# A Reliable Low-Latency Multipath Routing Algorithm for Urban Rail Transit Ad Hoc Networks
## 2 The Network Model  
In ad hoc networks, the common network structure is a planar network topology, whose characteristics include a high density of network nodes; all nodes have the same function, nodes can cooperate to complete the work, which has better robustness, and the node load is more balanced. In a flat network, using traditional proactive routing protocols requires all network nodes to maintain global routing information for a long time and to update routing information at a time when the network topology changes, so this requires a significant maintenance overhead. Using on-demand routing protocols, the route discovery process requires the multiple-forwarding of RREQ messages, resulting in a lot of time being consumed, which seriously affects the transmission efficiency. In this paper, combining the characteristics of an urban rail transit ad hoc network, the network is divided into several areas based on the maximum communication range of nodes, some trackside nodes are distributed in each area, and the routing information is stored in the routing table of trackside nodes in advance through using the characteristics of stationary trackside nodes, so the intermediate nodes have the routing information to reach the station sink nodes, and each node only needs to maintain the routing information of their respective paths, thus reducing the routing overhead. Finally, the network topology of the train and trackside nodes is shown in Figure 1.  
In the urban rail transit ad hoc network, the trackside nodes are static and linearly distributed around the trackside, the train nodes move along the tracks dynamically, and sink nodes are concentrated in the stations. In Figure 1, it can be seen that there are mobile nodes, trackside nodes, and sink nodes in the network, and each type of node has different ways of working. The functions and working modes of each node are as follows. 1. Mobile Nodes. This type of node essentially refers to the wireless terminal located at the front or rear of the train, which combines routing and control management func \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameters** & **AODV** & **AODDV** & **RLLMR** \\ \hline protocol type & on-demand & on-demand & hybrid \\ \hline discovery latency & long time & long long time & short time \\ \hline repair latency & NO & long time & short time \\ \hline reliability & average & excellent & slightly worse than AOMDV \\ \hline throughput & fair & better than AODV & better than AOMDV \\ \hline route repair method & NO & alternate path repair & local routing repair \\ \hline route selection & minimum hop count & minimum hop count & low hop count and good quality \\ \hline \hline \end{tabular}
\end{table} 
Table 1: A comparison of routing protocols. Figure 1: The network topology of the train and trackside nodes. tions. First, this type of node has the ability to aggregate the respective business data within the train carriages. Second, it is responsible for sending information to ground facilities. This type of node has a relatively small number in the network and is mainly responsible for carrying out various services of vehicle-ground communication. In the process of vehicle-ground communication, routing discovery is performed first, followed by data forwarding. 2. Trackside Nodes. It can also be called the relay node. This type of node is numerous, and different trackside signaling facilities can all serve as trackside nodes after adding routing functions. This type of node is statically distributed near the trackside in a linear pattern in the network, and the trackside nodes can be powered by underground cables. Therefore, there is no need to consider the issue of insufficient node energy during the research process. During the static configuration phase, routing information such as ground sink nodes and some special nodes will be written in advance in the routing table.