What is ZigBee?

First, let us tell you where the name ZigBee came from. When a bee finds a nectar-bearing flower, the bee brings the sample back to his hive and dances in a zigzag pattern to communicate the distance and the direction of the food source to other members of the colony. They liken the sensor networks created by ZigBee to bees flying from flower to flower around the hive and communicating information. ZigBee is a coined word from Zigzag and Bee.

ZigBee is a wireless communications protocol that works on IEEE 802.15.4 which specifications are defined by ZigBee Alliance. Sometimes, IEEE 802.15.4 is used as a synonym of ZigBee, but it is not quite true. The name ZigBee can only be used when a ZigBee stack operates on the MAC defined by IEEE 802.15.4. The ZigBee stack builds a network and communicates using the MAC and physical layers of IEEE 802.15.4 with a specified protocol. It is also possible to use an original stack or other stacks on the MAC of IEEE 802.15.4 , instead of using a ZigBee stack, or directly use IEEE 802.15.4. In this case, they are not ZigBee.

ZigBee is a standard developed to build a large-scale wireless sensor network (WSN). IEEE, the same standardization body of WiFi and Bluetooth, has defined the specifications for the physical and MAC layers. The IEEE 802 committee is in charge of management of local and middle-scale networks and the technology, and the 15 working group, which deals with wireless network technology, has established ZigBee/IEEE 802.15.4 and IEEE 802.15.1/Bluetooth. IEEE 802.11/WiFi, IEEE 802.15.1/Bluetooth, and ZigBee/IEEE 802.15.4 are not the technologies that compete each other, but are standardized by IEEE for each different purpose so that they can coexist. Upper parts than the physical and MAC layers have been established by ZigBee Alliance, a standardization body.

In ZigBee' s specifications, the number of terminals that can participate in a network is maximum 65535 (specifications of IEEE802.15.4). It is optimized to a wireless sensor network (WSN) where such a huge number of sensors can participate in a network and the number of terminals that can be connected is so large, which is a feature greatly different from WiFi (maximum 32) and Bluetooth (maximum 7). WiFi, which has a broad range of wireless usage, and Bluetooth, which has adopted frequency hopping, do not assume large-scale network building.

A ZigBee network consists of coordinators, routers, and end devices. In the standard, a mesh network, which requires complicated routing resolution, can be used. In addition, the highly functional specifications also consider coexistence and interconnectivity between the products offered by different companies.

Since ZigBee is a standard developed to establish a wireless sensor network (WSN), the data to be communicated are the ones from the sensor connected and the control signals to devices. It does not assume large-volume data communications. Therefore, the communications speed (data rate) of ZigBee is at the maximum 250 kbps, which is not so high compared with WiFi and Bluetooth. By optimizing the communications speed (data rate) to a sensor network, it keeps the power consumption low and the communications sensitivity high.

The fast connection to a network is also one of the features of ZigBee, where the end device can complete all the processing very quickly from waking up from a sleep status, connecting to a network, sending data, to returning to a sleep status. This is one of the contributors to low power consumption in the application of a wireless sensor network (WSN). The other communications methods such as WiFi and Bluetooth basically assume always-on connection, so it takes several seconds to connect to a network and they keep consuming power during that time. Therefore, even if you use WiFi and Bluetooth devices that are designed to consume less power, the difference in the procedures to connect to a network will be a disadvantage to those devices compared to ZigBee in achieving low power consumption. In this way, ZigBee has the specifications optimized for a wireless sensor network (WSN).

On the other hand, ZigBee is not suited to send music data with high-quality stereo sound, which is what Bluetooth is good at, or to send high-definition movie data, which is what WiFi is good at.

ZigBee shows the best performance in the application such as:
- Communications data are not so large. (e.g. sensor data and control data)
- Data communications are conducted intermittently.
- A terminal moves during operation. (Addition, deletion, moving)

ZigBee takes security of communications into consideration, which is also an important factor in constructing a wireless sensor network (WSN).

To implement a ZigBee protocol stack, which conducts highly functional, complicated network processing between a great number of nodes, requires relatively large resources such as memory and processing speed. If a ZigBee Module does not have sufficient performance, sometimes you may experience unstable operation when the number of nodes reaches several dozen. Such unstable operation is not attributed to the specifications of ZigBee. However, it conducts complicated communications (e.g. existence check of a device) all the time to maintain a mesh network wirelessly, so it may show unstable operation under a poor radio wave situation.

