## 5 WBAN and the Communications Protocol Stack
### MAC Layer
#### 5.3.3 WBAN Specific MAC Layer Schemes
[PERSON] _et al._[45] introduced an energy efficient MAC for WBANs based on single hop and central node based synchronization as a means to move network complexity away from distributed nodes and onto the central node. Once a node has joined the WBAN, collision is prevented within a cluster as all communications are initiated by the central node and addressed uniquely to a slave node. The main goal of the protocol is to reduce power consumption due to idle listening, overhearing, and collision using a CCA algorithm based on a Listen Before Transmit (LBT) and a \"wakeup fallback\" time to handle time slot overlaps. The disadvantage of the protocol is increased complexity at the central node although it is assumed the central node has more power and processing resources. Body Sensor Network MAC (BSN-MAC) [46] is an 802.15.4 based ultra low power adaptive MAC designed to exploit feedback information from nodes to deliver increased energy efficiency. To avoid collisions with nearby nodes, the protocol incorporates a CCA algorithm based on LBT. The concept of a wakeup fallback time is introduced to handle time slot overlaps. Energy efficiency over 802.11 and ZigBee is realized using single hop and centrally controlled sleep/wakeup times. The control algorithm enables the BSN coordinator to adjust parameters of the 802.15.4 superframe structure to avoid idle listening and achieve both energy efficiency and low latency on energy critical nodes. As in BSN-MAC, Distributed Queuing MAC (DQ-MAC) [47] uses a superframe structure to deliver further energy efficiency as a function of network load and packet length. DQ-MAC uses BSN-MAC and 802.15.4 as a benchmark to highlight specific energy saving improvements. DQ-MAC grants immediate access for light traffic loads (behaving as a random access mechanism) and seamlessly moves to a reservation system for high traffic loads, eliminating collisions for all data transmissions. The DQ-MAC superframe is bounded by a feedback packet (FBP) contained in the feedback frame. The FBP is used to synchronize the nodes to the BAN coordinator. Simulations have shown that DQ-MAC outperforms 802.15.4 MAC and BSN-MAC for all traffic loads in a generalized BSN scenario in terms of eliminating collisions while minimizing the control overhead and hence the overall energy consumption per bit. Schedule-based approaches to node communications within WBANs have the overhead of synchronization, but have the advantage of being collision free, low duty cycle in operation and do not suffer from high levels of overhearing. Nodes can only transmit data during their assigned time slots, therefore, collisions are avoided. Nodes can turn their radio on for their assigned time slots ensuring that low overhearing and low duty cycle operations can be achieved. TDMA-based protocols can reduce latency by guaranteeing dedicated time slots for each node to send its data. HeartBeat MAC (H-MAC) [10] is a MAC protocol designed for WBANs that exploits heartbeat rhythm to perform time synchronization for TDMA. Although heartbeat is a key life-sign that can be monitored as part of a WBAN via an ECG sensor, nodes can use the heart rate waveform peaks as a mechanism for node synchronization within a WBAN. Nodes can achieve synchronization by using the heartbeat rhythm without having to turn on their radio. This means that the energy cost for time synchronization can be avoided thereby increasing the lifetime of the network dramatically. H-MAC has the obvious limitation of a single point of failure, _i.e._, the reliance on the human heart for synchronization. Additionally, results may be affected by patients with a weak heart or those suffering from a cardiovascular condition where some sensor nodes may not be able to detect the synchronization data. Sensor nodes may request varying channel resources under different scenarios.