What Is The Best Network For Wireless Sensors?

This question has been asked over and over by clients and seen on forums. Unfortunately, there is no one answer. Each network has its own particular strengths and weaknesses. So the best depends on the application.

While there are many network technologies that can be used for wireless sensors, the most popular are – Wi-Fi, ZigBee, and Bluetooth. These networks have similarities in that each can accommodate and manage wireless sensors; each is designed for short-range wireless communication; and each operates in the open 2.4GHz Radio Frequency (RF) range. Beyond these similarities there are significant differences. Understanding the major differences between the network technologies makes the selection much easier.

There are three factors that are used as rules-of-thumb to quickly determine the best network for a given sensor application. They are network bandwidth, power consumption, and radio broadcast range. Bandwidth is the maximum throughput of data in the network as measured in bits per second. In terms of data flow, more and faster is usually better. Power consumption refers to how much energy is required to operate the sensor electronics and broadcast on a given network. Range is the effective distance the radio signal travels.

The relationship between these three technology factors is key to understanding their differences. Case in point, as the network bandwidth goes up so does the energy consumption. The same holds true for range. As the radio range increases so does the power requirement. Therefore it is essential to understand the sensor application requirements such as how many sensors, how far they are from each other, and approximately how much date the network is expected to manage.

Wi-Fi
Like all products, each network was designed to solve a specific problem. Wi-Fi’s main purpose was to provide a high-speed wireless replacement of Ethernet cables used to connect PC’s in a Local Area Networks (LAN). Wi-Fi has a bandwidth of 11 megabits per second while occupying five (5) channels in the 2.4 GHz band. This makes it well suited for PC’s and smart devices that transmit large data files such as videos, games, music, pictures, etc. The effective range of a Wi-Fi device is about 32 meters indoors and 70 outdoors.

The down side to Wi-Fi is that transmitting 11 megabits over five channels takes a lot of power. A laptop, tablet, or smart phone will only last about two hours if not plugged into an AC outlet. In relative terms, Wi-Fi is a power hog.

ZigBee
On the other hand, ZigBee was designed specifically for battery-powered wireless sensors. Energy conservation is an important component of ZigBee with features for managing energy designed into the platform. Battery-powered sensors may be required to run for months or even years without the need to recharge or change batteries. ZigBee accomplished this in several ways. First, sensor data packets are generally 250 bytes or less. This is a fraction of music files, games, and videos seen on a Wi-Fi network. Second, ZigBee devices can be put to sleep when not in use. This is referred to as Duty Cycling. For example, if a temperature sensor takes a measurement once a minute, the sensor will take a reading and transmit the results generally in less than 100 milliseconds. Then for the remaining 900 milliseconds the device is duty cycled to conserve energy.

ZigBee is idea for battery-powered wireless sensors where the number of nodes on the network is limited (generally less than 50) and network data traffic is fairly low. Network performance decreases dramatically as network traffic and the number of network devices increase. This is because ZigBee transmission rate is 250 kbits or about 1/44th of a Wi-Fi network and only uses one channel. The broadcast range is similar to Wi-Fi with an outdoors range of approximately 100 meters.

Bluetooth
Bluetooth is a platform developed by Ericson in 1994 as a Personal Area Network (PAN) for secure wireless communication between personal devices. Considering Ericson makes cell phones they needed a high-speed wireless means for connecting a wireless headset to a cell phone. They had two conflicting considerations, the headset was battery-powered and they needed a relatively high bandwidth.

Bluetooth solves this dilemma by making two adjustments. First, they set the bandwidth to three megabits per second. This rate is significantly lower that Wi-Fi but perfectly capable of handling audio communication between personal devices. However, three megabits could still drain a battery fairly quickly. So the second modification they made was to reduce the radio broadcast range to 10 meters. Broadcasting a signal 10 meters takes significantly less energy than the 70 meters seen in the other networks.

Bluetooth has emerged as a favorite technology for wireless communication between personal electronic devices such as Nintendo’s Wii, PlayStation 3, wireless mouse’s, keyboards, and printers. The advantage of Bluetooth is a relatively high bandwidth and low power consumption. The down side to Bluetooth in a wireless sensor network is that only seven nodes can be connected at once and they must be relatively close together.

So Which One Is Best?
After this brief explanation it would seem as though ZigBee would be used in most wireless sensor applications because it was built specifically for that purpose. More often than not this is true. Yet, there are a great number of applications where Wi-Fi or Bluetooth are a better choice. If the network has PC’s, smart phones, tablets, and sensors that require high bandwidth, then Wi-Fi is generally the best network. If the network is composed of wireless, battery-powered sensor then ZigBee is generally a better choice. If the network is connecting seven or less personal devices such as headsets to cell phone, controllers for a Wii, or a wireless keyboard to your PC, then use Bluetooth. Like any rule-of-thumb there are plenty of exceptions, but it’s a great place to start.