  This paper presents a crowd monitoring system based on the passive detection
of probe requests. The system meets strict privacy requirements and is suited
to monitoring events or buildings with a least a few hundreds of attendees. We
present our counting process and an associated mathematical model. From this
model, we derive a concentration inequality that highlights the accuracy of our
crowd count estimator. Then, we describe our system. We present and discuss our
sensor hardware, our computing system architecture, and an efficient
implementation of our counting algorithm -- as well as its space and time
complexity. We also show how our system ensures the privacy of people in the
monitored area. Finally, we validate our system using nine weeks of data from a
public library endowed with a camera-based counting system, which generates
counts against which we compare those of our counting system. This comparison
empirically quantifies the accuracy of our counting system, thereby showing it
to be suitable for monitoring public areas. Similarly, the concentration
inequality provides a theoretical validation of the system.