In ZigBee/IEEE802.15.4 Module TOCOS Wireless Engine, a ZigBee PRO stack is available without loyalty, and the powerful 32 bit CPU and the memory with large capacity built in a Wireless Module promise you reliable operation.

Network Topology

As for the nodes of ZigBee, three types of specifications are defined: for a coordinator, router, and end device.

One ZigBee coordinator is always needed for one network, as it establishes a network and manages the terminals that join the network.

A ZigBee router accepts an end device to participate in the network and relays the data from other terminals. It is also possible to have a router control sensors and do other jobs.

A ZigBee end device is a terminal with minimum network functions and implements sensor control and so on.

The basic operation of ZigBee is like this: a ZigBee end device is usually in a sleep mode for the power saving purpose, wakes up by a timer or a trigger signal from external circuits, sends data to a ZigBee router or ZigBee coordinator, and returns to a sleep status. By staying in a sleep status, ZigBee end devices contribute to power saving. During the sleep status, it turns off the wireless circuits, which has the largest consumption current, and therefore, a ZigBee router and ZigBee coordinator cannot wake up a ZigBee end device.

Usually, a ZigBee router and ZigBee coordinator stand by in a reception status all the time, waiting for data coming from a ZigBee end device. Therefore, a ZigBee router and ZigBee coordinator have to run the wireless circuits constantly in a reception status, so they need to use AC power supply, batteries with large capacity, or an energy harvesting generator (harvester).

If a ZigBee router or ZigBee coordinator sends a control signal to a ZigBee end device, a ZigBee end device makes an inquiry when it wakes up from a sleep mode. To improve the responsiveness, a ZigBee end device should make inquiries frequently.

The network topologies available in ZigBee include point to point, star, tree, and mesh networks.

A star topology consists of 1 coordinator and more than one end device (N), which builds a network in a star shape with 1 vs N.

A tree topology consists of 1 coordinator, more than one router and end device, which builds a network in the shape of branches.

A mesh topology can construct several communications paths with routers connected to each other like a mesh. The data from each terminal are relayed like a bucket brigade, but if some trouble happens in certain communications path, it automatically takes a detour to another router and continues the communications successfully.

In this way, ZigBee can take various network topologies, so you can choose the best option according to your intended application.

List of Frequency Used

There are three frequency bands currently assigned to ZigBee/IEEE 802.15.4: 868 MHz band (0ch), 915 MHz band (1 - 10ch), and 2.4 GHz band (11 - 26ch). The frequency band that can be used across the world, however, is the 2.4 GHz band alone.

When drawing up the specifications of ZigBee/IEEE 802.15.4, the sub Giga band (1 GHz or less) and 2.4 GHz band are assigned to a total of 27 consecutive channels (0ch - 26ch). The original purpose for this is to construct a robust wireless communications system by making them complement each other since each frequency band has different transmission properties. Sometimes, people discuss which is better; the sub Giga band (1 GHz or less) or 2.4 GHz band, but such discussion bears nothing because each has both good and bad points.

The sub Giga band (1 GHz or less) is also assigned to cell phones and the usage situation in each country is different. The 950 MHz band has just become available in Japan as a sub Giga band (1 GHz or less) for ZigBee/IEEE 802.15.4, but we will not be able to use it because the Ministry of Internal Affairs and Communications has changed the policy of assigning frequencies. Instead of 950 MHz, the 915 MHz band (920 MHz band) will be available soon, but exactly when ZigBee/IEEE 802.15.4 will start operating it or what the specifications will be like is to be determined.

Therefore, the frequency band of ZigBee/IEEE 802.15.4 currently available in Japan is the 2.4 GHz band only.

2.4 GHz Band

The advantages of the 2.4 GHz band of ZigBee/IEEE 802.15.4 over other frequency bands include the fact that it is available across the world, large number of channels, fast communications speed, consecutive data communications, and compact antennas thanks to the short wavelength.

The band is used by dividing the range from 2400 MHz to 2483.5 MHz into 16 channels. The lowest channel is Channel 11 and the highest one Channel 26. (The reason why it does not start with Channel 1 is because the 868 MHz band is designated to Channel 0 and the 915 MHz band to Channel 1 to 10.) Those channels in the 2.4 GHz band are spaced 5 MHz apart, and the sufficient distance helps it communicate without interfering each other. It is also possible that more than one node shares the same channel.

In ZigBee, a quiet channel when a coordinator initializes a network is to be assigned as a channel to be used.

Data Frame Size

Since ZigBee is a communications protocol that operates on IEEE802.15.4, the data frame structure is in accordance with the IEEE 802.15.4 standard.

