ZigBee overview
The ZigBee protocol was developed to provide low-power, wireless connectivity for a wide range of network applications concerned with monitoring and control. ZigBee is a worldwide open standard controlled by the ZigBee Alliance. ZigBee PRO was then developed as an enhancement of the original ZigBee protocol, providing a number of extra features that are particularly useful for very large networks (that may include hundreds or even thousands of nodes).
ZigBee features
The ZigBee standard builds on the established IEEE 802.15.4 standard for packet-based wireless transport. ZigBee enhances the functionality of IEEE 802.15.4 by providing flexible, extendable network topologies with integrated set-up and routing intelligence to facilitate easy installation and high resilience to failure. ZigBee networks also incorporate listen-before-talk and rigorous security measures that enable them to co-exist with other wireless technologies (such as Bluetooth and Wi-Fi) in the same operating environment.
ZigBee provides wireless connectivity that enables it to be installed on networks easily and cheaply. Its built-in intelligence and flexibility allow networks to be easily adapted to changing needs by adding, removing, or moving network nodes. The protocol is designed in such a manner that nodes can appear in and disappear from the network. Thus, it allows some devices to be put into a power-saving mode, when not active. This feature allows many devices in a ZigBee network to be battery-powered, making them self-contained and reduces installation costs.
Figure 1 shows a simple example of a ZigBee network in an HVAC (Heating, Ventilation, and Air-Conditioning) system.
Parent topic:ZigBee overview
ZigBee 3.0
ZigBee 3.0 employs the ZigBee PRO protocol and is designed to facilitate general wireless networks that are not market-specific. Thus, devices from different market sectors can belong to the same wireless network. For example, lighting and healthcare devices in a hospital may share a single ZigBee network, allowing data to be routed through any intermediate (routing) device, irrespective of the device functionality.
Connecting the network to the Internet brings the devices into the ‘Internet of Things’ (IoT), allowing the network devices to be controlled and monitored from IP-based devices such as computers, tablets, and smartphones.
Simple ZigBee Network (Heating and Air-conditioning)
Parent topic:ZigBee overview
ZigBee network nodes
A wireless network consists of a set of nodes that can communicate with each other by means of radio transmissions, according to a set of routing rules (for passing messages between nodes). A ZigBee wireless network includes three types of node:
Coordinator: This is the first node to be started and is responsible for forming the network by allowing other nodes to join the network through it. Once the network is established, the Coordinator has a routing role (is able to relay messages from one node to another) and is also able to send/receive data. Every network must have one and only one Coordinator.
Router: This is a node with a routing capability, and is also able to send/receive data. It also allows other nodes to join the network through it, so plays a role in extending the network. A network may have many Routers.
End Device: This is a node which is only capable of sending and receiving data (it has no routing capability). A network may have many End Devices.
The ZigBee PRO network topology describes deployment of these node types in a ZigBee PRO network. More detailed information about the node types is provided in ZigBee nodes.
Parent topic:ZigBee overview
ZigBee PRO network topology
ZigBee facilitates a range of network topologies from the simplest Star topology, through the highly structured Tree topology to the flexible Mesh topology. ZigBee PRO is designed primarily for Mesh networks.
A Mesh network has little implicit structure. It is a collection of nodes comprising a Coordinator and a number of Routers and/or End Devices, where:
Each node, except the Coordinator, is associated with a Router or the Co- ordinator - this is the node through which it joined the network and is known as its ‘parent’. Each parent may have a number of ‘children’.
An End Device can only communicate directly with its own parent.
Each Router and the Coordinator can communicate directly with any other Router/Coordinator within radio range.
It is the last property above that gives a Mesh network its flexibility and efficiency in terms of inter-node communication. A Mesh network is illustrated in the figure below.
Simple Mesh Network
Parent topic:ZigBee overview
Ideal applications for ZigBee
ZigBee is suitable for a wide range of applications, covering both commercial and domestic use. These applications include:
Point-to-point cable replacement (for example, wireless mouse, remote controls, toys)
Security systems (for example, fire and intruder)
Environmental control (for example, heating and air-conditioning)
Hospital patient monitoring
Lighting control
Home automation (for example, home entertainment, doors, gates, curtains, and blinds)
Automated meter reading (AMR)
Industrial automation (for example, plant monitoring and control)
ZigBee provides wireless communication that enables those applications to be developed, which currently cannot be implemented with cabled systems. Examples are applications that involve mobility, which must be free of cabling (such as long-term health monitoring, asset tracking in warehouses). Existing applications (such as lighting control and industrial plant monitoring) that currently rely on cable-based systems can be implemented more cheaply as ZigBee reduces or removes cable installation costs. ZigBee can also be beneficial in environments where cable-based solutions are difficult and expensive to install. Home security systems are examples of such systems. In these systems, the sensors should be easy to install (no cables or power supply wiring) and small and self-contained (battery-powered).
