ZigBee for monitoring and control.
The term ZigBee was born from the association with the movement of dancing bees, reminiscent of a zigzag — this is the way they exchange information.
This standard is the result of the joint work of a number of companies united in the ZigBee Alliance.
The standard provides specifications for the development of cheap, low-power wireless sensor networks that operate in a coordinated and autonomous manner.
The main advantages of ZigBee:
– reliability and self-organization;
– a large number of supported nodes;
– ease of installation;
– long (a year or more) battery life;
– security;
– low cost;
– wide range of applications;
– ensuring interchangeability of networks and nodes
– independence from the equipment manufacturer.
What is the IEEE 802.15.4 standard
IEEE 802.15.4 is an Institute of Electrical and Electronics Engineers (IEEE) standard for low-rate wireless personal area networks (WPANs) that defines the physical layer and the medium-range access layer.
Specifications: for the physical layer, or PHY, for low-power spread spectrum radio at 2.4 GHz with a basic rate of 250 kilobits per second, and for the 915 MHz and 868 MHz frequencies with reduced rates.
Detailed information is available on the IEEE 802.15 Working Group for WPAN website: http://ieee802.org/15/pub/TG4.html.
The IEEE 802.15.4 standard provides three frequency ranges
The difference between ZigBee and other wireless standards
There are many standards that define medium- and high-speed parameters for voice, wireless local area networks, video, etc.
However, until recently, there was no wireless networking standard that met the specific needs of sensor and control devices, which, while using long batteries and supporting a large number of devices on the network, require low latency and economical power consumption rather than high throughput.
Many specialized wireless systems with such characteristics are produced, but they exist outside the standards, which causes problems with their interaction with each other and compliance with new technologies.
Relative arrangement of wireless standards
Real areas of ZigBee use
ZigBee is perfect for solving a wide range of problems in automated building management, technological, medical control and monitoring.
These include:
– lighting control;
– technological and structural control;
– air conditioning and heating control;
– automatic meter reading;
– wireless smoke and CO detectors;
– room control, security systems;
– environmental parameter control;
– medical reading and control;
– asset management.
ZigBee Applications
For example, sensors installed in containers self-organize into a network in each of them, and individual networks – into the general network of the container ship and transmit data on the state of the cargo to the control center.
ZigBee from the manufacturer’s point of view
ZigBee solution providers
A manufacturer developing a ZigBee-compatible device must select ZigBee technology providers.
They in turn will provide the software (ZigBee stack) along with the radio transceiver and microcontroller.
The leading manufacturers of ZigBee chips are Freescale, Texas Instruments (TI) and Atmel. Among others, we note Ember, STMicroelectronics, Silicon Laboratories and Jennic.
The most popular processors on the market are ATmega1281 (Atmel), MSP430 (TI) and HC08 (Freescale).
Their flash memory sizes range from 64 KB to 256 KB (at least 128 KB is required to install the ZigBee stack).
Radio transceivers are available from Chipcon (such as the CC2420 transceiver), part of TI, and Freescale (MC13192). Ember's EM2420 radio is a licensed version of the CC2420. In June of this year, Atmel finally introduced a new radio transceiver, the AT86RF230.
The long wait has paid off, as the RF230 offers a range that is about three times greater than its competitors.
Most radio transceivers operate at 2.4 GHz, although lower frequency radio transceivers are also available (e.g. from ZMD).
Solutions based on so-called systems on a chip (System+on+Chip, SoC) have recently emerged. Jennic pioneered the JN512.
It was followed by Chipcon with the CC2430, Ember with the EM250 and Freescale with the MC1321. There is a tendency among the major players to expand the production of such solutions.
Example of a ZigBee module and a dual-chip sensor board
Module or System on a Chip
ZigBee vendors have developed modules designed to facilitate the process of creating applications. Such modules contain radio transceivers and microcontrollers, as well as the necessary passive elements.
Modules are not an intermediate stage between dual-chip solutions and a system on a chip. They occupy a separate niche.
Using a ZigBee module allows you to shorten the development cycle, since there is no need for additional radio expertise.
Modules are usually already certified (FCC/CE) and are easily integrated even on a two-layer board. They are more expensive than systems on a chip, but this is offset by a number of advantages.
Most manufacturers (such as Cirronet, Helicomm, MaxStream, Radiocraft, and Telegesis) use radio transceivers from Chipcon (TI) and Freescale.
A module with an Atmel AT86RF230 transceiver is currently only available from MeshNetics.
For the developer
Chip and module manufacturers usually offer demo kits and developer kits.
A typical kit includes sensor boards, radio, microcontroller with system software, accessories and software for wireless network management. The main purpose of the kit is to give an idea of the capabilities of the module or chip and provide conditions for developing simple applications.
Software factor
Software is a critical factor in developing ZigBee+ applications. The ZigBee+ stack determines the option for forming a network and exchanging data.
The stack architecture is a set of blocks called layers. Each layer is responsible for a specific set of functions of the layer above.
ZigBee stack diagram
Possible network configurations and node roles
There are three different network topologies supported by ZigBee: star, mesh, and cluster tree.
The «star» topology, being the simplest, is used quite often. The mesh topology provides a high degree of reliability. Data blocks can be transmitted over the network via various nodes and routers.
If there is interference on certain nodes, others can be used instead.
A «hybrid» network combines both topologies.
The main suppliers of ZigBee+ stacks are Figure8 Wireless (now part of TI), Freescale, Ember, CompXs (acquired by Integration Associates). The stacks are created by them in conjunction with the corresponding hardware solutions. There are also several suppliers of independent stacks, such as AirBee, MeshNetics and Mindtek.
Wireless data acquisition
The value of a wireless sensor network is obvious when receiving information from sensors.
Therefore, properly written software becomes a prerequisite for the full operation of the network. The functions of such software include network configuration, testing and monitoring of nodes, receiving data from sensors and controlling actuators.
The problem is that most enterprises already have one or another IT system in place, the replacement of which seems undesirable. An example is the SCADA dispatch control and data acquisition system.
Today, most of its variants are wired, and therefore have limited functionality, since only a small part of critical nodes and technologies are automated or monitored.
At the same time, the cost of cable installation can reach $300 per 1 m, and in addition, wired solutions are simply impossible in some remote, hard-to-reach and dangerous points and nodes. And wireless systems justify themselves perfectly in the automation of buildings.
Their sensors can be installed almost anywhere, which allows you to receive more accurate data on the actual state of objects and significantly reduce costs due to savings on the cost of wiring.
This happens, for example, when using ZigBee in an HVAC (heating, ventilation and air conditioning) control system.
Thus, the transition to wireless sensor networks allows us to overcome the limitations of wired solutions.
However, questions remain: how will existing IT systems receive information from wireless sensors and is it possible to painlessly integrate new wireless sensor networks with existing IT systems?
After all, companies that have invested heavily in installing such systems and training staff are not going to give them up, but at the same time would like to take advantage of the latest technologies.
For these purposes, Crossbow, MeshNetics and Tendril Networks already provide packages of so-called middleware.
It is usually installed on a server between the WSN (wireless sensor network) and the enterprise system.
The middleware filters the data and transmits the necessary information to the applications of the enterprise IT system.
