Wireless Technology and Networking Standards provide us with a very convenient way for information exchange. There is a vigorous development in the field of Wireless Communication Technology and you might have seen different technologies like WiFi, Bluetooth, Mobile Communications (Cellular Networks), RFID, NFC and WiMAX.
Hello Zigbee Technology!
So, where does Zigbee Technology fit into the existing pool of Wireless Technologies? The Zigbee Wireless Technology fits into a market that is not completely filled by other technologies. For example, other wireless technologies like WiFi and Bluetooth are striving to go faster, add more features, provide streaming of high definition content and run for hours. In contrast, the Zigbee Technology is designed for low data rates, fit into a tiny stack, controlling simple devices like LEDs or Thermostats and run for years. Although Zigbee Technology has many applications like Home Automation and Military, the main category that the Zigbee Technology has been aimed for is called as Wireless Sensor Network or WSN.
Wireless Sensor Network (WSN) is a collection (network) of wireless sensors that collect, store and process environmental information and also communicate with neighboring networks. More about WSN later. WSN has few unique requirements like low power and low cost and Zigbee Technology achieves these requirements with the help of a constraint: Low Data Rate.
What is Zigbee Technology?
Zigbee Technology is a Wireless Communication Standard that defines a set of protocols for use in low data rate, short to medium range wireless networking devices like sensors and control networks. The target of Zigbee Technology is low cost, low power, battery operated wireless sensors that do not need to constantly update its status and also allows sleep mode or low power mode for its electronics and radio. Zigbee Technology is based on IEEE 802.15.4 Standard and the Zigbee devices operate in the unlicensed 2.4 GHz ISM Band (ISM – Industrial, Scientific and Medical). Although 2.4 GHz band is commonly used worldwide for commercial Zigbee devices, there are devices that use a different set of frequency bands like 784 MHz, 868 MHz and 915 MHz in China, Europe and USA (and Australia) respectively. Since we are talking about low data rates of Zigbee Technology from the beginning, let us see the data rates supported by Zigbee. The data rates in Zigbee Technology are dependent on the frequency band. For example, the 868 MHz band supports a data transfer rate of 20 kbps while the more common 2.4 GHz band supports data rates up to 250 kbps. Before digging deep in to Zigbee Technology, let us take a look at two application, where Zigbee can be used. One application is the in-home patient monitoring, where a patient wears a Zigbee device, which periodically collects the information like blood pressure and heart rate. This data is then wirelessly transmitted to a local server in the patient’s home (local server – like a PC). The local server performs basic analysis and the vital information is sent to the Doctor over the internet. Another application of Zigbee is the building’s structural health monitoring. This application is very useful in earthquake prone areas. Several Zigbee based wireless sensors like accelerometers are installed throughout the build. These sensors, which form a wireless sensor network, collect information that can be used to detect signs of damage and evaluate whether the building is safe for public or not.
Why Low Data Rates in Zigbee?
When other wireless technologies like WiFi and Bluetooth are aiming to provide high speed data transfers, why the data rates are low in Zigbee standard? The answer is very simple. The intention behind the development of Zigbee Technology is to use it in wireless monitor and control. The amount of information and the frequency of communication in such applications is very less. Even though it is possible for an IEEE 802.15.4 network to achieve higher data rates, the Zigbee Technology, which is based on IEEE 802.15.4, is not designed to do so.
Zigbee and IEEE 802.15.4 are not the same
Yes. You’ve read it right. IEEE 802.15.4 is a technical standard, defined and maintained by IEEE (IEEE 802.15 Working Group, to be specific) that defines the operations of low data rate WPANs (Wireless Personal Area Networks). Zigbee, on the other hand, is a network protocol, designed developed and maintained solely by the Zigbee Alliance and uses the physical and MAC layers of IEEE 802.15.4. There are other networking protocols based on IEEE 802.15.4 like MiWi, SNAP, WirelessHART, etc.
Zigbee vs. Bluetooth and IEEE 802.11
Comparing IEEE 802.11, Bluetooth and Zigbee Technologies will help you understand how Zigbee Technology is different from other Wireless Technologies that operate in the same 2.4 GHz band. NOTE: There are many protocols in IEEE 802.11, but we will be considering IEEE 802.11b as it operates in the 2.4 GHz band. The data rates of IEEE 802.11 can be as high as 11 Mbps and the typical application of this protocol is wireless internet connection. The range of IEEE 802.11b is up to 35meter indoors and about 100 meters outdoors. The Bluetooth Technology is a very famous and one of the most commonly use wireless communication technologies. The popular application of Bluetooth is Wireless Headphones. The data rates of Bluetooth are lower that IEEE 802.11 and are less that 3 Mbps. The indoor range of Bluetooth is less than 10 meters. Coming to the Zigbee Technology, it has a data rate of 250 kbps, which is the lowest among the three and the typical range of Zigbee is between 10 meters and 100 meters.
Zigbee Architecture
Although understanding the Architecture of Zigbee Standard is a very good idea, it is not the aim of this article. Even then, we will take a look at the Zigbee Architecture or often called as the Zigbee Stack. The following image illustrates the Zigbee Stack Architecture. The PHY (Physical Layer) and MAC (Medium Access Layer) layers are defined by the IEEE 802.15.4 standard. On this foundation, the Zigbee Alliance provides the NWK (Network Layer) and the framework for application layer. Application Support (APS) Sub-layer, Zigbee Device Objects (ZDO) and the application objects for manufacturers are all part of the Application Framework, which is under control of the Zigbee Alliance.
