## 3 MAC Protocols for HCWSNs  
The provision of quality of service (QoS), measured in terms of reliability, latency and energy efficiency, is tightly coupled with the selection of a suitable MAC protocol for the HCWSN. To ensure the longevity of the network and to attain an acceptable end-to-end packet delay, a MAC protocol must strike a balance in the tradeoff that exists between energy efficiency and end-to-end packet delay. For health care applications, it is also important that a MAC protocol account for the mobility that patients may exhibit while under care in their treatment environment. Therefore, when choosing a MAC protocol for a HCWSN, it must be able to adapt to the mobility of nodes that can subsequently result in the change of network size, node density, and network topology. CSMA based MAC protocols aptly adjust to these changing conditions and, thus, are selected for review in this paper. To this end, we briefly review several CSMA-based protocols that also display energy saving and low latency characteristics, and discuss their respective advantages and disadvantages as they relate to their application in a HCWSN. _S-MAC_[22]: In Sensor-MAC (S-MAC), neighboring nodes form virtual clusters and utilizes local synchronization to set up common sleep schedules. One key feature of S-MAC is its use of message-passing, where a long message is divided into frames and sent sequentially when a node seizes the channel for transmission. Using this technique yields energy savings because less communication overhead is incurred, but at the cost of increased unfairness in the sharing of the medium. The main advantages of S-MAC are that it reduces the energy waste caused by idle listening and also prevents time synchronization overhead with sleep schedule announcements. On the other hand, the sleep and listen periods are predefined and constant, and thus the efficiency of S-MAC is decreased under variable traffic loads. In general, WSNs for health care do not generate very large amounts of data at a time [13, 19]. Most of the data is either event-driven or generated periodically in small amounts [13, 19]. Therefore, it is possible that S-MAC would not see significant performance degradation in a HCWSN. _DS-MAC_[23]: Timeliness is a key objective to the success of WSNs for health applications. In Dynamic Sensor-MAC (DS-MAC [23]), a dynamic duty cycle feature is implemented in S-MAC with the goal of decreasing the latency for delay-sensitive applications. Initially, all nodes start with the same duty cycle. During the SYNC period, each node transmits a packet that holds the schedule table which contains the sleep-wakeup cycle of all its neighbors. The header of the packet maintains a one-hop latency value that measures the time difference between when a packet is queued and when the packet is subsequently delivered to the one-hop neighbor of the transmitting node. If a receiver node receives the packet and determines that its latency value exceeds a specified threshold, it doubles its duty cycle by shortening its sleep period length, without modifying its listening period. The main benefit of the dynamic duty cycle adjustment scheme is that it results in lower latency and better scalability than S-MAC, while also delivering more efficient power consumption per packet. _MD-SMAC_[24]: The Mobility-Aware Dynamic S-MAC (MD-SMAC) protocol is a combination of the mobile version of S-MAC (MS-MAC [25]) and the previously described DS-MAC [23]. The main objective of MD-SMAC is to provide mechanisms to satisfy the constraints posed by delay sensitive applications while also handling mobility conditions in an energy efficient manner. [PERSON] _et al._[24] propose modifications to both DS-MAC [23] and MS-MAC to achieve the aforementioned objective. For the DS-MAC [23] component of MD-SMAC, the duty cycle of a node is reverted back to itsoriginal duty cycle (_i.e._, duty cycle upon power up) when the energy level threshold \(T_{E}\) has been exceeded.