ZigBee, a proposed standard that is a short-range, low-power, wireless personal-area network technology optimized for low-cost sensor and control devices used in homes, office buildings, and medical and industrial settings, is based on the IEEE (Institute of Electrical and Electronics Engineers) 802.15.4 packet radio standard. IEEE 802.15.4 defines the low-rate physical layer and the media access control part of the data link layer to communicate over unlicensed areas of the spectrum that cordless telephony and microwave ovens use. Most chips and modules use the 2.4GHz band, and some use the longer-range, better penetration 900MHz band. To the direct-sequence spread-spectrum radio specification, ZigBee adds a network layer that includes self-organizing multi-hop routing. Within a few years, an average house could possibly have 100 or more devices, while a large office building or plant could have tens of thousands of ZigBee nodes. A security toolbox for ZigBee includes use of access control lists, packet freshness timers, and encryption based on AES-128 from NIST (National Institute of Standards). ZigBee was designed to use very little energy, and this is accomplished with ZigBee protocols, which minimize radio on-time. ZigBee, was also designed to work well with hostile radio frequencies generally found in industrial and commercial applications, a feature that increases its reliability. The ZigBee Alliance was formed in 2002 to coordinate activities and to regulate and promote the proposed standard and related technologies. More than 175 companies have signed on with the alliance.