## 5 End-to-End Solutions for M2M Communication  
The goal of an mHealth application is to provide a bridge between the patient and the medical personnel. Hence, the M2M system must provide end-to-end connectivity, connecting the medical sensor devices via the M2M gateway to the Internet, and ultimately to the application server. This role is played by the access communication network that is usually based on WLAN/WMAN wireless technologies such as WiFi (IEEE 802.11), WiMAX (IEEE 802.16) and LTE/LTE-A. Since standardization efforts on M2M communications are still under way, there are many different approaches to achieve end-to-end connectivity.  
In this section, we examine end-to-end solutions for M2M communications. In particular, we first discuss theoretical works that focus on the high-level integration of WLAN/WMAN access communication technologies with WBANs. Then, we give some practical examples of end-to-end solutions by presenting testbed implementations for healthcare monitoring. Finally, we present some current research projects for mHealth applications.  
\begin{table}
\begin{tabular}{l l l} \hline
**MAC Mechanism** & **Proposed Approach** & **Related Works** \\ \hline  & \multicolumn{3}{c}{_Energy-efficient channel access_} \\ \hline Collision-free access & TDMA for periodic allocations & [79, 80, 90, 91, 94, 97, 103, 104] \\  & Tree-based distributed scheduling & [81] \\ \hline Hybrid access & Contention phase for access request & [82] \\ (continon-based/collision-free) & Distributed queuing with collision-free data transmission & [83] \\  & Energy-aware polling and probabilistic hybrid access & [84, 85] \\ \hline Preamble sampling, & Polling at predefined wake-up times & [86, 87] \\ minimizing idle listening & Polling with dynamic learning of wake-up times & [88] \\  & Polling through secondary receiver & [89] \\ \hline  & Timestamp through control packets & [79, 80, 82, 83, 87, 88] \\  & Predefined wake-up fallback time & [86] \\ Synchronization & Secondary channel & [89] \\  & Dynamic guard bands and clock drift correction & [90] \\  & Heartbeat-based time reference & [91] \\ \hline  & \multicolumn{3}{c}{_QoS Provisioning_} \\ \hline Access & Two-level priorities (normal, alarms) & [86, 87, 92] \\ priorities & Multiple priorities & [29, 93] \\ \hline Cross-layer & Channel-aware & [94, 95] \\ scheduling & Routing-aware & [81] \\  & Energy-aware & [83, 96, 97, 98] \\ \hline  & \multicolumn{3}{c}{_Reliability_} \\ \hline Retransmission mechanisms & ACK packets and retransmissions & [80, 82, 83, 86, 87, 89, 91, 96] \\ \hline  & Transmission of NC packets & [99] \\ Diversity with & Cooperative diversity coding & [100, 101] \\ network coding (NC) & Cooperative diversity coding with QoS & [102] \\  & Cloud-assisted NC & [30] \\ \hline  & \multicolumn{3}{c}{_Interference and coexistence mechanisms_} \\ \hline Interference-aware & Cognitive radio approach & [103] \\ channel selection & Channel selection based on link quality & [29, 95] \\ \hline Coexistence & Coexistence with IEEE 802.11 & [104] \\ mechanisms & Coexistence in hospital environments & [105] \\ \hline \end{tabular}
\end{table}
Table 4: Overview of Medium Access Control (MAC) layer mechanisms for Wireless Body Area Networks (WBANs).