A communications protocol determines the type of signal being transmitted and how it encodes the information contained within the signal. While there are many different wireless communications protocols, three stand out for wireless sensing: Wi-Fi (IEEE 802.1), Bluetooth (IEEE 802.15.1), and ZigBee (IEEE 802.15.4). Each protocol is backed by a consortium of companies, with ZigBee gaining favor thanks to its simplicity, low cost, and low power dissipation (see the table).
Two special frequency bands have been set aside for wireless-sensor-net communications: the Industrial, Scientific and Medical (ISM) band and the Unlicensed Network Information Infrastructure (U-NII) band. However, many potential interference sources can plague wireless sensor nets, including microwave ovens and electric motors. Often, they will simply overwhelm sensor communications.
Another method of communications, RF ID tagging, requires no power in the RF ID tag for transmission. The passive technology is used for labeling and tracking materials. Message size, though, is one limitation. Present RF ID tags can hold only 96 bits of information. Work is under way to increase that amount to 128 and 256 bits.