Wireless LAN, or Wi-Fi, is based on the IEEE 802.11 standard first approved in July, 1997. It was originally designed to provide wireless Ethernet connectivity for computers, eliminating the need for wired connections. Later revisions of the standard have focused on increased data rates (802.11g) and operation at a higher frequency (802.11a) to improve network performance in an electrically "noisy" environments.
The initial focus of 802.11 was for computer-to-computer connectivity, but the technology has been adapted to industrial and embedded applications. ZigBee, on the other hand, is a protocol that was designed from the ground up for device connectivity. ZigBee, an adaptation of the IEEE 802.15.4 protocol, was finalized in December 2004, and is controlled by the ZigBee Alliance.
It is intended to provide a networking solution for small devices and to provide wireless remote monitoring of sensors and simple input devices, such as light switches. These devices typically have very low bandwidth and power requirements.
Typical performance characteristics of ZigBee and WLAN networking devices are summarized in Table 1.
Both ZigBee and 802.11 are deployed in the 2.4GHz frequency worldwide. Connection ranges are very similar between the two technologies.
The major differences between the technologies are data rate, power consumption, and network topologies. 802.11 provides fast networking connections (11-54 Mbits/s), suited to its original purpose of computer networking, while ZigBee has a defined rate of 250 Kbits/s. So while 802.11 is capable of streaming large amounts of data and supports web-based applications, ZigBee is best suited for periodic or intermittent data, or a single signal transmission from a sensor or input device.
While it is possible to use 802.11 in a peer-to-peer mode, the most common method is to deploy 802.11 wireless devices to communicate to a wired network infrastructure through wireless access points. This allows the immediate connection of wireless devices to an existing network.
Because of ZigBee's lower data rates, it can support much lower power consumption rates as well. ZigBee-based battery-operated end devices can often "sleep" for much of their lives and wake on activation or for periodic timed status updates. Low active duty cycles of 1% can provide several years of battery life:
Connection of ZigBee devices is achieved through mesh networking. Networks can scale to hundreds and thousands of devices and all will communicate using the best available path for reliable message delivery. ZigBee networks are self-forming and self-healing, meaning that if one path stops working, a new path is automatically discovered and used without stopping the system operation. This mesh networking formation makes it well suited for products which are installed in large numbers throughout a facility, such as a sensor network.
There are a variety of ways to implement ZigBee or WLAN networking in an embedded device. While it is possible to implement a radio design using networking chipsets, this approach requires RF design expertise and a great deal of time and expense. Radio components may not be available in the smaller quantities needed for embedded products, and the design must still be certified for worldwide deployment.
Figure 1: ZigBee module from MaxStream.
A better approach is the use of modules which can be easily integrated into an embedded product design. Modules such as the MaxStream XBee module for ZigBee (Figure 1) or the Digi Connect Wi-ME(Figure 2) for 802.11b .
Figure 2: WiFi module from Digi International.
Both of these modules provide wireless network co-processing in an embedded product. The modules include wireless radios and microcontrollers, offloading network processing from the systems main microprocessor. A simple serial interface provides communication between the main processor and the networking module.