Parent topic:ZigBee overview
Wireless radio frequency operation
The IEEE 802.15.4 protocol, on which ZigBee is built, provides radio-based network connectivity operating in one of three possible RF (Radio Frequency) bands: 868 MHz, 915 MHz, or 2400 MHz. These bands are available for unlicensed use, depending on the geographical area (check your local radio communication regulations).
The characteristics of these RF bands are shown in the table below.
Total number of channels
RF Band |
Number of Channels |
|---|---|
863 MHz - 876 MHz |
63 |
915 MHz - 921 MHz |
27 |
Total |
90 |
Channel distribution across pages
Channel Page |
Description |
|---|---|
863-876 MHz |
63 |
915-921 MHz |
27 |
Total |
90 |
The internal representation of the channels in our stack is as follows:
A 32-bit mask is used to represent the channel mask.
The top 5 bits are used for page number and the lower 27 bits are the channel masks.
In 2.4G, page number is 0 channel range 11-26. Thus, it will be 0x00000800 (page 0, Channel 11). In Sub Gig Page 28 channel 0, is 0xE0000001. The 868 MHz and 915 MHz bands offer certain advantages such as fewer users, less interference, and less absorption and reflection, but the 2400 MHz band is far more widely adopted for a number of reasons:
Worldwide availability for unlicensed use
Higher data rate (250 kbit/s) and more channels
Lower power (transmit/receive are on for shorter time due to higher data rate)
Band more commonly understood and accepted by the marketplace
Therefore, the ZigBee standard assumes operation in the 2400-MHz band, although it is possible to implement ZigBee networks in the other IEEE 802.15.4 bands. ZigBee includes measures to avoid interference between radio communications. One is its ability to automatically select the best frequency channel at initialization. It is also possible to adapt to a changing RF environment by moving the network to another channel, if the current channel proves problematic - this ‘frequency agility’ is a core feature of ZigBee PRO. Other measures are described in Highly reliable operation. The range of a radio transmission is dependent on the operating environment - for example, indoors or outdoors. Using an NXP JN518x or K32W041/K32W061/K32W1/MCXW71/MCXW72/RW612 standard module fitted with an external dipole antenna, a range of over 1 km can typically be achieved in an open area, but inside a building this can be reduced due to absorption, reflection, diffraction and standing wave effects caused by walls and other solid objects. A high-power module (greater than 15 dBm output power) can achieve a range which is a factor of five greater than that of a standard module. In addition, the range between devices can be extended in a ZigBee network since the network topology (see Network topology) can use intermediate nodes (Routers) as stepping stones when passing data to destinations.
Parent topic:ZigBee overview
Battery-powered components
There are many wireless applications that benefit from battery power, including light-switches, active tags and security detectors. The ZigBee and IEEE 802.15.4 protocols are specifically designed for battery-powered applications. From a user perspective, battery power has certain advantages:
Easy and low-cost installation of nodes:No need to connect node to separate power supply.
Flexible location of nodes:Nodes can be installed in difficult places where there is no power supply, and can even be used as mobile devices.
Easily modified network:Nodes can easily be added or removed, on a temporary or permanent basis.
Since these devices are generally small, they use low-capacity batteries and therefore battery use must be optimized. This is achieved by restricting the amount of time for which energy is required by the device.
Since the major power drain in the system is the operation of the radio, data may be transmitted infrequently (perhaps once per hour or even once per week), which results in a low duty cycle (transmission time as proportion of time interval between transmissions).
When data is not being sent, the device may revert to a low-power ‘sleep’ mode to minimize power consumption.
In practice, not all nodes on a network can be battery-powered, notably those that need to be switched on all the time for routing purposes (and therefore cannot sleep). These devices can often be installed in a mains-powered appliance that is permanently connected to the mains supply (even if not switched on) - for example, a ceiling lamp or an electric radiator. This avoids the need to install a dedicated mains power connection for the node. Only End Devices are normally battery-powered.
Note:
A network device can also potentially use “energy harvesting” to absorb and store energy from its surroundings - for example, the use of a solar cell panel on a device in a well-lit environment.
Parent topic:ZigBee overview
Easy installation and configuration
One of the great advantages of a ZigBee network is the ease with which it can be installed and configured.
As already mentioned, the installation is simplified and streamlined by the use of certain battery-powered devices with no need for power cabling. In addition, since the whole system is radio-based, there is no need for control wiring to any of the network devices. Therefore, ZigBee avoids much of the wiring and associated construction work required when installing cable-based networks.
The configuration of the network depends on how the installed system has been developed. There are three system possibilities: pre-configured, self-configuring, and custom.