If you look at the Zigbee Stack in the above picture, it doesn’t exactly fit the OSI Networking Model. The bottom three layers i.e. Physical, Data Link and Network Layers are present in the Zigbee Stack in the form of PHY, MAC and NWK. The last four layers i.e. Transport, Session, Presentation and the Application layers are covered in the Application Support Sublayer (APS) and Zigbee Devices Object (ZDO). Between the layers of the Zigbee Stack, there are Service Access Points (SAPs), which isolates the working of a layer from the layers above and below it. We will now see about the layers in the Zigbee Architecture.
Physical (PHY) Layer
As mentioned earlier, the lowest two layers i.e. the PHY and the MAC are defined by the IEEE 802.15.4 Specification. The PHY layer is closest to the hardware and it directly controls and communicates with the Zigbee radio. The PHY layer translates the data packets in to over-the-air bits for transmission and vice-versa during reception.
MAC Layer
The MAC layer is responsible for interface between PHY and NWK layers. The MAC Layer is also responsible for providing PAN ID and also network discovery through beacon requests.
Network (NWK) Layer
The Network Layer (NWK) acts as an interface between MAC Layer and the Application Layer. It is also responsible for mesh networking (network formation and routing). In addition to the above tasks, the NWK Layer provides security to the Zigbee Networks i.e. the entire data in the NWK Frame is encrypted.
Application Layer
The Application Layer in the Zigbee Stack is the highest protocol layer and it consists of Application Support (APS) sub-layer and Zigbee Device Object (ZDO). It contains the manufacturer defined applications. The APS Sub-layer is responsible for discovery and binding services. Zigbee Device Object (ZDO) looks over the local and over-the-air management of the network. The Application Framework consists of Application Objects that control and manage the protocol layers in a Zigbee device. The Application Framework can contain up to 240 Application Objects.
Zigbee Devices
The IEEE 802.15.4 Specification defines two types of devices: FFD or Full-Function Devices and RFD or Reduced-Function Devices. An FFD Device can literally do it all. It can perform all the tasks described in the IEEE 802.15.4 Standard and can take up any role in the network. An RFD Device, as the name suggests, has limited capabilities. The number tasks performed by an RFD Devices are limited. An FFD Device can communicate with any device in the network and it must active and always listening in the network. An RFD Device can only communicate only with an FFD Device and is intended for simple applications like turning on or off a switch. The FFD and RFD devices in an IEEE 802.15.4 Network can take three different roles: Coordinator, PAN Coordinator and Device. Coordinator and PAN Coordinator are FFD Devices and the Device can be either an FFD or an RFD Device. A Coordinator is capable of relaying messages, a PAN Coordinator is the main controller in a Personal Area Network (PAN) and if the device is not a coordinator, then it is simply called as Device. Based on the concept of FFD and RFD devices and Coordinator, PAN Coordinator and Device in the IEEE 802.15.4 Specification, the Zigbee Standard has created three Zigbee Protocol Devices. They are
The Zigbee Coordinator The Zigbee Router and The Zigbee End Device
Zigbee Coordinator
A Zigbee Coordinator is a PAN Coordinator in the IEEE 802.15.4 Network (an FFD Device) and it is responsible for forming the network. After establishing the network, it allocates network address for the devices that are allowed to join the network. It also routes the messages between the end devices.
Zigbee Router
A Zigbee Router is an IEEE 802.15.4 Coordinator (an FFD Device) and is enables the range of the Zigbee Network. With the help of a Zigbee Router, more devices can be added to the network. A Zigbee Router can sometimes act as a Zigbee End Device.
Zigbee End Device
A Zigbee End Device is neither a Zigbee Coordinator nor a Zigbee Router. A Zigbee End Device physically interfaces to a sensor or executes a control operation. It can be either An FFD or an RFD depending on the application.
Zigbee Network Topologies
The Zigbee Network Layer in the Zigbee Stack is responsible for formation of the network. There are three Zigbee Network Topologies: Star, Cluster Tree and Mesh. These three Zigbee Topologies come under one of the two network topologies mentioned in the IEEE 802.15.4 i.e. Star and Peer-to-Peer.
Star Topology
In a Star Network Configuration, there is one Coordinator and any number of End Devices. All the End Devices are connected to the Coordinator and the individual End Devices are isolated, both physically and electrically i.e. no direct communication between end devices.
All the information must pass through the Coordinator, even the information from one End Device to another.
Cluster Tree Topology
The Cluster Tree Topology is a type of Peer-to-Peer Topology. In Zigbee Cluster Tree Topology, the End Devices join the network via the Coordinator or the Router. As the Zigbee Router extends the range of the Zigbee Network, the End Device need not be in the range of the Coordinator.
Even in Tree Topology, the End Devices cannot communicate with each other directly but the Router can communicate with other Routers and the Coordinator.
Mesh Topology
The Zigbee Mesh Topology is also a Peer-to-Peer Topology and is an extension to the Cluster Tree Topology. The End Devices that are configured as FFD can directly communicate with other FFD Devices (either Routers or End Devices). But End Devices configured as RFD, still need to communicate through Routers or Coordinators.
Applications of Zigbee Technology
Zigbee Networking and Zigbee Technology has a wide range of application like Home Automation, Healthcare and Material Tracking. Let us see few Applications of Zigbee Technology, where Zigbee Devices can increase efficiency and reduce cost.
Home Automation Security Systems Meter Reading Systems Light Control Systems HVAC Systems Consumer Electronics Gaming Consoles Wireless Mouse Wireless Remote Controls Industrial Automation Asset Management Personnel Tracking Livestock Tracking Healthcare Hotel Room Access Fire Extinguishers
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