# A Reliable Handoff Mechanism for Mobile Industrial Wireless Sensor Networks
## 5 Effective Handoff Mechanism
### Mobility Awareness  
Each node selects a parent node with a good connection to form a multi-hop spanning tree topology. After joining the network, each node collects metrical information from each transmission and reception. Being an initiative aware mechanism, the node can extract metrics from the acknowledgement of transmission and the reception of command. In addition, each node broadcasts for network management at every superframe. At the end of the superframe, nodes compute the degree of triggering according to the collected metrics within the time window calculated by (2), which is denoted by \(\mu(Tr_{W})\). To obtain an accurate trigger decision from the OWA operator, the definition of the membership function is required to map the trigger degree. Moving State, Channel Condition and Packet Delivery are converted to a corresponding degree by defining the values respectively. The respective member functions of three metrics are the linear relationship between the modified measurements and the corresponding degree of metric. To describe the relationship clearly, the importance is to define the key degree of the measurements. These key values of measurements originate from the experimental data and the existing results. Thus, the diagram of the member function can intuitively perform their relationship. For MS, tests are carried out to describe the relationship between \(k\) and mobile speed as shown in Figure 4. Since the movement of welder machine is generally caused by a worker who need to change the current position. Therefore, we select two values of \(k\) corresponding to 1 m/s and 2 m/s mobile speed as the key values in membership function \(\mu_{MS}\), which is shown in Figure 5a. As for **CC**, there are a number of studies providing a relationship curve between bit error rate and SNR. The membership function of \(\mu_{CC}\) is described in Figure 5b, where 3 and 8 are selected as the boundaries of the good channel and bad channel respectively. As a transmitter-side measurement, **PD** is evaluated by counting the number of transmissions and retransmissions and its membership function \(\mu_{PD}\) is shown in Figure 5c. Based on the representative standard WirelessHART, the maximum retransmissions is three. Thus it is considered the bound of a bad link when RNP reaches three. RNP = 1 is the bound of the good link where packets are all received successfully.  
The linear relationship between collected information and corresponding degree is enough to represent individual metrics. Moreover, it also decreases the complexity of computing and energy consumption. Finally, after the conversion between metrical information and degree, we can obtain the current degree of triggering handoff by Equation (7) according to Equation (5).  
\[\begin{split}\mu(T_{W})&=\beta\cdot min(\mu_{MS}(T_{ W}),\mu_{CC}(T_{W}),\mu_{PD}(T_{W}))\\ &+\frac{1-\beta}{3}(\mu_{MS}(T_{W})+\mu_{CC}(T_{W})+\mu_{PD}(T_{W }))\end{split} \tag{7}\]  
Obviously, once such degree is less than the given threshold \(\mu_{T}\), the node is aware of its movement and begins the handoff procedure. To provide a more comprehensive assessment, the result is changed to the range in \([0...100]\) in accordance with  
\[\mu=100\cdot\mu(T_{W}) \tag{8}\]