# A New Association Scheme for Handling Node Mobility in Cluster-Tree Wireless Sensor Networks
## 2 Related Works
### Modification in the Association Process  
The association process of IEEE 802.15.4 standard is defined in [2]. This subsection presents some relevant state-of-the-art proposals that modify this process.  
In [PERSON] et al. [14] is proposed an association mechanism similar to FastA [2]. This scheme was called Simple Association Process (SAP). The main similarity is that the coordinator does not include the associating response in the list of pending transactions, i.e., the device just waits for the join response command sent by the coordinator, eliminating the need for request command data.  
[PERSON] and [PERSON] [9] proposed an association mechanism intended to support real-time applications. The Fast Association Mechanism (FAM) modified both the initialization and the channel scanning procedures of the standard association mechanism. In channel scanning, FAM stops a scanning process whenever a suitable Personal Area Network (PAN) has been found, reducing the time interval required to finish this process. Additionally, as in the previously cited proposal, FAM uses direct transmission to forward the join response command to the node.  
In [PERSON] et al. [11], the authors present a new association scheme, in which coordinators transmit their beacon frames using a dedicated channel. This proposal allows a node to acquire network information from all coordinators in its vicinity by checking only one channel. They also proposed to use the Link Quality Indicator (LQI) as a means to anticipate a node's loss of connectivity.  
[PERSON] et al. [15] reported that one of the main problems of association mechanisms is the delay caused by the full channel scanning process, that can reach 90% of the total handover interval and contributes to high packet loses and to the random loss of connection. Basically, they propose a new mechanism that eliminates the channel scanning delay, where the MN creates, and continually updates, a table to store the MAC address, the available channel number and the LQI of the neighbor coordinators. Then, whenever the LQI of the MN drops below a predefined threshold, the MN directly triggers the re-association instead of running a new scanning process.  
Similarly to the previous work, [PERSON] and [PERSON] [16] also presented an association technique to remove the full scanning process. They also proposed that the MN must maintain a list of operating channels for all the coordinators in the vicinity. Then, whenever the MN needs to find a new coordinator, it scans only those channels, reducing the number of channels to scan and, consequently, decreasing the total association time.  
Figure 1: Improvements in the association process.  
In [PERSON] et al. [17], the authors proposed a set of algorithms to define the Contention Window (CW) parameter of the Carrier Sense Multiple Access with Collision Avoidance (CSMA-CA), according to the measured content level. A Markov chain is used to model the number of devices disassociated with a coordinator and the operation of the devices is modeled based on statistical concepts. These algorithms have been proposed to reduce the total association time.