# Routing in Mobile Wireless Sensor Networks: A Leader-Based Approach+
## 1 Introduction  
Wireless Sensor Networks (WSNs) [1] are used for monitoring and data gathering from the physical world in many applications, such as environment monitoring, farming management, tracking animals or goods, health care, transportation and ubiquitous home networks. Nowadays, WSNs are attracting great attention in research (for a recent survey, see, for example, [2]). A WSN consists of a usually big number of _sensor nodes,_ also called _notes,_ deployed in the application scenario. Motes are equipped with the specific sensors demanded by the application, and gather information about the environment, which is transmitted towards one or more _sink nodes_ (also called base stations). Sink nodes collect and process the received data in order to make it available to the user. Although in a small WSN one-hop communication to the sink can be implemented, in general, a multi-hop schema must be considered. In this case, regular motes are in charge of executing a routing protocol in order to forward the information towards the sink. Since motes must usually operate unattended for a long time, they have severe energy constraints. This has a deep impact on the design of a WSN and specifically on the routing protocol. Since communication is a costly resource in terms of energy consumption, a brute-force message forwarding mechanism (i.e., flooding) is in general inadvisable. Instead, the design of the routing protocol [3] is a critical aspect that should consider trade-offs between transmission power and forwarding strategies in order to provide reliability and quality of service. Additionally, since a motecan crash due to battery exhaustion or other reasons, an efficient routing protocol should be flexible enough to react to a failure by reconfiguring the network [4]. Traditional application scenarios of WSN include _static_ deployments. In a static WSN, routing can be initially configured to optimise communication and reduce latencies. However, some particular nodes, specifically the ones closer to a sink are especially active in message forwarding, which results in the drop of battery level, and henceforth in a higher probability of message loss due to the reduction of its communication range. Thus, even in static WSN, maintaining an efficient level of operation requires routing reconfiguration. This can be done on the basis of the information about the _quality of links_[5]. As technology matures, WSN are gaining opportunities for application in _mobile_ deployments [6]. There are two ways to introduce mobility in a Mobile WSN (MWSN). The simpler one is to have static sensor nodes while the sink nodes are the ones moving. For example, crops on a farm may have sensors that take measurement about the humidity or temperature and whenever a farmer walks by, his smartphone acts a sink node in order to download that information. The second approach is to maintain static sinks while sensor nodes are mobile, for example, when attached to animals in tracking applications. In this case, a static sink can be used to collect tracking information stored in the sensor nodes when the animals are in its range. Finally, both approaches can be combined, letting all nodes in the WSN be mobile. For example, in a residential environment for aged people or for people with disabilities, sensors attached to them can provide information to the mobile devices of the assistant personnel [7]. Despite the additional complexity of routing protocols for MWSN, mobility brings the opportunity of reducing the number of hops to the sink node. According to [8], the probability of having at least a sensor node in the range of a sink increases with the communication radius, the velocity of the node and the number of sinks, resulting in a reduction of the latency. However, high mobility conditions could prevent many transmissions to successfully deliver messages [9]. On the one hand, any wireless communication requires a minimum time to be completed.