Pre-configured system: A system in which all parameters are configured by the manufacturer. The system is used as delivered and cannot readily be modified or extended. Examples: vending machine, patient monitoring unit.
Self-configuring system: A system that is installed and configured by the end-user. The network is initially configured by sending “discovery” messages between devices. Some initial user intervention is required to set up the devices - for example, by pressing buttons on the nodes. Once installed, the system can be easily modified or extended without any re-configuration by the user. The system detects when a node has been added, removed, or simply moved, and automatically adjusts the system settings.
Example: off-the-shelf home security or home lighting system in which extra devices can be added later.
Custom system: A system that is adapted for a specific application/location. It is designed and installed by a system integrator using custom network devices. The system is usually configured using a software tool.
As indicated above, system commissioning (individually configuring the network nodes) can be performed in either of the below modes:
By using an IO interface (for example, buttons or a keypad) on the node in a self-configuring system.
By using a commissioning tool (for example, by running on a lap-top PC) that interacts with the node in a custom system.
In the latter case, ZigBee PRO allows commissioning to be conducted in a secure way - for example, using a security key to gain access to the configurable parameters of the node, and using encryption in any wireless communication between the commissioning tool and the node. For more information on system security, refer to Secure operating environment.
Parent topic:ZigBee overview
Highly reliable operation
ZigBee and IEEE 802.15.4 employ a range of techniques to ensure reliable communications between network nodes - that is, to ensure communications reach their destinations uncorrupted. Corruption could result, for example, from radio interference or poor transmission/reception conditions.
Data Coding: At a first level, a coding mechanism is applied to radio transmissions. The coding method employed in the 2400-MHz band uses QPSK (Quadrature Phase-Shift Keying) modulation with conversion of 4-bit data symbols to 32-bit chip sequences. This coding results in a high probability that a message reaches its destination intact, even if there are conflicting transmissions. (A conflicting transmission implies that more than one device transmits in the same frequency channel at the same time).
Listen Before Send: The transmission scheme also avoids transmitting data when there is activity on its chosen channel - this is known as Carrier Sense, Multiple Access with Collision Avoidance (CSMA-CA). Put simply, this means that before beginning a transmission, a node listens on the channel to check whether it is clear. If activity is detected on the channel, the node delays the transmission for a random amount of time and listens again - if the channel is now clear, the transmission can begin, otherwise the delay-and-listen cycle is repeated.
acknowledgments: Two systems of acknowledgments are available to ensure that messages reach their destinations:
End-to-End: When a message arrives at its final destination, the receiving device sends an acknowledgment to the source node to indicate that the message has been received. End-to-end acknowledgments are optional.
Next Hop: When a message is routed via intermediate nodes to reach its destination, the next routing node (or ‘next hop’ node) in the route sends an acknowledgment to the previous node to indicate that it has received the message. Next-hop acknowledgments are always implemented.
In both cases, if the sending device does not receive an acknowledgment within a certain time interval, it resends the original message (it can resend the message several times until the message has been acknowledged).
Frequency Agility: When a ZigBee network is initially set up, the ‘best’ channel in the relevant radio band is automatically chosen as the operating channel. The operating channel is normally the quietest channel detected in an energy scan across the band. However, it might not always remain the quietest channel if other networks that operate in the same channel are introduced nearby. For this reason, ZigBee includes an optional frequency agility facility. If the operating channel becomes too noisy, this feature allows the whole network to be moved to a better channel in the radio band.
Route Repair: Networks that employ a Mesh topology (see ZigBee PRO network topology) have built-in intelligence to ensure that messages reach their destinations. If the default route to the destination node is down, due to a failed intermediate node or link, the network can ‘discover’ and implement alternative routes for message delivery. ZigBee PRO is designed for Mesh networks and therefore incorporates “route repair” as a core feature.
The above reliability measures allow a ZigBee network to operate even when there are other ZigBee networks nearby operating in the same frequency band. Therefore, adjacent ZigBee networks do not interfere with each other. In addition, ZigBee networks can also operate in the neighborhood of networks based on other standards, such as Wi-Fi and Bluetooth, without any interference.
Parent topic:ZigBee overview
Secure operating environment
ZigBee networks can be made secure - measures can be incorporated to prevent intrusion from potentially hostile parties and from neighboring ZigBee networks. ZigBee also provides privacy for communication between nodes of the same network.
ZigBee PRO security includes the following features:
Access control lists
Key-based encryption of communications
Frame counters
These security measures are outlined below.
Access control lists
An access control list allows only pre-defined ‘friendly’ nodes to join the network.
Parent topic:Secure operating environment
Key-based encryption
A very high-security, 128-bit AES-based encryption system (built into the device as a hardware function) is applied to network communications, preventing external agents from interpreting ZigBee network data.
