## 5 Open Research Issues, Challenges and Opportunities
### Communication and Networking Protocols Challenges  
Existing MAC protocols, which claim to be effective for WBANs, are not completely compliant with the key attributes and characteristics described in Section 4.2.2. Furthermore, the MAC layer of the IEEE 802.15.6 standard [9] needs to be maximally exploited to meet the required design constraints. MAC layer should be robust enough to support multiple WBANs reliably and has to be  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Categories** & **Monitoring** & **Sensors** & **Other Devices** \\ \hline \multirow{4}{*}{Health} & Vital signs: heartbeat, temperature, oxygen saturation, _etc._ & Pulse count,heart rate, respiratory count, & \\  & Specialized Signals: ECG, EEG, EMG, _etc._ & temperature, SpO\({}_{2}\), ECG, EEG & Smart Phone \\ \hline Movement \& & & Accelerometer, \\   Activity & Speed and direction & gyroscope, etc. & GPS \\ \hline Safety and Protection & Fall detection, unusual activity detection, toxic gasses and hazards & Accelerometer, gyroscope, gas sensor & RFID, BLE \\ \hline \hline \end{tabular}
\end{table}
Table 15: Monitoring requirements for workers safety scenario.  
energy efficient. Moreover, QoS requirements for transmission of data as soon as possible need to be handled in appropriate manner. In order to achieve many design objective through proposed IEEE 802.15.6 MAC protocol is still to be investigated and is an open challenge as discussed above in the previous section.  
With regards to effective networking for WBANs, appropriate mobility modeling is very important. Since the human body is constantly moving, sensor nodes on or inside the body have mobility features. With the body movements, the distance between sensor nodes will be constantly changing. This will cause serious topological partition problems in that some links between nodes are broken and built frequently. This kind of network is called Delay Tolerant Network (DTN), whose routing protocol is very difficult to design and realize [89]. Existing approaches in the Mobile _Ad hoc_[128] or the Delay Tolerant Networking (DTN) [129] fields need to be evaluated and coupled with Wearable WBANs. For example, [130] deals with a limited multi-hop WBAN and proposes an epidemic routing strategy coupled with a TDMA MAC scheduling over a single BAN.  
For low-range communication, an opportunistic and on-demand routing scheme coupled with service discovery may be sufficient to enable collaboration across WBANs. Regarding long-range communications, work such as WASP and CICADA [131] use cross-layer data-forwarding schemes to extend the structure of the WBAN in a tree-like topology. However, depending on the considered applications, the WBANs' communication architecture cannot always be organized around a central WBAN coordinator. In this context, a promising research trend is the development of opportunistic and mobility aware communication stack.  
Last but not least, energy efficiency is a paramount issue to guarantee that WBAN systems can operate continuously for up to several days between batteries recharge cycles. Energy consumption can be divided into three categories, _i.e._, radio communication, data processing, and sensing. Hence, special attention should be paid on designing energy efficient WBAN communication stacks where available radio communications and data processing at the WBANs' nodes are efficiently managed. In this context, promising research trends concern the development of low control overhead, low delay transmission, and cross-layer MAC and Networking layers. Finally, as will be discussed further in Section 5.4, the proper management of interference is also extremely important to improve energy efficiency and enable the emergence of green WBANs.