10-04-2017, 08:37 PM
Zig Bee technology
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Why is ZigBee needed?
There are a multitude of standards that address mid to high data rates for voice, PC LANs, video, etc. However, up till now there hasn t been a wireless network standard that meets the unique needs of sensors and control devices. Sensors and controls don t need high bandwidth but they do need low latency and very low energy consumption for long battery lives and for large device arrays.
There are a multitude of proprietary wireless systems manufactured today to solve a multitude of problems that also don t require high data rates but do require low cost and very low current drain.
These proprietary systems were designed because there were no standards that met their requirements. These legacy systems are creating significant interoperability problems with each other and with newer technologies.
The ZigBee Alliance is not pushing a technology; rather it is providing a standardized base set of solutions for sensor and control systems.
The physical layer was designed to accommodate the need for a low cost yet allowing for high levels of integration. The use of direct sequence allows the analog circuitry to be very simple and very tolerant towards inexpensive implementations.
The media access control (MAC) layer was designed to allow multiple topologies without complexity. The power management operation doesn t require multiple modes of operation. The MAC allows a reduced functionality device (RFD) that needn t have flash nor large amounts of ROM or RAM. The MAC was designed to handle large numbers of devices without requiring them to be parked .
The network layer has been designed to allow the network to spatially grow without requiring high power transmitters. The network layer also can handle large amounts of nodes with relatively low latencies.
ZigBee/IEE 802.15.4 - General Characteristics
Dual PHY (2.4GHz and 868/915 MHz)
Data rates of 250 kbps (@2.4 GHz), 40 kbps (@ 915 MHz), and 20 kbps (@868 MHz)
Optimized for low duty-cycle applications (<0.1%)
CSMA-CA channel access
Low power (battery life multi-month to years)
Multiple topologies: star, mesh
Addressing space of up to:
18,450,000,000,000,000,000 devices (64 bit IEE address)
65,535 networks
Optional guaranteed time slot for applications requiring low latency(tmp inactive)
Range: 50m typical (5-500m based on environment)
ZigBee/IEE802.15.4 - Typical Traffic Types Addressed
Periodic data
Application defined rate (e.g., sensors)
Intermittent data
Application/external stimulus defined rate (e.g., light switch)
Repetitive low latency data
Allocation of time slots (e.g., mouse)
Each of these traffic types mandates different attributes from the MAC. The IEE802.15.4 MAC is flexible enough to handle each of these types.
Periodic data can be handled using the beaconing system whereby the sensor will wake up for the beacon, check for any messages and then go back to sleep.
Intermittent data can be handled either in a beaconless system or in a disconnected fashion. In a disconnected operation the device will only attach to the network when it needs to communicate saving significant energy.