\section{System Design}
\label{sec-design}

\begin{figure}[t!]
  \centering
  \includegraphics[width=0.45\textwidth]{./figures/system-crop.pdf}
  \vspace{2mm}
  \caption{\textbf{System Components}}
  \label{fig:system}
  \vspace{-6mm}
\end{figure}

\PS{} collects two types of measurements from clients---spectrum utilization,
and network performance and healthiness information---in two different
ways---synchronously and asynchronously. Figure~\ref{fig:system} shows
the main components of \PS{}.

Idle smartphones can be used to improve nearby device's network performance. For
example, in Figure~\ref{fig:system}, when \PS{} Access Point (AP) sends a
synchronous query about spectrum condition of the active device (e.g., a
laptop), nearby \PS{} client (smartphone) will perform detailed measurements
\textit{on behalf of} the laptop. This information can then be fed into AP
adaption (e.g., channel assignment, rate and power control, etc.) algorithms for
better network performance---all without disrupting the current network session
of the active client.

On the other hand, to cope with rapidly-changing network environment caused by
mobility, smartphones already perform aggressive network exploration and thus
naturally generate a flow of measurements of high temporal resolution.
Harnessing this behavior for network monitoring purpose only requires to deliver
the measurements to those who can make use of it. Besides, lightweight network
performance tests can be conducted using smartphones' idle cycles without
consuming noticeable amount of energy. All these measurements can be uploaded
asynchronously in a energy-neutral way (e.g., by only uploading when phone is
charging) for long-term network monitoring purpose.