# Review and Comparison of Emerging Routing Protocols in Flying Ad Hoc Networks
## 4 Routing Protocols
### Beaconless Opportunistic Routing Protocol  
The opportunity network is a kind of ad hoc network that does not need a complete path between the source node and the destination node, and realizes network communication by means of the encounter opportunities brought by node movement. Traditional multi-hops wireless networks, such as MANET, wireless sensor network, and wireless mesh network, are all lacking a specific solution to deal with the interruption of network connection in difficult environments. Therefore, when the connection is interrupted, the network performance will decline significantly, even causing the network running to fail. However, in practical applications, the topology structure may change at any time, and its connectivity is often not guaranteed. Therefore, we need to study the routing protocol to find the data transmission path to ensure the efficiency of task completion. The advantage of beaconless opportunity routing is supporting and maintaining network connectivity under the high delays and intermittent links.  
4.1 Cross-Layer Link Quality and Geographical-Aware Beaconless Opportunistic Routing Protocol (XLinGO)  
[PERSON] et al. [51] introduced a cross-layer link quality and location-aware beaconless opportunistic routing protocol (XLINGO). Considering the instability of UAV network, the location information of nodes is adopted in the routing design, and the link quality, geographical position, and residual energy are taken into comprehensive consideration in the selection of nodes for relay. When choosing transmission mode, the author introduced dynamic forwarding delay (DFD) to define link status, which is calculated by the following equation,  
\[DFD=DFD_{Max}\times(\alpha\times link\ quality+\beta\times progress+\gamma\times queue\ length). \tag{11}\]  
where \(\alpha,\beta,\gamma\) are coefficients of each metrics, and they are equal to 1. If multiple video streams are transmitted at the same time, buffer overflow will occur in the intermediate nodes. In this paper, the sequence length of the intermediate nodes is considered to prevent the overload of the nodes. Therefore, it ensures load balancing of nodes, reduces packet loss rate and transmission delay.  
Then, the link recovery mechanism is set to deal with link failure by detecting link quality. For this special self-organizing network, the efficiency of video stream transmission is improved.  
**Advantages:** (1) A forwarding node with a minimum dynamic delay and a high threshold energy is selected to ensure successful forwarding of data packets. (2) In order to reduce delay during video data transmission, this paper introduced a persistent transmitting mode; the timeliness and reliability of video transmission are guaranteed.  
**Disadvantages:** (1) The impact of UAV mobility has not been effectively addressed. If only consider a single factor of distance, then the communication defects, delays and packet loss during transmission cannot be avoided. (2) Equal importance is given to each type of environmental information, and no analysis of which environmental indicators has a greater impact on video quality  
\begin{table}
\begin{tabular}{l c c c c c c} \hline \hline  & **PAKET** & **CLUSTER** & **CLUSTER** & & & \\
**PROTOCOL** & **DELIVERY** & **BUILDING** & **LIFE** & & & \\  & **RATIO** & **TIME** & **TIME** & & & \\ \hline
**EECA**[47] & x & x & x & ✓ & ✓ & ✓ & ✓ \\
**EALC**[49] & x & ✓ & ✓ & ✓ & x & x \\
**BICSF**[50] & ✓ & ✓ & ✓ & ✓ & x & x \\ \hline \hline \end{tabular}
\end{table}
Table 4: Comparison of metrics used in cluster-based routing protocols.  
levels. (3) The proposed prediction mechanism does not consider predicting the change in the distance from the current node to the next hop, so routing failures cannot be predicted in advance.