# Evaluation of BLE Star Network for Wireless Wearable Prosthesis/Orthosis Controller +
## 3 Materials and Methods  
### Experimental Set-Up  
The maximum number of sustained peripheral connections, average and maximum power consumed, and latency were measured for all combinations of connection interval values of 10, 20, 30, 40, 50, and 100 ms and event length values of 2500, 5000, and 7500 \(\upmu\)s. These values cover the range of appropriate values for real-time prosthesis/orthosis control, as connection intervals above 100 ms would introduce noticeable lag into the control system [6]. The minimum values selected for the connection interval and event length \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \begin{tabular}{c} **Connection Interval (ms)** \\ **[MTU Size]** \\ \end{tabular} & **Event Length (\(\upmu\)s)** & \begin{tabular}{c} **Theoretical Max.** \\ **Number of** \\ **Connections** \\ \end{tabular} & \multicolumn{3}{c}{**Achieved Connections at Specified Distance**} \\ \cline{3-6}  & & **Connections** & **0.5 m** & **4.5 m** & **9.0 m** \\ \hline \multirow{4}{*}{\begin{tabular}{c} 10 \\ [40 bytes] \\ \end{tabular} } & 2500 & 4 & 4 & 3 & 2 \\ \cline{2-6}  & 5000 & 2 & 2 & 2 & 2 \\ \cline{2-6}  & 7500 & 1 & 1 & 1 & 1 \\ \cline{2-6}  & 2500 & 8 & 8 & 2 & 1 \\ \cline{2-6}  & 5000 & 4 & 4 & 4 & 1 \\ \cline{2-6}  & 7500 & 2 & 2 & 2 & 1 \\ \hline \multirow{4}{*}{\begin{tabular}{c} 30 \\ [80 bytes] \\ \end{tabular} } & 2500 & 12 & 11 & 6 & 3 \\ \cline{2-6}  & 5000 & 6 & 6 & 3 & 1 \\ \cline{2-6}  & 7500 & 4 & 4 & 4 & 2 \\ \hline \multirow{4}{*}{\begin{tabular}{c} 40 \\ [100 bytes] \\ \end{tabular} } & 2500 & 16 & 12 & 6 & 1 \\ \cline{2-6}  & 5000 & 8 & 8 & 8 & 2 \\ \cline{2-6}  & 7500 & 5 & 5 & 6 & 2 \\ \hline \multirow{4}{*}{\
\begin{tabular}{c} 50 \\ [120 bytes] \\ \end{tabular} } & 2500 & 20 & 12 & 5 & 2 \\ \cline{2-6}  & 5000 & 10 & 10 & 7 & 3 \\ \cline{2-6}  & 7500 & 6 & 6 & 6 & 1 \\ \hline \multirow{4}{*}{
\begin{tabular}{c} 100 \\ [247 bytes] \\ \end{tabular} } & 2500 & 40 & 10 & 7 & 1 \\ \cline{2-6}  & 5000 & 20 & 10 & 7 & 2 \\ \cline{1-1} \cline{2-6}  & 7500 & 13 & 11 & 2 & 2 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Number of connections sustained vs. distance. Also shown is the theoretical maximum number of connections. represent the lower limits available in the version of the software development kit (nRF Connect SDK v2.3.0) used. The minimum connection interval available is 7.5 ms, which was rounded up to 10 ms. Event lengths shorter than 2500 us were not supported by the software development kit used. The MTU value was scaled to maximize the number of analog-to-digital converter (ADC) samples sent for a given connection interval. When sampling at 1 kHz (sampling period of 1 ms), the number of ADC samples captured within a connection interval is equal to the duration of that connection interval in ms. Thus, to remain current with the capture of ADC samples in a real-time system, the number of bytes transmitted per connection interval must scale with the connection interval duration. In this case, each 12-bit sample is stored in 2 bytes. Twenty bytes of each BLE data packet were reserved for header information. Therefore, the MTU size used, in bytes, is equal to two times the value of the connection interval plus twenty bytes of header information. For the longest connection interval considered, 100 ms, a MTU size of 247 bytes was selected, as it is the maximum number of bytes permitted in a single packet [30]. Setting the MTU size based on the number of ADC samples collected minimized the delay between when the data were sampled and when they were ready for BLE transmission. This approach mimics embedded MCU schemes. All MTU sizes used are presented in Table 1, alongside the corresponding connection interval. A transmit power of +0 dBm was used for all measurements. A single central node was connected to as many peripheral devices as possible for each configuration tested.