# Cross-Layer Methods for Ad Hoc Networks--Review and Classification
## 3 Cross-Layer Method Publications Review
### One Layer
#### 3.1.3 Network Layer  
Cross-layer methods concentrated on the network layer consist of cross-layer routing protocols and cross-layer routing metrics, which can be used to turn a one-layer routing protocol into a cross-layer routing protocol. _Network layer (routing metrics)_  
Cross-layer routing metrics are composite metrics based on metrics collected from multiple layers (Table 6). Cross-layer routing metrics are used for existing routing protocols, turning routing protocols into cross-layer routing protocols. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Publication** & **Year** & **Citations** & **Layers** & **Goal** \\ \hline
[55] & 2005 & 54 & 1, 2 & line throughput \\
[56] & 2008 & 32 & 1, 2 & line throughput \\
[57] & 2008 & 100 & 1, 2 & line throughput \\
[58] & 2011 & 52 & 1, 2, 3 & line throughput \\
[59] & 2011 & 67 & 1, 2, 3 & line packet error rate, energy consumption \\
[60] & 2014 & 26 & 1, 2 & line throughput, line latency \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Cross-layer methods concentrated on channel layer cooperative transmission. [PERSON] et al. [61] propose a route reliability metric as the probability of successful packet delivery across the route. Successful packet delivery rate calculation is based on the received packets' signal-to-noise ratio from the physical layer. [PERSON] et al. [62] propose two metrics: the metric of expected transmission time (ETT) and the weighted cumulative expected transmission time (WCETT). The ETT metric improves the expected transmission count (ETX) metric [74] (ETX is equal to the sum of hops, including retransmissions) by measuring the transmission speed. The ETT metric is equal to the ETX metric multiplied by the ratio of the packet size to the transmission speed. The WCETT metric improves the ETT metric by giving a lower score to routes in which subsequent lines use the same channel resources (data about channel resources is gained from the channel layer) to reduce user interference. However, this metric does not account for interference from other routes. [PERSON] et al. [63] propose the metric of interference and channel-switching (MIC). The MIC metric improves the WCETT metric by accounting for interference from other routes. The metric uses the number of adjacent nodes for each line of the route to evaluate interference from other routes. The standard 802.11 s [64] uses the ALM metric (Airtime Link Metric). This metric measures the time required to deliver a packet of 8224 bits between the sender and receiver. To obtain this metric, the network layer uses the following data from the channel layer: number of bits in the frame, line transmission speed, probability of frame error, channel access delay, and frame service field size. [PERSON] et al. [65] propose the metrics for estimating the probability of route packet loss, the probability of line packet loss, and the remaining battery charge of the node from the power of the received signal. [PERSON] et al. [66] propose a metric defined as the ratio of the time taken to transmit a packet to the sum of the transmission time and the time taken to wait for the channel to be free. If this metric is small, the channel is less congested, and the line has a low transmission delay; in this case, the line is more suitable for real-time applications. [PERSON] et al. [67] offer a route metric for ad hoc networks with radio links with multiple transmission rates. The metric takes into account the maximum allowable transmission rate and the number of packet retransmissions. The selection of the lowest metric route ensures a high bandwidth route and prevents the selection of low bandwidth routes. A small metric value means that the route has a few intermediate nodes. The metric is based on the data of a number of packet retransmissions from the channel layer and the available line transmission speeds from the physical layer. [PERSON] et al. [34] use a weighted sequence of the signal-to-noise ratio and packet loss probability data from the channel layer to estimate line reliability.