# A Reliable Low-Latency Multipath Routing Algorithm for Urban Rail Transit Ad Hoc Networks
## 3 The Improved Multipath Routing Algorithm
### Adaptive Multipath Selection Algorithm Based on Service Requirements  
As a variety of services need to be carried out in the urban rail transit vehicle-ground communication system, a unipath routing algorithm is used for data transmission, which cannot meet the communication needs of different services. on the one hand, and on the other hand, network congestion easily happens, which causes data loss and affects the efficiency and reliability of the vehicle-ground communication system. Therefore, how to meet the communication needs of different services is an important issue. Multipath routing technology has the features of providing different paths for communication services with different QoS requirements and providing multiple paths for data of the same communication services. So, using multipath routing algorithms, it is possible to flexibly carry out several services and also meet the communication requirements of different services. However, traditional multipath routing algorithms, such as the AOMDV, take a lot of time to flood RREQ messages in the network during route discovery. So, the algorithm proposed in this paper addresses the above problem and speeds up the route discovery. Generally, end-to-end delay includes transmission delay, processing delay, queuing delay, and propagation delay. Existing routers are generally high-speed routers with the processing latency of microseconds or even lower, so it is unnecessary to consider it. In the process of route discovery, it is necessary to first send RREQ to the destination node and then reply to RREP to the source node; the latency of route discovery [15] is shown in Equation (1). \[T_{f}=2*hop(i)*(\frac{L_{R}}{B}+T_{queue}+T_{propagation}) \tag{1}\]  
in which \(hop(i)\) is the number of hops from the source node to the destination node, \(i\) represents the number of independent disjoint paths that are allowed to be discovered, \(L_{R}\) is the control message size, \(B\) is the transmission bandwidth, \(T_{queue}\) is the queue time of each hop, and \(T_{propagation}\) is the propagation time of each hop. In the route discovery process of the on-demand routing protocol, the source node needs to send RREQ to find the routing information of the destination node, and when the routing information is found, the node replies to the source node with a route reply (RREP) message, which completes the establishment of the link. Therefore, combined with Equation (1), it is known that the number of hops is the main influencing factor on the route discovery time. So, this paper proposes a static configuration routing scheme for urban rail transit networks. The scheme enables the source node to obtain the routing information of the destination node from the routing table of the intermediate node during the route discovery process to reduce the number of control message forwarding and decrease the route discovery latency. Providing a different number of paths for different services can more fully utilize routing resources and relieve the network load effectively. In this paper, a multipath selection algorithm based on service requirements is proposed. The algorithm constrains the number of paths according to the service latency requirements and then selects transmission paths based on the link state. Now, assume that the nodes in the network have the samerouting performance, and because the network range is small, when the mobile node runs at 120 km/h, the propagation delay is also in microseconds, so the transmission delay and queuing delay are mainly considered in the estimation of delay. The packet arrivals obey Poisson distribution. According to the knowledge of queuing theory, \(\lambda\) represents the average amount of data arriving in unit time, \(\mu\) represents the average amount of data processed in unit time, and each link can be considered as a standard M/M/1 model. Thus, the average arrival rate and service rate with the multipath parallel transmission mode are \(\lambda M\) and \(\mu M\), and \(M\) is the number of parallel paths.