## 5 LoW Mob Mobity Support Scheme
### Handoff Support  
The proposed handoff support for MN considers the sleep state of SNs. Thus, as shown in Figure 3, once the current SN observes that its link quality with the MN has degraded beyond a certain threshold value, it then concludes that the MN is moving. Hence the current SN needs to activate the next appropriate SN for the handoff process. In order to activate the next appropriate SN, the associated SN must know the direction in which the MN is moving. The direction can only be found through localization procedure, where a series of MN's locations depict its course of movement, i.e., tracking. There are various methods to obtain MN location like, trilateration/triangulation method, AoA (Angle of Arrival), and ToA / TDoA (Time {Difference} of Arrival) method, signal strength indicator etc.  
In order to determine the direction of movement, we propose to use AoA method [21]. AoA is defined as the angle between the propagation direction of an incident wave and some reference direction, which is known as orientation. Orientation, defined as a fixed direction against which the AoAs are measured, is represented in degrees in a clockwise direction from the North. When the orientation is 0\({}^{\rm o}\) or pointing to the North, the AoA is absolute otherwise it is relative.  
As shown in Figure 3, 'the star' shows MN position at time-stamps TS\({}_{1}\), TS\({}_{2}\), and TS\({}_{3}\) respectively. The associated SN measures the AoA from the signal that it receives from the MN. Moreover, the associated SN also estimates the distance between the MN and itself by using received signal strength of the packets.  
As shown in Figure 3, let \(\alpha_{1}\), d\({}_{1}\) ; \(\alpha_{2}\), d\({}_{2}\) ; \(\alpha_{3}\), d\({}_{3}\) be the angles (in degrees) and the distances between the SN1 and the MN, at time-stamps TS\({}_{1}\), TS\({}_{2}\) and TS\({}_{3}\) respectively. Based on the angle and distance information, the SN can obtain the MN's location coordinates. At the time-stamp TS\({}_{3}\), the SN1 awakes the next SN2 for the handoff. Also SN1 sends a _new_ _node_ message to SN2 that contains MN's ID.  
When the next SN receives the _new_node_ message, it transmits a _hello_packet_ at some intervals. Also, at the same time, it sends an LU to the GW. When the next SN detects the MN, it again sends a _new_node_ message to the previous SN. The purpose of this message is to create a tunnel between the next SN and the previous SN, so that as soon as the previous SN receives the _new_node_ message, it starts forwarding the MN's packets to the new location. The tunnel's outer header contains the next SN as a destination and previous SN as a source. When the MN's packets stop arriving at the previous SN, the SN switches to the sleep mode after a short interval of time. The handoff support mechanism is depicted in Figure 4. If the next SN does not detect the MN within a certain amount of time, it will consider that the MN is lost. So the next SN will request, by sending a radio triggered broadcast, to all of its neighboring SNs to suspend their duty cycle and keep awake until the MN is found.  
Figure 4: MN’s handoff support scenario.  
Figure 3: Next node activation process using AoA method.