Wireless Control That Simply Works
Companies looking for reliable and secure wireless monitoring and control solutions have a new alternative –- ZigBee™ (www.zigbee.org). ZigBee provides the network, security and application profiles layers for the IEEE 802.15.4 global standard for reliable, low-power, wireless data communications.
ZigBee’s standards-based approach simplifies wireless design, accelerates product development and reduces project risk. In addition, customers will soon be able to enjoy interoperable wireless products from multiple vendors, for example, lighting systems, utility meters, and heating and cooling systems can be interconnected to simplify operation and optimize energy consumption. The ZigBee Alliance, a rapidly growing international association of semiconductor companies, software providers, systems integrators, and original equipment manufacturers (OEMs), is defining the ZigBee specification, along with the requirements for certification and product compliance testing. The goal of the ZigBee Alliance is to provide "Wireless Control That Simply Works."
The IEEE 802.15.4 standard (www.ieee802.org/15/pub/TG4.html), ratified in May 2003, specifies the physical (PHY) and medium access control (MAC) layers for multiple RF bands, including 868 MHz, 915 MHz, and 2.4 GHz. The standard is designed to provide reliable data transmission of modest amounts of data up to 100 meters or more while consuming very little power. IEEE 802.15.4 is a simple packet protocol, typically less than 32 kb in size, featuring a 64-bit address space, source and destination addressing, error detection, and advanced power management. ZigBee extends the capabilities of this new radio standard by defining a flexible and secure network layer that supports a variety of architectures to provide highly reliable wireless communications in harsh or dynamic RF environments.
Highlights of IEEE 802.15.4 Standard
The three license-free frequencies of the IEEE 802.15.4 standard include sixteen channels at 2.4 GHz, ten channels at 915 MHz, and one channel at 868 MHz, to support global or regional deployment. The maximum data rates for each band are 250 kbps, 40 kbps and 20 kbps, respectively. The air interface is direct sequence spread spectrum (DSSS) using binary phase shift keying (BPSK) for 868 MHz and 915 MHz and offset-quadrature phase shift keying (O-QPSK) for 2.4 GHz. Other features of the IEEE 802.15.4 PHY include receiver energy detection, link quality indication and clear channel assessment. Both contention-based and contention-free channel access methods are supported. Maximum packet size is 128 bytes, including a variable payload of up to 104 bytes. IEEE 802.15.4 employs 64-bit IEEE and 16-bit short addresses, which supports over 65,000 nodes per network. The IEEE 802.15.4 MAC also enables network association and disassociation, has an optional superframe structure with beacons for time synchronization, and a guaranteed time slot (GTS) mechanism for high priority communications. The access method is carrier sense multiple access with collision avoidance (CSMA-CA).
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