# A Reliable Handoff Mechanism for Mobile Industrial Wireless Sensor Networks
## 5 Effective Handoff Mechanism
### Handoff  
The sequence of handoff procedure is shown in Figure 6. After handoff triggering, the \(MN\) should not leave the network or register a new path immediately, since ping-ping effect during handoff can cost extra communication resource and delivery delay. Therefore, an important feature of our  
Figure 4: Relationship between \(k\) and mobile speed. Figure 5: Definition of membership functions: (**a**) is the member function of MS, (**b**) is the member function of CC and (**c**) is the member function of PD. proposed mechanism is that _MN_ continuously evaluating its mobile state before registering a new path. The evaluations aim to distinguish between the handoff triggered by interference and real movement. Once _MN_ realizes that its state is close to static, it begins a new registration. While on the move, it makes an evaluation every superframe. In Figure 7, we show the procedure of handoff which is designed to avoid the problem of ping-pong effect. At time \(t_{0}\), \(v_{1}\) becomes _MN_ and begins to move along the moving path until time \(t_{4}\). As soon as _MN_ triggers handoff procedure, it opens the scan mode from \(t_{0}\). To save energy, _MN_ only listens to the broadcast nearby, instead of idle listening, since it is still synchronized with the network. We append additional information to the broadcast of each node such as \(\varphi_{i}\). Thus, _MN_ has a neighbor table \(N=\{n_{1},\ldots,n_{i},\ldots,n_{m}\}\) during scan mode. Each neighbor can be represented as \(n_{j}:\langle v_{i},\varphi_{i},r^{c}_{i},r^{p}_{i}\rangle\), where \(r^{c}_{i}\) and \(r^{p}_{i}\) are the RSSI values of \(v_{i}\)'s broadcast measured in the current and previous superframe respectively. The moving state is evaluated by the following equation. \[R=\frac{1}{m}\sum_{i}|r^{c}_{i}-r^{p}_{i}| \tag{9}\]  
Figure 6: The sequence of handoff procedure. Figure 7: Movement of the mobile node from time \(t_{0}\) to \(t_{4}\). \(R\) is the mean of all neighbours' RSSI variation and \(m\) is the number of neighbours detected during the last two superframes. When \(R\) is below the threshold \(R_{T}\), \(MN\) immediately begins to register the new path. Just as \(v_{2}\) realizes its link \(v_{2}\longrightarrow v_{1}\) has broken up at \(t_{1}\), it can immediately request for a new link from the Network Manager. Another innovative feature is that \(MN\) establishes temporary links for transmissions if \(f_{MN}^{p}\) and \(f_{MN}^{C}\) are generated during the movement. A node is still sampling the data from the welding machine while it is moved to another place. The measurements of moving state evaluation also can be utilized for selecting one of the temporary links. It is different from the WirelessHART standard that the resource of communication will be released by the Network Manager when it is beyond the communication range of its parent. Therefore, the temporary link can improve the delivery, which requires the reliability and timeliness in industry. The segments slot allocation proposed in [31] is scheduled, which makes full use of the sequence of segment and shared slots in this paper. During \(t_{0}\) to \(t_{4}\), see Figure 7, we assume that \(f_{MN}^{p}\) will be generated. While evaluating the moving state, \(MN\) selects a better infrastructure node to be its parent by comparing the signal strength and the broadcasted parameters. \(v_{4}\), \(v_{5}\), \(v_{6}\) and \(v_{7}\) are selected to be parents around time \(t_{1}\), \(t_{2}\), \(t_{3}\) and \(t_{4}\) respectively. The scheduling of the superframe is displayed in Figure 8, which is divided into segments. According to \(\varphi_{i}\), broadcasted by the selected parent, the temporary link is allocated in the shared slot which belongs to the previous segment \(\varphi_{i}-1\). For example, \(\varphi_{4}\) and \(\varphi_{5}\) are both allocated to #5 at the beginning. Thus, the temporary link is assigned in #4 when \(v_{4}\) and \(v_{5}\) are selected by \(MN\) around \(t_{1}\) and \(t_{2}\) respectively.