## 3 MAC Protocols for HCWSNs
The purpose of this modification is to give priority to extending the lifetime (_i.e._, conserve the remaining energy) of the node by using a small duty cycle, at the cost of increased latency. In MS-MAC, mobile nodes moving from an old cluster to a new cluster maintain both the schedules for the old cluster and that for the new cluster, which is an inefficient use of energy. In MD-SMAC, a modification is made so that the old schedule is removed when a new schedule has been received in the new cluster. Another modification made to MS-MAC is the adaptive change of the neighbor discovery frequency. Normally, when a mobile node moves from one cluster to another, its neighbor discovery frequency increases from once per 5 min (default frequency) to once per 30 s. However, the neighbor discovery frequency never reverts back to the default frequency. In MD-SMAC, a mobile node that has already had its neighbor discovery frequency increased waits to receive its new schedule from the new cluster before reverting back to the default frequency. The collective modifications made to MS-MAC and DS-MAC [23] allows MD-SMAC to possess mobility-aware, delay-sensitive, and energy-efficient characteristics, thus making it strong a candidate for consideration in HCWSNs. _DS-MAC_[26]: Patients admitted to health care institutions typically have illnesses or injuries of different severity. Therefore, the manner in which traffic is characterized for medical updates from patient to doctor must garner special consideration. The Differential Service Medium Access Control (DS-MAC [26]) protocol accounts for the different traffic types by contextualizing them as normal (Class 2--routine data), warning (Class 1--high priority), and emergency traffic (Class 0--highest priority). This allows DS-MAC [26] to enable prioritized channel provisioning for differentiated service based on the class type. DS-MAC [26] extends the 802.11e QoS MAC protocol by utilizing a preemptive service scheduling algorithm to realize class-based channel access. Each node in the WSN maintains a separate queue for each traffic type. A node contends for channel access with each information frame that it wishes to send. When channel access has been acquired, all packets in the information frame are transmitted successively, unless the service gets interrupted by a higher class traffic type within the same node. By rule, routine data traffic can only be transmitted if both the warning and emergency traffic queues are empty. Otherwise, emergency traffic is always given immediate access for channel service while warning traffic always maintains precedence over routine data traffic. Although this classification of traffic may result in a starvation scenario for routine data, it is still imperative that data concerning medical emergencies are addressed first, because lives may be at stake. The MAC protocols reviewed in this section offer advantages and disadvantages toward their use in a HCWSN. Table 2 provides a summary of what mechanisms the reviewed MAC protocols utilize in regards to addressing energy efficiency, timeliness, and robustness to mobility. Overall, the MD-SMAC protocol strikes a good balance between energy efficiency and end-to-end packet delay while also handling mobile scenarios. The performance evaluation of MD-SMAC conducted in [24] has shown that it consistently outperforms S-MAC and DS-MAC [23] in terms of disconnection duration, average end-to-end delay, and average energy consumption. In scenarios that evaluated the amount of time a mobile node has been disconnected from the network, both S-MAC and DS-MAC [23] performed poorly because they do not have specific mechanisms to deal with mobility. Although not all nodes in a health care setting will be mobile, it is quite evident that mobility must still be supported. Therefore, future work towards enhancing HCWSN performance via advanced MACprotocols should account for the effect of mobility.