The document discusses Received Signal Strength Indicator (RSSI) in wireless sensor networks. RSSI is a measurement of the power of a received radio signal. It can be used to estimate node distance and connectivity. Some cautions of using RSSI in TinyOS include that RSSI values are platform-specific and may not be in dBm units. An example Java application called RSSIDEMO outputs RSSI measurements from a sending node to a RSSI base station node. Advantages of RSSI for localization include not requiring directional sensors and working indoors or outdoors, while difficulties include effects of multipath propagation, fading, and bad frontend devices.