# Adaptive Downward/Upward Routing Protocol for Mobile-Sensor Networks
## 1 Introduction  
Wireless Sensor Networks (WSNs) have been widely developed in the last decade, and new applications are emerging rapidly. Most of these applications are used for surveillance and recording environmental or physical conditions [1]. In these applications, data collection is the basic traffic mode, where all traffic in the network is destined to a predefined destination called the sink node. We use the word upward to describe the transmission direction of packets from sensor nodes towards the sink. Thus data collection is also called upward routing in this paper. If we consider the Internet of Things (IoT) application domains, upward routing is not the only traffic mode in the network. The sink node needs to send commands to certain sensor or actuator nodes to perform actions according to received information. In these applications, another traffic mode coexists with the upward routing which is from the sink node to certain sensor nodes, which we call downward routing.  
Compared with upward routing, downward routing is much less studied in WSNs. The major downstream methods use flooding as the basic dissemination method, which propagates data or control messages to the entire network. This operation costs a lot of resources (energy, bandwidth, computation, etc.) during transmission and causes network congestion. Some routing protocols, like Routing Protocol for Low-Power and Lossy Networks (RPL), support downward routing by unicasting data to the destination through a route path embedded in data-packet headers [2]. Our proposal is based on a similar technique.  
Nowadays, mobility is an important requirement in many applications. In a monitoring application, if the observed object is mobile, the network topology is dynamic. In our previous work [3], we have already proven that traditional upward routing protocols do not support mobility well. Due to the movement of nodes, topology continuously changes and links between nodes are unstable. Nodes need to quickly react to adapt to the movement before making a decision for the next-hop. It is also a hard task for downward routing to cope with mobility. Each time the topology changes, most of the paths to reach every node by the sink are different. Thus, the sink needs to quickly reconstruct the topology and rebuild the routes to reach every node in the network, or data are delayed or lost.  
In this paper, we concentrate on mobile scenarios where all nodes except the sink are free to move, and need to send periodic data to the sink. Simultaneously, the sink needs to periodically send command packets to randomly selected nodes in the network. We propose a routing protocol, Adaptive Downward/Upward Protocol (ADUP), which supports both upward and downward routing. Mobility support of upward routing is achieved according to RRD+ (RSSI, Rank and Dynamic) [4].  
The remainder of the paper is organized as follows. In Section 2, we present related work concerning downward routing in WSNs. Section 3 describes our contribution: the ADUP protocol. In Section 4, we present and analyze simulation results that show the efficiency of ADUP. Finally, we conclude the paper and give some future investigations in Section 5.