The maximum size of the data frame is 127 byte, of which 6 byte is for the header size of the physical layer, 9 - 25 byte for the header size of the MAC layer, and the rest 96 - 112 byte for outgoing data. Therefore, the maximum data volume that can be transmitted at once is 112 byte. The header size of the MAC layer varies depending on the addressing mode to be used. (Please refer to the table below.)

Addressing Mode	MAC Header Size (Byte)
No address for source and destination of transmission (PAN ID only)	9
16 bit address used for source and destination of transmission	13
32 bit address used for source and destination of transmission	25

Note: For an addressing mode, combinations other than the above are also possible.

CSMA/CA (Carrier Sense)

ZigBee/IEEE 802.15.4 offers a mechanism to allow the same frequency to be shared by more than one device.

ZigBee/IEEE 802.15.4 has a mechanism to avoid collision of radio waves, called CSMA/CA. With the function, any interference with other ZigBee/IEEE 802.15.4 devices that are communicating on the same frequency can be avoided. The specifications have allowed more than one ZigBee/IEEE 802.15.4 device to communicate on the same frequency. CSMA/CA is the specifications also implemented on WiFi (wireless LAN), but because of the different modulation system, they do not concern each other.

Each node of ZigBee/IEEE 802.15.4 implements CCA (clear channel assessment) to check that the same frequency is in use or not every time before transmitting data. Every node conducts CCA at a random interval before starting transmission to check the power level of the same modulation wave on the same frequency. If it is under the threshold value, it transmits data, and if it is over the threshold value, it will wait for randomly chosen time and conduct CCA again. It takes 0.128 ms to implement CCA.

For your information, Ethernet also has a mechanism to avoid collision of signals, called CSMA/CD, as multiple devices share the same signal line. Since Ethernet is wired communications, it is easy to detect signal collision. Therefore, it conducts collision detection (CD), instead of collision avoidance (CA). If there is collision, it will wait for about several milliseconds and transmit the data again.

CCA (Clear Channel Assessment)
CSMA (Carrier Sense Multiple Access)
CA (Collision Avoidance)
CD (Collision Detection)

ZigBee Alliance

ZigBee® Alliance has over 250 members at this point, and it defines and implements ZigBee® technology on the IEEE 802.15.4 standard.

The ZigBee PRO Stack can be used in ZigBee/IEEE 802.15.4-compliant Wireless Module TOCOS Wireless Engine, but there are conditions as shown below if you wish to use it for a commercial purpose.

If you create a product using intellectual properties of the ZigBee Alliance (in the case of using our products, ZigBee PRO stack software library provided with SDK), it requires you to become a member of the ZigBee Alliance.

In addition, to mark the ZigBee logo on your product, you need to obtain a ZigBee logo certification, which requires costs to be incurred. Even when you do not mark the ZigBee logo on your product, you need to join the ZigBee Alliance because the ZigBee communications standard itself is the intellectual property of the ZigBee Alliance.

If you use IEEE 802.15.4 API or JenNet that comes with SDK, instead of ZigBee PRO Stack software, membership in the ZigBee Alliance is not required.

The following is an excerption from http://www.zigbee.org/Join/MembershipFAQ.aspx, which explains the requirements of the ZigBee Alliance.
(As of 2011/12)

1. My company wants to develop ZigBee products for sale, does it need to be a member of the Alliance?
   

   Membership in the ZigBee Alliance is required if an organization uses the ZigBee brand (name, logos, interoperability icons). It is also required to request ZigBee Certified status for products or to participate in the development of Alliance standards and specifications.

2. I am a subcontractor for a ZigBee member company, do I need to be a member?
   

   No. If you have been contracted by a member company to perform work on their behalf, you are not required to join the ZigBee Alliance.

3. I am a homeowner, end-user, academic, student, installer of ZigBee products, do I need to be a member of the Alliance?
   

   No, membership in the Alliance is only required if you are using ZigBee intellectual property for commercial gain.

4. If my company uses a ZigBee module in my product, do I need to be a member?
   

   Yes.

For more details, please contact the ZigBee Alliance.

Tokyo Cosmos Electric is a member of the ZigBee® Alliance.

ZigBee® SIG Japan

ZigBee® SIG Japan (ZigBee® SIG-J) is an incorporated association by domestic promotion members with the goal of spreading "ZigBee", a global standard for wireless networks in a short distance and at low power consumption, in the Japanese market, and developing the specifications. To become a member, you need to be a member of the ZigBee Alliance.