This encryption is key-based. Normally, the same ‘network key’ is used for all nodes in the network. However, it is possible to use an individual ‘link key’ between a given pair of network nodes, allowing communications (possibly containing sensitive data) between the two nodes to be private from other nodes in the same network.
Keys can be pre-configured in nodes in the factory, commissioned during system installation or distributed around a working network from a central ‘Trust Centre’ node. A Trust Centre manages keys and security policies - for example, changing the network key on all network nodes, issuing link keys for node pairs and restricting the hours in which certain events or interactions can occur. Any node can be nominated as the Trust Centre, but it is by default the Coordinator.
A distributed security model can alternatively be used, which does not have a Trust Centre - instead, security is managed by the Router nodes in the network.
Parent topic:Secure operating environment
Frame counters
The use of frame counters prevents sending the same message twice, and freshness checking rejects any such repeated messages, preventing message replay attacks on the network. An example of a replay attack would be someone recording the open command for a garage door opener, and then replaying it to gain unauthorized entry into the property. Frame counters are described in more detail in the Appendix A, Appendix C: Implementation of frame counters.
Parent topic:Secure operating environment
Parent topic:ZigBee overview
Co-existence and interoperability
ZigBee is an open standard devised by the ZigBee Alliance. Any device designed for use in a ZigBee network must comply with the standard. This ensures “co-existence” and, to a certain extent, “interoperability” of ZigBee devices:
Co-existence: The ability of a device to operate in the same space and radio channel as devices in other wireless networks (which possibly use protocols other than ZigBee) without interfering with them
Interoperability: The ability of a device to operate in the same ZigBee network as devices from other manufacturers - that is, to communicate and function with them.
The ZigBee Alliance coordinates the compliance issues for products based on the ZigBee protocol. It defines two levels of compliance:
ZigBee Compliant Platform (ZCP) applies to modules or platforms intended as building blocks for use in end-products. All NXP products based on the supported chips are designed to be ZigBee Compliant Platforms. Refer to the section Chip compatibility.
ZigBee Certified Product applies to end-products that are built on ZigBee Compliant Platforms, and that use public ZigBee Alliance device types and clusters. After successful completion of the ZigBee Alliance Certification program, the ZigBee Certified Product logo can be applied to the product.
Note: End-products based on manufacturer-specific device types and clusters can also obtain ZigBee Certified Product status, but such products cannot carry the ZigBee Certified Product logo.
Test service providers are authorized by the ZigBee Alliance to undertake testing and certification. For details of authorized test houses, contact the ZigBee Alliance.
In addition, products using an NXP ZCP must also be checked against the radio regulations of the country or countries where they are to be marketed (these checks can often be performed by the same test house).
Parent topic:ZigBee overview
Device types and clusters
For the purpose of interoperability (described in Section 2.9), the ZigBee Alliance employs the concepts of a device type and a cluster, which define the functionality of a network node. Clusters and device types are introduced below (but more detailed information can be found in Section 3.4).
Note: The ZigBee ‘application profile’ (which collects together the device types for a market sector) is not so
prevalent in ZigBee 3.0. However, application profiles are still supported for backward compatibility.
Clusters
A cluster is a software entity that encompasses a particular piece of functionality for a network node. A cluster is defined by a set of attributes (parameters) that relate to the functionality and a set of commands (that can typically be used to request operations on the cluster attributes). As an example, a thermostat uses the Temperature Measurement cluster that includes attributes such as the current temperature measurement, the maximum temperature that can be measured, and the minimum temperature that can be measured. However, the only operations that needs to be performed on these attributes would be reads and writes.
The ZigBee Alliance defines a collection of clusters in the ZigBee Cluster Library (ZCL). These clusters cover the functionalities that are most likely to be used. The NXP implementations of these clusters are provided in the ZigBee 3.0 Software Developer’s Kit (SDK) and are described in the ZigBee Cluster Library User Guide (JNUG3132).
Parent topic:Device types and clusters
Device types
The complete functionality of a network node is determined by its device type. This defines a collection of clusters (some mandatory and some optional) that make up the supported features of the device. For example, the Thermostat device uses the Basic and Temperature Measurement clusters, and can also use one or more optional clusters. A device is an instance of a device type.
A network node can support more than one device type. The application for a device type runs on a software entity called an endpoint and each node can have up to 240 endpoints.
All ZigBee 3.0 nodes must implement the ZigBee Base Device (which does not occupy an endpoint), which handles fundamental operations such as commissioning.
The ZigBee device types and ZigBee Base Device are detailed in the ZigBee Devices User Guide (JNUG3131).
Parent topic:Device types and clusters
Parent topic:ZigBee overview