# Internet of Drones: Routing Algorithms, Techniques and Challenges
## 5 Future Research Directions
### Packet Mobility in IoDs  
Seamless mobility of packets is a key concern in IoDs. Telecommunication models identify and create methods for hop-by-hop data communication in the form of bits and  
\begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Category** & **Routing Technique** & **Facilities** & **Challenges** \\ \hline \multirow{8}{*}{Network Structure Based Routing} & Flat Routing & Wake-up schedule of sensor nodes and trajectory of UAV & Reliable data collection in the fading channel \\ \cline{2-4}  & Linear Sensor Routing & Nodes remain between two parallel lines that stretch for a long transmission distance & Multi-hop routing can cause high energy dissipation \\ \cline{2-4}  & Cluster-Based Routing & All nodes are allowed to make independent decisions, coordinated clustering & Finding new routes results in significant routing overhead \\ \cline{2-4}  & Tree-Based Routing & Parent node acts as sink node & Overhead in broadcasting and errors in data reconstruction process \\ \cline{2-4}  & Location-Based Routing & UAV broadcast geographical location and clock time & Time required to empower sensor nodes unconsidered \\ \hline \multirow{8}{*}{Protocol Operation Based Routing} & Swarm Intelligence Routing & Determining the network topology and use of UAV for data collection & Wind effect is travel time of UAVs to be taken into account [2] \\ \cline{2-4}  & Multi-Path Routing & Aims to reduce the distance between senders and receivers to obtain better channel quality & Efficient data gathering is a challenging task [2] \\ \cline{2-4}  & Shortest Path Routing & Voronoi diagram provides feasible UAV routing path & Minimizing the UAV overall trajectory distance [2] \\ \hline \hline \end{tabular}
\end{table}
Table 3: Summary of routing techniques and challenges.  
Figure 9: Summary of IoD routing protocols.  
frames. The OSI model is widely accepted by TELCOs around the globe. For data packet organisation and delivery, the OSI model is the most viable and widely used. It has seven-layer architecture, but for IoDs, only the lower three layers are considered for our IoD applications. For packet mobility, there are three OSI model layers to be considered. First is the physical layer for a better connectivity; the second layer is the data link layer for network access, and third is the network layer for a lossless data transfer between IoD nodes. Consider Figure 10 in which a flow diagram is given for packet mobility inside a UAWSN.  
It is evident from the Figure 11 that each upper layer is dependent upon the bottom layer and vice versa. Considering the conventional fashion of OSI layers, the physical layer allows multiplexing techniques, namely frequency division multiplexing (FDM), time division multiplexing (TDM), or space division multiplexing (SDM), and allows data to be aggregated. The data link layer is responsible for frame interleaving, and the network layer identifies the nodes, the packet path, and its organisation from symbols. In IoD, there can be a large number of nodes present so the data is organised and forwarded accordingly. Figure 11 shows a layer-wise packet transfer in IoT.  
Multiplexing is a requirement for transmission systems in order to utilize the available bandwidth and decrease the delay. For the physical aspect, the frequency, time, or space division multiplexing takes place. At the data link layer, the formation of bits into frames takes place. Data then get transmitted in frames for set periods of time. The network layer defines the nodes, the paths, and the packets in the form of symbols.