Blowin' in the Wind

ZigBee

The ZigBee technology was initially designed to make a mesh network of sensors for use in building automation for lighting and temperature control. The name of the brand is a reference to the behavior of worker honey bees after they return to the beehive.

When food is discovered by scout workers, they return to the hive. Shortly after their return, many foragers leave the hive and fly directly to the food. The remarkable thing about this is that the foragers do not follow the scouts back (the scouts may remain in the hive for hours). So the scout bees have communicated to the foragers the necessary information for them to find the food on their own by a dance. The scouts zig and zag back and forth in a certain manner and number of repetitions to convey to the foragers information about the odor of the food, its direction from the hive, and its distance from the hive.

In the same manner, ZigBee wireless devices have the ability to create ad hoc networks dynamically, so that messages can get through even if the environment or the different nodes in the network are changing or adversely affected in some way.

There are three different types of ZigBee devices:

ZigBee coordinator (ZC): The most capable device, the coordinator forms the root of the network tree and might bridge to other networks. There is exactly one ZigBee coordinator in each network since it is the device that started the network originally. It is able to store information about the network, including acting as the trust center and repository for security keys.

ZigBee Router (ZR): As well as running an application function, a router can act as an intermediate router, passing on data from other devices.

ZigBee End Device (ZED): Contains just enough functionality to talk to the parent node (either the coordinator or a router); it cannot relay data from other devices. This relationship allows the node to be asleep a significant amount of the time, thereby extending battery life. A ZED requires the least amount of memory, and therefore can be less expensive to manufacture than a ZR or ZC.

Technical Specs

ZigBee is also referred to as the IEEE 802.15.4.

Low Power—Individual devices must have a two-year battery life to pass ZigBee certification.

ZigBee operates in the 2.4 GHz region also, in most of the world, with an additional spectrum at 915 MHz and 868 MHz in Europe. It uses direct-sequence spread spectrum (DSSS) modulation and a very powerful 128-bit AES encryption scheme. The ZigBee network can have as many as 65,000 nodes, all of which can be with routers, or end users, but only one coordinator! With all the traffic going along its network, collisions are avoided and corrected when they do occur using CSMA/CA—Carrier Sense Multiple Access/Collision Avoidance. This means that whenever a device has a need to transmit, it listens for a free carrier, and then transmits. This method avoids collisions, rather than trying to sort them out later.

One advantage of the low power (multiyear) specification is that it includes a very fast “wake-up time” for end devices.

Wi-Fi

The term Wi-Fi is almost always associated with the IEEE 802.11x standard. The term Wi-Fi was originally coined as a twist on Hi-Fi, and references to Wireless Fidelity appear in some early documents from the Wi-Fi alliance. The Wi-Fi alliance serves a purpose similar to both the Bluetooth SIG and the ZigBee alliance—to provide testing and assurance of conformity to the standard. It is almost synonymous with computer wireless communications.

Wi-Fi is used for a Local Area Network of PCs. These can be connected in an ad hoc network or as a more static method using either MAC addresses or fixed IP addresses.

Technical Specs

There are 14 channels designated in the 2.4 GHz range spaced 5 MHz apart (with the exception of a 12 MHz spacing before Channel 14). Because the protocol requires 25 MHz of channel separation, adjacent channels overlap and will interfere with each other. Consequently, using only channels 1, 6, 11, and 14 is recommended to avoid interference.

Potential Wireless LAN uses of this range are documented by IEEE 802.11a, b, g and n. Countries apply their own regulations to the allowable channels, allowed users, and maximum power levels within the frequency range. (See Potential Wireless LAN Uses chart.)

So, which is best? It depends on the application. (See Comparison Chart).

Need an easy-to-set-up, on-the-fly network with a limited number of nodes that you can simply pair up? Use Bluetooth.

Need a very low-power network to reliably send small amounts of data over a wide range regardless of network conditions? Use ZigBee.

Need a fast network that can be expanded, can send large amounts of data quickly, can be scaled quickly, and is future-proof? Use Wi-Fi.

Why Bluetooth?

If you are in your car, it is convenient to set up your cell phone, your car’s GPS, and your hands-free headset to “talk” to each other. That way, if you have your headset with you, as you get in the car, your headset can talk to the phone. If you forgot your headset, the GPS can be paired with the phone to allow you to talk on the phone via the microphone in the GPS. Since they are close to each other and need to be paired up differently each time you use them, it is convenient to pair them once, and when they are in range, have them connect automatically. 

In a scale application, you could use Bluetooth for a master device that occasionally
has to travel around to different scales and get the readings from them when the operator is close by.

Why ZigBee?

The main advantage to ZigBee is its ability to self-form a “mesh” network to get the message through. A good example of this would be in a building where you wanted to collect the temperature in every office on multiple floors, to balance the heat or air conditioning. If someone rearranged his or her office and unknowingly placed a big steel file cabinet in front of a node, it might be cut off from the rest of the network. However, only the messages from that node would be lost. The rest of the messages that used that node as a routing device would simply form a new route around it by using another path. It would also make it very obvious where the problem was, rather than a whole section of the network going down and having the old “Christmas tree light” problem of locating the fault or faults.

A load cell (and its associated A/D circuitry) could be connected to a ZigBee end device and transmit weight on a regular basis to an indicator that was within range of either the load cell or a coordinator node to extend the distance.

Why Wi-Fi?

Wi-Fi is rapidly growing in use. The advantages are the ability to handle large amounts of data with a long range between devices. PCs use this regularly to communicate on the Internet, at home, or in coffee houses and other establishments that offer free Wi-Fi. Hotels use this since it is much easier to equip an existing hotel with Wi-Fi than it is to hard wire Ethernet® to every room.

A scale indicator can be equipped with Wi-Fi to communicate wirelessly to either an existing Ethernet network or the Internet.

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ZigBee is a registered trademark of the ZigBee Alliance.