## 4 Network Communication
### State-of-the-Art Research on Network Communication
#### 4.1.3 Physical Layer  
The physical layer, as the bottom layer of BSN structure, is mainly responsible for encoding and decoding of signals, preamble generation and removal for synchronization, bit transmission and reception, and specification of the transmission medium. In the past few years, the research on BSN physical layer has focused on two aspects, which are channel selection and channel characterization. (1) Channel Selection  
The physical channel, as the data transmission medium, plays a key role in the BSN. Its characteristics directly affect the performance of the whole network. Therefore, in order to achieve the best transmission performance, the most suitable channel must be chosen according to different data transmission scenes. Figure 3 presents all the bands in a BSN. The Human Body Communication (HBC) band which takes the human body as the data transmission medium is only used by BSNs. Its primary technology is electric field coupling which includes capacitive coupling and galvanic coupling [105,106]. An HBC channel of the capacitive coupling type is developed based on the near electric field around body which is induced by a transmitter terminal, and a receiver terminal is used to detect the weak coupling changes of the near electric field along the human body channel. Research has shown that the data rate of capacitive coupling is up to 2 Mb/s [107]. In addition, Reference [108] has put forward a kind of electromagnetic coupling technology which takes human body as the \begin{table}
\begin{tabular}{p{142.3 pt} p{142.3 pt} p{142.3 pt}} \hline \hline  & **Star Topology** & **Mesh Topology** \\ \hline \multirow{4}{*}{Path Loss} & Nodes on the same side with low path loss. Nodes on the different sides with high path loss. & Reducing path loss caused by diffraction though multiple hops. \\ \hline \multirow{2}{*}{Radio Transmission Range} & Not suitable for small radio propagation range. & Adjusting radio propagation range by changing the number of nodes \\ \hline \multirow{2}{*}{Energy Consumption} & Nodes closer to sink node consume lower power. & The nodes nearer to sink node consume more energy, as they have to forward not only their data but also data from other nodes. \\ \hline \multirow{2}{*}{Transmission Delay} & Sensors connect with sink node directly take the least possible delay in transmission. & Nodes closest to sink node get their data quickly, without any intermediate delay. \\ \hline \multirow{2}{*}{Inter-User Interference} & Nodes farther away from sink node need higher power to transmit data with more interference to other nodes. & As each node only transmits to its neighbors, the energy of transmission is low and hence with smaller interference. \\ \hline \multirow{2}{*}{Node Failure and Mobility} & Only the failed node is affected and the rest nodes of network perform well. & The whole network including nodes with errors need to be reset. \\ \hline \hline \end{tabular}
\end{table} 
Table 2: A comparison between star topology and mesh topology. electromagnetic waveguide to finish communications. Medical Implant Communications Service (MICS), which has a transmission power of 25 \(\upmu\)W, is also one of communication bands suitable for low data rate networks [109]. The Wireless Medical Telemetry Service (WMTS) is mainly used in wireless telemetry in hospitals. Its band is similar to some parts of the local TV spectrum in the United States, but no research on interference between the two bands has ever been done so far. Band F is the 2.36 GHz medical band newly approved by the Federal Communication Commission (FCC) in the United States. The Industrial Scientific Medical (ISM) band commonly used in BSNs operates at 2.4 GHz. Hence, for the sake of avoiding interference, corresponding solutions which could deal with interference must be designed [110]. UWB communication is believed to have great advantages and is promising in WBAN applications. It is a technique with low-power and high data rate features.