# A Survey on Mobility Support in Wireless Body Area Networks
## 3 Mobility Model and Communication Protocol
### Group Mobility
#### 3.2.2 Protocols for General Mobility  
The Random Contention-based Resource Allocation (RACOON) [44] is a bandwidth control system to support multi Quality of Service (QoS) in MAC layers. It has two important components: Central Processing Node (CPN)-based resource allocation and random contention-based inter-CPN negotiation. Unlike typical WSN, CPN-based protocol has two separate channels. Inter-WBAN channel is used to exchange resource negotiation message while Intra-WBAN channel is for polling and data packets. CPN sends a polling message to each sensor node according to the polling slot. When the coverage of two nodes is duplicated, CPN resource negotiation is performed in the Inter-WBAN channel. In this protocol, different priority and bandwidth are given to each WBAN. Figure 6 illustrates an example of resource allocation in RACOON. In Figure 6a, if two WBANs have duplicate coverage areas, inter-WBAN channel is used to perform CPN resource negotiation. After negotiation, a superframe slot for the Intra-WBAN channel is used for data communications as shown in Figure 6b. As an approach in the MAC layer, Time-Synchronized Channel Hopping (TSCH) can be applied in WBAN to provide collision-free and interference-avoiding communications. In order to reach a steady state in early time without mobility, decentralized time- synchronized channel swapping (DT-SCS) was proposed in [45] to achieve high bandwidth utilization, and connectivity was also achieved. The other protocol to use group mobility to extend WBAN network lifetime was introduced in [46]. When the coordinate node on the body is intended to deliver packets to access points, intermediate coordinate nodes operate as relay nodes if connection is not available due to a person's movement. Another disconnection occurs when the energy level of the coordinate node becomes lower than the threshold. In this case, the coordinate node notifies sensor nodes to change the coordinate node. After identifying the coordinate node's status, a sensor node searches for available coordinate nodes around it. In order to handle mobility in time-efficient ways, the interval of HELLO messages is proportional to mobility speed. This implies that more HELLO messages are generated to update topology information in highly dynamic networks. The factor, \(HelloP\), is computed by the two following Equations (1) and (2), where \(highmobSpeed\) represents the highest mobility speed and \(lowmobSpeed\) the lowest mobility speed, respectively. \[HelloP=\ coverage/mobSpeed, \tag{1}\]  
\[mobSpeed=\ \Big{(}highmobSpeed+lowmobSpeed\Big{)}/2. \tag{2}\]Figure 7 illustrates the scenario for cooperation among four WBANs. When the battery of coordinator nodes in both WBAN1 and WBAN4 is almost drained out, a sensor node in WBAN1 transmits packets to new coordinators in WBAN3 while a node in WBAN2 sends packets to the nearest node in WBAN4 through node-node communications. Consequently, a receiving sensor node in WBAN4 sends packets to coordinators in WBAN3 instead of one in WBAN4, which suffers from low battery power. For the handover issue, there is a good technical paper [47] to maintain the accessibility of sensor nodes in healthcare wireless sensor networks (HWSN). [PERSON] et al. presented main principles and intra-mobility schemes in HWSN. In addition to approaches in WSNs, a fuzzy based handoff mechanism [48] was proposed to switch links between APs for WBANs. In this model, there are two users: Primary Users (PUs) and Secondary Users (SUs). Specific QoS threshold should be guaranteed for PUs. To ensure QoS, fuzzy logic determines whether handoff is performed. This decision is initiated by the level of signal strength. The MAC layer approach without backup server for WBAN  
Figure 6: Example of channel allocation in Random Contention-based Resource Allocation. Figure 7: Inter-WBAN communication. was proposed in [49].