# A New Association Scheme for Handling Node Mobility in Cluster-Tree Wireless Sensor Networks
## 1 Introduction  
A Wireless Sensor Network (WSN) may be composed of a large number of low-cost devices distributed within a particular area of interest. Over the last few years, WSNs have been used for agriculture, environment, industry and health monitoring purposes [1]. IEEE 802.15.4 standard [2] defines one of the most widely used protocols for deploying WSNs, specifying the physical layer and medium access control sublayer for low-data-rate wireless connectivity. An IEEE 802.15.4 network operates either in one of two topologies, depending on application requirements: star topology or peer-to-peer topology. A cluster-tree topology is a special peer-to-peer topology that has been shown to be one of the most suitable for deploying wide-scale WSNs [3]. It is noteworthy that, regardless of the topology used, every node in an IEEE 802.15.4 network must be associated with the network, through an association procedure, before it can exchange messages (for more information about association and re-association process of the IEEE 802.15.4, please refer to [2]). Many WSN monitoring applications require mobility of sensor nodes; therefore, enabling node mobility has become a significant challenge in WSN research [4; 5; 6]. Many wireless standards and technologies do not efficiently support node mobility [7], impairing their use, especially if WSNs are being used for supporting critical applications with real-time constraints and reliability requirements. Ideally, mobility implies that devices moving along the monitored area do not suffer connection losses, requiring the use of appropriate association/re-association mechanisms. This means that nodes must decide at appropriate time instants to disconnect from their coordinators in order to associate with another with a better signal, via a mechanism known as a handover. As mobility support involves several relevant issues, such as the best moment to start the handover procedure and the time required to complete this operation [8], various design issues must be carefully considered, such as adequate beacon scheduling [3], node speed upper bounds and direction [9; 10; 11], and time spent in the scanning phase [9; 12]. However, regarding the time and energy costs of the handover procedure, the most critical aspect is the association algorithm due to the scan phases and the Media Access Control (MAC) involved [13]. This paper aims to improve mobility support for WSN nodes, acting upon the association and handover procedures. These improvements aim to reduce the time interval during which a Mobile Node (MN) remains out of communication with the network. We consider an existing WSN monitoring communication infrastructure organized as a cluster-tree beacon-enabled topology. We also consider that one or more mobile nodes are able to join to the network, at any instant, in order to transmit messages with real-time requirements. In this context, there are several issues that must be taken into account. For example, how to integrate mobile source nodes into a pre-established cluster-tree network (association and re-association phases); how to guarantee connectivity without packet losses (handover, buffers); and how to guarantee timing requirements (resource allocation).