# Energy-Efficient Wireless Hopping Sensor Relocation Based on Prediction of Terrain Conditions
## 3 The Proposed Relocation Protocol for Rugged Terrains
### Descriptions of the Proposed Protocol
Therefore, it is a good example to see that the use of the above probability could reduce the number of request messages, when the cluster header of sensing hole is requesting members to the neighbor zones. Actually, the number of REQ messages of the first scenario is greater than that of the second scenario. Message flow charts for the above described two scenarios could be explained easily, as shown in Figure 6. The cluster header H\({}_{\text{B}}\) of the zone B first detects its sensing hole occurred, and it transmits the first REQ message to the cluster header H\({}_{\text{A}}\) of the neighboring zone A, here the REQ includes that the number of member sensor node needed is one (i.e., \(cnt=1\)). In fact, a direct communication between the cluster headers is impossible, but for convenience, transmission and reception of their messages with a RELAY node are omitted in the middle. The header H\({}_{\text{A}}\) selects M1 appropriately among its member nodes, and it sends a MOVE message to relocate to the sensing hole, zone B. Member node M1, which received MOVE, was able to recover the sensing hole without any problems for moving. First, the message flows for the first scenario are depicted in Figure 5(a). The cluster header of B, H\({}_{\text{B}}\), detects a sensing hole occurred again, and it sets two members (\(cnt\) = 2) for recovery in the REQ message, and sends the message (the second REQ) to the neighbor header H\({}_{\text{A}}\). H\({}_{\text{A}}\) appropriately  
Figure 6: Message flows for two scenarios. selects two members (M2, M3) and transmits MOVE message to them to relocate, and the member node M2 of them is moved to the zone B without any problems. However, the other member M3 is not able to move to the zone B due to an unexpected obstacle. After a certain time, the cluster header H\({}_{\text{B}}\) determines that the sensing hole is not recovered. H\({}_{\text{B}}\) transmits a message (the third REQ) to request one member needed to the zone A. H\({}_{\text{A}}\) directly sends MOVE message to the member M4, but M4 unfortunately crashes into an obstacle and fails to recover the sensing hole. After a certain amount of time again, the header H\({}_{\text{B}}\) checks the current status of zone B, and it inserts that the value of _cnt_ is 1 (i.e., requires one member) in the requesting message and sends the message (the fourth REQ). The member M5 receiving MOVE from its cluster header H\({}_{\text{A}}\) successfully is moved to the sensing hole zone B without colliding with obstacles. Next, the message flows for the second scenario are depicted in Figure 5(b). The header H\({}_{\text{B}}\) detects a sensing hole occurred again and sends a message to request some hopping sensors. Here, H\({}_{\text{B}}\) appropriately calculates the number of members required using that the movement success rate \(p\) is one and sets the value of _cnt_ to two. H\({}_{\text{A}}\) chooses two members, M2 and M3, to transmit MOVE for relocation, and then [PERSON] is successfully moved to the neighbor zone B. However, another hopping sensor M3 is unable to move because of unexpected obstacles. The cluster header H\({}_{\text{B}}\) determines that the sensing hole is not recovered after broadcasting HELLO message repeatedly. The header of zone B transmits a message (the third REQ) to request some members needed from the header of zone A. At this time, since the movement success rate for the previous REQ message is \(p=1/2\) (50%), the header of B could request two members calculated by Ceiling[\((5-4)*(1+(1-1/2))\)], even though the zone B currently needs one member to recover the sensing hole. That is, H\({}_{\text{B}}\) recognizes that the level of obstacles between cluster zones is high and requests a large number of sensors from H\({}_{\text{A}}\) for rapid recovery. As expected, member M4 collided again with an obstacle, but member M5 successfully moved to neighbor zone B.