Tokyo Cosmos Electric is a member of ZigBee® SIG Japan.

ZigBee PRO

Here is the basic function of ZigBee PRO.

ZigBee PRO is a ZigBee Feature Set of the latest ZigBee 2007 and has the following features. As a conventional version of a ZigBee standard, there are ZigBee 2004 specification and ZigBee 2006 specification.

Mesh Network

Without the need for fixed network design, ZigBee automatically establish a network if you place nodes within the range where radio waves can reach.

Frequency Agility

The function automatically changes the frequency (channel) that the ZigBee system is using to the one less crowded, when it gets too crowded due to wireless LAN and Bluetooth, to avoid interference and ensure stable wireless communications.

Packet fragmentation/re-assembly

IEEE802.15.4 uses the specifications of 1 packet consisting of 127 byte. When sending large data over ZigBee, it divides them into several packets. It can properly reassemble them even when the packets are received in a random order.

Security

ZigBee provides secure communications with a common key and public encryption key.

ZigBee Profile

ZigBee specifies and uses ZigBee Profile as common specifications with the requirements defined and implemented according to an application. By creating a product using ZigBee Profile, a network can be constructed between the products using the same ZigBee Profile. The use of ZigBee Profile helps maintain the interconnectivity between different manufacturers and coexistence of dissimilar models.

There are two types of ZigBee Profile: Stack Profile and Application Profile.

ZigBee Stack Profile defines the structure of a network such as a type, shape, and security.
The ZigBee Alliance has defined:
- Home Controls
- Building Automation
- Plant Control
- Network Specific

ZigBee Application Profile is specified for each specific application. In addition to the Public Application Profile to be specified by the ZigBee Alliance, each manufacturer is also allowed to specify its own profile as Private Application Profile. Here is a list of Public Application Profiles released.

ZigBee Smart Energy (SE)

With the aim of promoting efficient and reliable use of energy, the ZigBee Profile is used for such application as readout of a meter, distribution of power load, display of a usage amount, and coordination with price setting.

ZigBee Home Automation (HA)

It targets at automation at home. The ZigBee Profile is used for such applications as on-off switches, thermostat, various sensors, and lighting control.

ZigBee Remote Control (RF4CE)

This has been standardized to replace infrared ray (IR) technology. It is for making various remote controls wireless. There were limitations in infrared ray; we had to limit the range to where the eye can reach and had to point it to that direction. By making it wireless, there are many advantages including omnidirectionality, control through walls, long-distance communications, highly frequent transmission (e.g. pointing device), and bidirectional communications. ZigBee Input Device (to make a mouse and a keyboard wireless) and ZigBee 3D Sync (to make 3D glasses and other devices wireless) and other subordinate standards have also been defined.

ZigBee Health Care Profile

The ZigBee Profile established as a common communications standard for health care and fitness related devices.

ZigBee Telecom Services

ZigBee Retail Services

ZigBee Network Devices

ZigBee Gateway

ZigBee Light Link

ZigBee Light Link is an open standard for lighting control for general houses, which specifications are being developed by the ZigBee Alliance. ZigBee Light Link enables on-off control and dimming of lighting wirelessly. By complying with ZigBee Light Link, interoperability will be ensured between different manufacturers and models. ZigBee Light Link is a standard specialized in lighting and will be separate specifications from ZigBee Home Automation (HA

ZigBee IP

ZigBee IP is a next generation ZigBee standard to connect a ZigBee network to an IP network, which specifications are currently being developed by the ZigBee Alliance. It is a communications protocol to communicate over UDP/TCP and allow IPv6 packets to be sent to and received from over a ZigBee network. It uses 6LoWPAN (IPv6 over Low power Wireless Personal Area Networks) established by IETF. 6LoWPAN reads six-low-pan and is a standard to adapt IPv6 in wireless communications.

ZigBee SE 2.0 will be implemented on ZigBee IP.

History of ZigBee

Oct. 21, 2002

The ZigBee Alliance founded.

Dec. 14, 2004

1.0 specification fixed, and ZigBee 2004 released.

Sep. 27, 2006

ZigBee 2006 released.

Oct. 2, 2007

ZigBee PRO released.

Nov. 5, 2007

ZigBee Home Automation (HA1.0) profile released.

June 23, 2008

ZigBee Smart Energy (SE1.0) profile released.

Dec. 16, 2009

ZigBee remote control 1.0 (RF4CE) released.

April 5, 2010

ZigBee health care profile released.

Related Information

What is ZigBee?

Related Products

ZigBee/IEEE802.15.4 Module