The IEEE STD 802.15.4 speifies the RF, PHY and MAC layers. There are a variety of custom and industry-standards based networking protocols that can sit atop this IEEE STD.These networking protocols allow the rapid creation of mesh networks that are also self-healing. IEEE 802.15.4 STD defines an RF and PHY layer with a Phase-Shift-Key (PSK) tranceiver capable of over-the-air speeds of up to 250 kilobits per second(kbps), operating on a subset of 27 available radio channels in specific unlicensed 800, 900 and 2400 MHz bands.
The IEEE 802.15.4 STD uses two channel access methods : CSMA/CA and TDMA using synchronization beacons and Guaranteed Time Slots (GTS).There are four packet frame types : Data, Acknowledgement, MAC command and beacon.Each frame contains a receiver synchronization sequence, a packet length field, source and destination address, various
frame control bits, the data payload, and an error-detecting Frame Check Sequence (FCS). MAC layer generates beacon if the device is a coordinator, synchronizes to others’ beacons, support Personal Area Network (PAN) association and disassociation, manages the channel access, handles and maintains the GTS mechanism, and provides a reliable link between two peer MAC entities.
The IEE 802.15.4 STD specifies and controls only the RF, PHY dan MAC layer. It suggest, but does not describe, networking methods and techniques.The STD defines two types of physical device: The Full Function Device(FFD) and The Reduced Function Device (RFD). There are three logical network devices : PAN Coordinator, the router, and the end device.
RF Link :
The nominal transmitter power output specified is 0.5mW(-3dBm), again to limit power consumption but also because the standard is expected to be used in short-range (10-50 meter) applications.
PHY layer :
The PHY uses CSMA/CA to access the radio channel. CSMA-CA can provide nearly a 36% channel usage, but in practical environments where all stations cannot hear one another, the channel usage efficiency is as low as the traditional ALOHA mechanism, about 18%.
MAC Layer :
The MAC layer is responsible for generating network beacon. This beacon may be set in increments from approximately 15.83ms to over 4 minutes, as defines by the equation :
Beacon Interval = 15.83ms * (2^2) ; where n = 0 to 14.
A device may be allocated one or more GTS intervals in order to transfer network traffic – during that time no other device may use channel. When a GTS is used, the device does not use the CSMA-CA to access the channel, hence it named “guaranteed time slot”.
An Introduction to IEEE STD 802.15.4, by Jon T.Adams – Freescale Semiconductor, inc. Arizona USA.
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October 16, 2008
High Speed Network Lab