast routes are trees. Sometimes, a node can have multiple children, and then an omnidirectional antenna pattern is preferable. Sometimes, a node can have one child, and then a directive pattern is preferable. It should be noted that thorough research was conducted regarding potential ad hoc network capacity using directive antennas such as sector antennas and beamforming antennas [118; 119; 120], the channel layer protocols development [55; 56; 57; 58; 59; 60; 121; 122; 123; 124], and fast neighborhood discovery protocols for directive antennas [125; 126]. However, channel layer protocols optimize line performance, not flow performance. Furthermore, cross-layer methods concentrated on routing protocols rarely use estimated data flow rates from application or transport layers. If the flow rate is low, the routing protocol can be lenient in choosing the route because a low rate can hardly cause network congestion. The contrary situation occurs for high bandwidth flows. Cross-layer methods with external entity multiple-layer control and optimization based on fuzzy logic lack a theoretical base because, in all publications, to our knowledge, fuzzy rules are chosen empirically. Automated heuristics are much needed for fuzzy rule set creation. As for dual decomposition optimization applications, there are many more heuristic optimization methods that are still not applied to multiple-layer control.