# A Survey on Mobility Support in Wireless Body Area Networks
## 3 Mobility Model and Communication Protocol
### Individual Mobility
#### 3.1.2 Protocols for General Mobility
To perform routing procedure based on the prediction model, each node builds backbone links with fixed neighbors to maintain constant distance in spite of body gestures. This new backbone link is updated with link quality. Based on the prediction model, the next hop is selected if it has the highest link quality in the current time slot as well as the previous one. However, whenever the prediction model is not available in the case of the random mobility model, a packet is transmitted over the shortest path tree on the backbone link. In addition to the delay issue, temperature and amount of energy are taken as routing cost in Energy efficient Thermal and Power Aware (ETPA) [23], which performs a forwarding task under the store and forward principle. Instead of delay or link information, the HELLO message containing a node's temperature and remaining energy is exchanged between neighbor nodes. These values are used to determine a node's cost function by taking received power as well as neighbor's temperature and energy level in a HELLO message. A node either sends a packet to one of neighbor nodes with a minimum cost value or keeps the packet in the buffer only if there is no suitable node to forward. Additionally, a packet is discarded when hop count of packet exceeds maximum hop count. This procedure continues until a packet is eventually delivered to the sink node. As far as the authors are aware, the Mobility-supporting Adaptive Threshold-based Thermal-aware Energy-efficient Multi-hop ProTocol (M-ATTEMPT) [24] is the most applicable protocol for mobility in WBANs. It is a temperature aware protocol under tree structure. Whenever a person changes his/her gesture, the tree is rebuilt by a node's joining to the new parent node. In M-ATTEMPT, a sink node locates at the center of body and the nearest node from a sink node is set to the parent node, which is root of the tree. When a node changes location, a larger amount of energy will be consumed if the current tree path is maintained. To prevent this unsuitable waste, a node connects to the new parent node after disconnecting from the link. Through these procedures, a new tree is constructed and a data packet is delivered along the links on the tree. Furthermore, M-ATTEMPT provides two major functions to handle hot spot and mobility at the same time as adjusting the tree. Figure 3 shows the procedure to prevent the hot spot. In this example, node e reaches the temperature threshold due to the forwarding task. Whenever each node becomes the hot spot, the parent node disconnects all links from child nodes during the next few rounds. When a node's temperature status turns normal, a link is reestablished. If a hot spot parent node receives a data packet from the child, it returns the data packet to the child. If this case happens, the child node marks its parent as a hot spot in order to not deliver the packet next time. The mobility function provided by M-ATTEMPT is illustrated in Figure 4. If a child node moves to another place, it disconnects the link to the parent node in order to reduce the communication overhead. Upon being disconnected, a node searches the nearest parent node and rebuilds the tree dynamically by sending a joint-request message. These sequential procedures are explained in order of from Figure 4a-c.  
IM-SIMPLE [25] is extended from the original Stable Increased-throughput Multi-hop Protocol for Link Efficiency (SIMPLE) protocol to take mobility into account. Its objective is to improve reliability and stability as well as achieve low energy consumption and high throughput by defining new cost function that takes a node's residual energy as well as its relative distance. Each sensor node delivers a packet to a sink node via a forwarder node which is set to one with maximum residual energy and minimum distance. However, data delivery via a forwarder node may lead to rapid energy consumption on the forwarder. When energy levels of the forwarder node is lower than the threshold, a node sends a packet to the sink node directly.