## 4 The WBANs Communication Stack
### Medium Access Control Layer
#### 4.2.3 Existing State-of-the-Art on the MAC Protocols
Three types of bandwidth allocation mechanisms allowed for flexible and efficient data and control communications. An efficient sleep mode is introduced to reduce the idle listening duration, especially for low duty cycle \begin{table}
\begin{tabular}{c c c} \hline \hline
**Performance Metrics** & **CSMA/CA** & **TDMA** \\ \hline Energy Efficiency & Low to medium & High \\ Synchronization & Not Required & Needed \\ Bandwidth Utilization & Low & High \\ Scalability & High & N/A \\ Reliability & Low to Medium & High \\ QoS & Low & High \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Comparison of medium access mechanisms. nodes in the network. Simulation results show superior performance of BodyMAC compared to the IEEE 802.15.4 MAC. Heartbeat Driven MAC protocol (H-MAC) [55], is another WBAN-specific TDMA-based protocol which originally was proposed for a star topology. The energy efficiency is improved by exploiting heartbeat rhythm information in order to synchronize the nodes. The nodes do not need to receive periodic information to perform synchronization. The heartbeat rhythm can be extracted from the sensory data and hence all the rhythms represented by peak sequences are naturally synchronized. The H-MAC protocol assigns dedicated time slots to each node to guarantee collision-free transmission. This protocol is supported by an active synchronization recovery scheme supported by two resynchronization schemes. Although H-MAC protocol reduces extra energy cost required for synchronization, it does not support sporadic events. Since the TDMA slots are dedicated and are not traffic adaptive, H-MAC protocol encounters low spectral/ bandwidth efficiency in case of low traffic. A Medical Medium Access Control (MedMAC) protocol is proposed in [56] for energy efficient and adaptable channel access in body area networks. The MedMAC incorporates a novel synchronization mechanism in which only a multi-superframe beacon has to be listened by the nodes. An optional contention period is also available for low-grade data, emergency operation, and network initialization procedures. Initial-level simulations show that the MedMAC protocol performs better than the IEEE 802.15.4 protocol for two classes of medical applications. _TDMA-based Low Duty Cycle:_ Duty cycling is one of the most energy efficient and famous techniques for low-power devices. The energy efficient low duty cycle MAC protocol for WBANs presented in [57], is based on multi-channel and centrally controlled synchronized protocols. In this work, master-slave based network topology is used, where master is synchronized with slave based on time slots for fixed traffic. Although it achieves duty cycle in the order of 10% but the protocol is efficient only for very low static traffic. The authors in [58] proposed an energy efficient low duty cycle TDMA-based MAC protocol. It enables access to the physical layer for a hierarchical topology consisting of nodes communicating with master nodes, which in turn communicate with the monitoring station. The hierarchy removes the need for the sensors to expand power by transmitting to the monitoring station. It considered a TDMA frame of a duration of 1 s, packet size of 1250 bits with 80 bits of overhead having sampling rate of 125 samples/s. It achieves 4.51% duty cycle with each node requires 48.3 ms to completely transmit its data packet. This total time includes packet transmission, waiting time, ACK reception, and guard time. More recently, [PERSON] _et al._[59] presented a statistical MAC protocol for heterogeneous-traffic human body communication with periodic synchronization for use in heterogeneous traffic networks based on human body communication (HBC). The MAC protocol is designated to ensure energy efficiency by means of flexible time slot allocation and a statistical frame. The statistical frame is intended to increase the sleep time and keep low duty cycles in each beacon period.