## 3 Design Challenges for mHealth Applications  
The previously described M2M system architecture can be employed in the context of wireless mHealth applications, aiming to form a communication bridge between the patients and the healthcare providers. At the patient's side, we consider WBANs, defined as low-power short-range networks deployed in the vicinity of the human body, as the prevalent technology for the M2M area (or capillary) network. The nature of WBANs poses inherent limitations not present in regular sensor networks, due to the specific characteristics and requirements of the employed M2M sensor devices. Furthermore, to provide end-to-end connectivity, the integration of different medium and long range technologies must be ensured, guaranteeing reliable and secure communication. The key design characteristics and challenges that must be taken into account at different stages of the M2M system, in order to design efficient techniques and communication protocols are summarized next [15, 16, 21, 22]:  
* _Heterogeneous devices and traffic_: In mHealth applications, the M2M devices are usually medical sensors that can be either attached on (wearable or on-body), or placed inside (implantable or in-body) the human body. These devices are capable of establishing wireless communication links, in order to enable continuous patient monitoring and provide real-time feedback to the responsible \begin{table}
\begin{tabular}{p{113.8 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt}} \hline \hline
**Sensor** & \begin{tabular}{c} **Battery Lifetime** \\ \end{tabular} & \begin{tabular}{c} **Data Rate** \\ \end{tabular} & \
\begin{tabular}{c} **Criticality** \\ \end{tabular} &
\begin{tabular}{c} **Application** \\ \end{tabular} \\ \hline Deep brain stimulation & \(>\)3 years & 1 Mbps & High & Therapeutic benefits for [PERSON]’s disease, chronic pain, tremor, and dystonia \\ \hline Hearing aid & \(>\)40 h & 200 kbps & High & Sound amplification \\ \hline Accelerometers/Gyroscopes & \(>\)1 week & 1 Mbps & Low & Measurements on motion detection, acceleration and angular velocity \\ \hline Pulse oximeter (\(SpO_{2}\)) & \(>\)1 week & 2 kbps & Low & Measurement of hemoglobin oxygen saturation \\ \hline Capsule endoscope & \(>\)24 h & 1 Mbps & High & Imaging of the digestive tract \\ \hline Temperature & \(>\)1 week & 2.4 bps & Low & Body or environmental temperature measurements \\ \hline Electrocardiogram (ECG) & \(>\)1 week & 9.6 kbps & Medium & Heart waveform characteristics \\ \hline Electromyograph (EMG) & \(>\)1 week & 100 kbps & Medium & Muscle movement \\ \hline Gate/Falls & \(>\)1 week & 250 kbps & High & Fall detection \\ \hline Electroencephalo-gram (EEG) & \(>\)1 week & 100 kbps & High & Brainwave activity \\ \hline Video/Medical imaging & \(>\) 12 h & \(<\)10 Mbps & High & Digital video transmission \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Technical characteristics of selected Wireless Body Area Networks (WBAN) sensors for mHealth applications [18, 23]. healthcare provider. Based on their operation, they can be classified as sensors or actuators. Sensors are used to measure external (e.g., motion, location, environmental temperature, _etc._) or internal (e.g., heart beat, body temperature, muscle movement, brain activity, _etc._) parameters of the human body. Actuators perform specific actions according to data received from the sensors or inserted by the end user (e.g., a pump for the administration of insulin based on blood glucose measurements). Unlike typical sensor networks, the number of devices in a WBAN is relatively small (_i.e._, usually restricted within the range of 20-50), with each device having a unique function. Table 1 contains a list of commonly employed sensors, emphasizing their diverse communication characteristics and requirements.