Introduction
There has
been tremendous growth in the field of wireless communication during the last
decade. The wide acceptance of 802.11 standards for wireless local area network (WLAN)
and cellular phone networks have proved that low cost wireless solutions are feasible and acceptable. There are many
applications that require low cost,
low data rate, low power and inexpensive solution to network within a small area thus requiring a low rate
wireless personal area network (LR- WPAN). There
are many proprietary solutions that address these needs, but they are expensive and incompatible between manufacturers.
The IEEE 802.15.4 is a new standard
for LR-WPAN providing a low cost and less complicated solution. The expected
applications are home/office automation, industrial sensors and control, distributed sensor networks and
environment monitoring. The ZigBee networking standard has slowly but surely
taken up an important share of the wireless
market. Vendors of ZigBee products are increasing, and more and more types are coming out.
The ZigBee
alliance, an association of companies working together to develop ZigBee
standard based products for monitoring and control, has led to the increased
adoption of the IEEE 802.15.4 standard. The ZigBee standard defines upper layer that are built on the IEEE 802.15.4
standard. The aim of the ZigBee alliance is to replace every switchbox,
electrical outlet and various sensors in a
building by wireless nodes that communicate with each other even though manufactured by different
manufacturers. This standard is being
widely adopted by the industry and the numbers of products based on the standard are increasing exponentially. The ZigBee
alliance is forecasting that in the next few years time, there could be
50 ZigBee devices per home and eventually
as many as 150.
Network Topologies
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| Star, Tree and Mesh Topology |
Device
types
There are
three different types of ZigBee devices
ZigBee
coordinator (ZC): The most capable device, the coordinator forms the root of the network tree and might bridge to
other networks. There is exactly one ZigBee coordinator in each network since
it is the device that started the network originally. It is able to store information
about the network, including acting as the Trust Centre and
repository for security keys.
ZigBee
Router (ZR): As well as running an application function a router can
act as an intermediate
router, passing data from other devices.
ZigBee End Device (ZED): Contains just
enough functionality to talk to the parent
node (either the coordinator or a router); it cannot relay data from other devices. This relationship allows the node to be
asleep a significant amount of the time thereby giving long battery
life. A ZED requires the least amount of memory,
and therefore can be less expensive to manufacture than a ZR or ZC.
ZigBee Protocol Stack
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| Protocol Stack |
The ZigBee specification is a standard that defines a stack protocol enabling the interoperability of wireless devices in a low-cost, low-power consumption and low-data-rate network. The ZigBee stack is founded over the IEEE 802.15.4 standard, which defines the multiply accumulate (MAC) and physical (PHY) layers of the protocol. MAC and PHY layers define the RF and communications components of neighboring devices. ZigBee stack layers, on the other hand, include a network layer, an application layer and a security service provider (SSP).
Devices are defined by the profiles and are implemented
as application objects. Each application object is
connected to the rest of the ZigBee stack by an endpoint, which is an addressable component within a device.
Communication is made from endpoint
to endpoint and through data structures called clusters. Clusters
contain a set of attributes needed to share information between application objects. Clusters used in a specific
application are defined within its profile.
Each interface can receive or send data in the form of a cluster.
There
are two special endpoints: 0 and 255. Endpoint 0 is used for the configuration
and management of the entire ZigBee device. Through this endpoint, the application can communicate with
other layers of the ZigBee stack to
initialize and configure them, attach points with data transmission, security
and binding. Binding is the ability
to match different but compatible devices together, such as a switch and lamp.
The network layer enables devices to communicate with one another. It is involved in the initialization of the device
within a network, routing of messages
and network discovery. The application support sublayer also provides these
services. The application can configure and access the parameters of the network layer through the ZDO.
Application Example - A Reference Design
This
reference design describes the design of light dimming devices for light bulbs,
based on Freescale's MC1321x microcontroller with an 802.15.4 RF modem and controlled by the MC33794 E-Field Imaging device. The communication is
based on a low power 2.4 GHz radio frequency transceiver using Simple MAC
(SMAC) based on ZigBee.
The system
consists of two kinds of applications:
·
Intelligent switch is intended to
replace a standard wall switch and provides
dimming function on one phase of AC power. The control interface is an E-field contactless touch panel
with various shapes of electrodes.
The intelligent switch can also control four intelligent outlets through a 2.4 GHz radio frequency transceiver
using SMAC.
·
Intelligent
outlets are intended to replace standard wall power outlets. The device needs the full AC power and dims the light in
standard desk lamps and other
portable devices. There are two versions of intelligent outlet boards,
implementation in a wall outlet case and implementation in an adaptor case
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| System Block Diagram |
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