Using Sensor Motes for Real Time Measurement of Social Networks
Our mote-based social-network-capture system uses two basic elements. Individual healthcare workers will wear rechargeable badge-like motes, while additional motes, placed in a number of static locations in the hospital unit, will serve as fixed-point beacons and data transfer stations. Beacons, which can be used to triangulate position, are functionally identical to motes, but are usually configured with larger batteries or are simply plugged into an AC power outlet. Both types of motes are programmed to periodically broadcast a timestamped identifying packet at low power. As these message packets are broadcast packets, they are not destination-specific, and can be received by any other badge or beacon. When a packet is received, its originating device identifier, received signal strength (RSSI), and timestamp are logged in on-board memory by the recipient. Here, RSSI is used as a proxy for distance; its precision is sufficient to discriminate between short distances (up to a few feet away) and longer distances (up to the transmission limit, typically no more than 200 feet, depending on broadcast power). Since the process is symmetric, both motes will record the contact. Thus the merged mote contact logs constitute an explicit contact trace; using these data collected over time, it is easy to infer a social network in the form of a contact graph, with nodes representing healthcare workers, and weighted edges representing the rate of contact between the connected nodes over some standard unit of time.
Motes are quite inexpensive (less than approximately $100 each, with prices expected to decline significantly as the technology improves), and hold a number of technical advantages over, e.g., RFID-based systems. First, motes record data locally in memory, which can then either be offloaded for analysis after the fact, or may even be opportunistically offloaded when passing near a fixed beacon. Second, with proper beacon placement, sensor motes promise much higher location resolution than that offered by RFID systems, and motes will continue to record interactions with each other even outside the range of any fixed beacon infrastructure. In contrast, RFID systems cannot directly record interactions, but merely the co-location of multiple devices, typically only to room-level resolution only in rooms where RFID sensors have been installed. Finally, because the mote infrastructure consists only of beacons, it is portable and easily reconfigured, meaning it can quickly be retargeted to fit new locations.
Our first-generation design consists of a commercially-available wireless sensor board paired with a rechargeable iPod battery in a repur posed pager case. This badge-like mote is worn by individual healthcare workers. The USB port is for data transfer, including maintenance and development uploads of custom mote control software. The round connector on the right side of the pager case accommodates a charging cradle.