ZigBee, a technical overview of wireless technology

Wireless is hot but is it more then just a buzz? About 2 years ago I would have said that it is more a buzz then practical. The progress made with wireless technology has been very big. Today it’s not a buzz; it’s something to really take into consideration!

Many companies that are planning a new product just want it wireless. When a product migrates from a stand-alone product to a network product, tons of new possibilities arise. With all the new possibilities a knowledge upgrade within the sales and support organizations of the company is highly needed. This change, from stand alone to networks, is a VERY big change. The small step of removing the wires is very small compared to that. My personal advice is to start thinking networked first.

Why wireless? They claim it saves costs because no wires need to be installed, saving on installation costs. This view is a bit short. Wireless adds new problems! There can be many kind of problems: devices do not respond to each other, noise disturbance, neighbor interference, and many more. The biggest problem however is that if it does works today, it might give you problems tomorrow. Most progress on the technical part has been in prevention of all possible problems. So at this moment my personal experience is that it’s very good already. However, you really need ‘site survey tooling’ to be able to pinpoint problems at customer locations. Remember that there are no wires to follow and measure!

In this document I will focus primarily on Zigbee but there are other wireless protocols which could suite your application even better. I have reviewed many wireless protocols and I also have practical experience with many. But hey, I have to start somewhere! So let’s start talking Zigbee and stop the boring intro. Just drop me an e-mail if you have a question not answered on my website.

Zigbee….
In very few words I would say: “low cost, low power, very diverse possibilities”. 

Let’s go into more detail.

What is Zigbee? Zigbee is a wireless networking standard that is aimed at remote control and sensor applications which is suitable for operation in harsh radio environments and in isolated locations. It builds on IEEE standard 802.15.4 which defines the physical and MAC layers. Above this, Zigbee defines the application and security layer specifications enabling interoperability between products from different manufacturers. In this way Zigbee is a superset of the 802.15.4 specification.

Zigbee is organized within the Zigbee Alliance. Many companies (>150) already adapted this technology, to get an impression just look here. 9 companies are called ‘promoters’ and they are the actual promoters of the Zigbee standard. These companies are: BM Group, Chipcon, Ember, Freescale, Honeywell, Mitsubishi, Motorola, Philips and Samsung. On the right you will find a direct link to those compagnies.

If we take a closer look at protocoll we can make the following picture.

The 802.15.4 standard is primarily aiming at monitoring and control applications. Low power consumption is the most important feature that makes battery operated devices operate for a long time. The amount of data throughput (bandwidth) is relativily low compared to wireless lan for example, but with 250kbps for many applications more than enough. The distance between 2 nodes can be up to 50 meters but be aware the each node can relay data to the next making a very big network, covering significant distances, possible.

Hardware (Physical and MAC layers)
At the moment all solutions work on 2.4GHz but specified is also 915MHz for North America and 868MHz for Europe. The 2.4GHz frequency band is a license free band, so a ZigBee product may be used all over the world. All current products seem to be using the 2.4GHz band at the moment. Take a look at the next table for a few differences between the bands:

In all bands DSSS (Direct sequence spread spectrum) is used. 868 and 915 MHz are using Binary Phase Shift Keying and 2.4GHz uses O-QPSK (Offset Quadrature Phase Shift Keying).

These license free frequencies are becoming more and more crowded and noisy. The 802.15.4 specification has many feature to ensure a reliable operation under the worst environmental conditions. Some keywords: Clear Channel Assesment, Quality Assesment and Receiver Energy Detection. To prevent problems caused by itself, a technique called Carrier Sense Multiple Access (CSMA) is used to only transmit when this does not cause problems (collisions).

Like in any network data is transmitted in packets. ZigBee's packets have a maximum size of 128 bytes including protocol overhead. In total there is room for a maximum of 104 bytes. Compared to ethernet this is rather small but for most applications that ZigBee will be used for this is more than enough (how many bytes do you need to switch on a light? (no, this is not a lightbulb joke)).

For realtime features, ZigBee has the possibility to define high priority messages. This is achieved by use of a guaranteed timeslot mechanism so that the high priority messages can be send as fast as possible.

ZigBee uses 2 kinds of addressing. There is a 64 bit IEEE address that can be compared to the IP address on the internet. There is also a 16 bit short address. The short addresses are used once a network is setup so this makes a total of 2^16 = ~64000 nodes within one network possible. This is enough for almost anything imaginable. If you need more than you can offcourse design a gateway node.

The ZigBee upper layers
The layers above that what 802.15.4 specifies is what we call the ZigBee standard (look above for a graphical overview). Many aspect of the network are specified in this layer, like: Application profiles, security settings and the messaging.

ZigBee is known because of it's mesh network architecture but it does also support a star topology or cluster tree or hybrid architecture. Depending on the application or situation each kind of topology has it's own advantages and disadvantages. A star topology is very simple, all nodes directly communicate with one central node (like a star...). The mesh topology is more complicated, each node may communicate with any other node within range. It's easy to understand that this gives many possible routes through the network, this makes it a very robust topology because bad performing routes can be ignored. The cluster tree topology is basically a combination of star and mesh.

There is much more to tell but these are the basics of ZigBee. If you need more detailed information you can download the ZigBee specification here. Google also found some interesting links presented just below here. With questions you can contact me and if you want to know more about me, just check out my website. Note that I work for companies that can develop and produce your product! On zigbee.pagina.nl you can find many more links!
.

ZigBee Promotors: