## 5 WBAN and the Communications Protocol Stack
### MAC Layer
#### 5.3.2 MAC Layer Adaptation Schemes  
One of the most referenced low power MAC protocols in the literature is Sensor-MAC (S-MAC) [33]. S-MAC is a synchronous protocol where the basic idea is for nodes to sleep periodically and have each node somehow aware of all other nodes sleeping patterns. S-MAC is designed to save energy by periodically switching between active and sleep states, thereby making tradeoffs between energy and latency according to traffic conditions. Energy consumption is minimized by utilizing a synchronization mechanism that maintains a common sleep schedule between neighbor nodes in low duty cycle operations. S-MAC uses Request to Send (RTS)/Clear to Send (CTS) and a virtual carrier sense technique similar to 802.11 to avoid overhearing and provide adaptive listening. The protocol presents a message passing technique that separates a long message into small fragments and transmits them in a burst to loosely coupled nodes. S-MAC uses three techniques to achieve low power duty cycling: _periodic sleep_, _virtual clustering_ and _adaptive listening_. S-MAC delivers significant energy improvements over 802.11 but the duty cycle is required to be synchronized to a specific traffic load. Thus its performance suffers under varying traffic loads. Timeout-MAC (T-MAC) [35] introduces an adaptive duty cycle that dynamically ends the active part of the cycle to reduce the energy wasted on idle listening. T-MAC improves the design of S-MAC \begin{table}
\begin{tabular}{c c c} \hline
**Performance Metric** & **CSMA/CA** & **TDMA** \\ \hline Power consumption & High & Low \\ Efficiency at light traffic & Good & Poor \\ Efficiency at high traffic & Poor & Good \\ Bandwidth utilization & Low & Maximum \\ Scalability & Good & Poor \\ Effect of packet failure & Low & Latency \\ Synchronization & Not applicable & Required \\ \hline \end{tabular}
\end{table} 
Table 1: Comparison between CSMA/CA and TDMA. by shortening the active period if the channel is idle. In S-MAC the nodes will remain awake through the entire active period even when not receiving or transmitting data. T-MAC improves on S-MAC by listening to the channel for only a short time after the synchronization phase. The node will return to sleep mode if no packet is received during this window. T-MAC also uses an adaptive listening technique where nodes are able to immediately pass the data avoiding the timeout introduced in S-MAC. This adaptation of the duty cycle improves the energy and throughput performance under varying traffic loads. T-MAC uses one fifth of the energy used by S-MAC. However these gains come at the cost of reduced throughput and increased latency. [PERSON] _et al._ developed DMAC [36] and identified inefficiency in S-MAC and T-MAC concerning idle listening where nodes remain active or asleep for longer than they need to. The protocol identified efficiency (low latency and low energy) in WSNs using data gathering trees. DMAC was designed to solve the interruption problem where nodes that are out of hearing range of both sender and receiver are unaware of ongoing data transmissions and will, therefore, go to sleep until the next cycle. DMAC allows continuous packet forwarding by giving the sleep schedule of a node an offset that depends upon the depth of the tree. DMAC also adjusts the duty cycles adaptively according to the traffic load in the network. Simulation results show that DMAC achieves both energy savings and low latency in comparison to other protocols when used with data gathering trees in WSNs. Some MAC protocols such as Berkeley-MAC (B-MAC) [37] and WiseMAC [38] use the preamble element of a packet to adapt the duty cycle to ensure nodes only wake up when absolutely necessary. B-MAC is an example of an asynchronous technique where the protocol appends a long enough preamble to the data packets to ensure the destination is active at least once while the preamble is transmitted.