Customers have asked this question over and over again and it has been seen on the forums. Unfortunately, there isn’t an answer. Each network has its own particular strengths and weaknesses. So what is 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 2.4 GHz radio frequency (RF) range. Beyond these similarities there are significant differences. Understanding the main differences between network technologies makes the selection much easier.
There are three factors that are used as general rules of thumb to quickly determine the best network for a given sensor application. They are network bandwidth, power consumption, and radio transmission range. Bandwidth is the maximum data throughput on the network measured in bits per second. In terms of data flow, faster and faster is usually better. Power consumption refers to how much power is required to operate the sensor electronics and transmit on a given network. Range is the effective distance that the radio signal travels.
The relationship between these three technological factors is key to understanding their differences. For example, as network bandwidth increases, so does power consumption. The same is true for the range. As the range of the radio increases, so does the power requirement. Therefore, it is essential to understand the sensor application requirements, such as how many sensors, how far apart they are, and approximately how much data the network is expected to handle.
wireless
Like all products, each network was designed to solve a specific problem. Wi-Fi’s primary goal was to provide a high-speed wireless replacement for Ethernet cables used to connect PCs on local area networks (LANs). Wi-Fi has a bandwidth of 11 megabits per second while occupying five (5) channels on the 2.4 GHz band. This makes it ideal for PCs and smart devices that transmit large data files, such as videos, games, music, images etc The effective range of a Wi-Fi device is about 32 meters indoors and 70 outdoors.
The downside to Wi-Fi is that transmitting 11 megabits over five channels requires a lot of power. A laptop, tablet, or smartphone will only last about two hours if it’s not plugged into an AC outlet. In relative terms, Wi-Fi consumes a lot of power.
ZigBee
On the other hand, ZigBee was specifically designed for battery-powered wireless sensors. Power conservation is an important component of ZigBee with power management features built into the platform. Battery powered sensors may be required to operate for months or even years without the need to recharge or change batteries. ZigBee accomplishes this in several ways. First, sensor data packets are generally 250 bytes or less. This is a fraction of the music, game and video files viewed on a Wi-Fi network. Second, ZigBee devices can be put to sleep when not in use. This is known as the duty cycle. For example, if a temperature sensor takes a measurement once a minute, the sensor will take a reading and transmit the results typically in less than 100 milliseconds. Then, for the remaining 900 milliseconds, the device performs a duty cycle to conserve power.
ZigBee is an idea for battery-powered wireless sensors where the number of nodes in the network is limited (usually less than 50) and the network data traffic is quite low. Network performance drops drastically as network traffic and the number of network devices increase. This is because ZigBee’s transmission speed is 250 kbits or about 1/44 of a Wi-Fi network and it only uses one channel. The transmission range is similar to Wi-Fi with an outdoor 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. Since Ericson makes mobile phones, they needed a high-speed wireless medium to connect a wireless headset to a mobile phone. They had two conflicting considerations: the headphones were battery-powered and they needed 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 than Wi-Fi but perfectly capable of handling audio communication between personal devices. However, three megabits could still drain a battery pretty quickly. So the second modification they made was to reduce the radio transmission range to 10 meters. Transmission of a 10-meter signal requires significantly less power than the 70 meters seen on other networks.
Bluetooth has become a favorite technology for wireless communication between personal electronic devices such as Nintendo Wii, PlayStation 3, wireless mice, keyboards, and printers. The advantage of Bluetooth is relatively high bandwidth and low power consumption. The disadvantage of Bluetooth in a wireless sensor network is that only seven nodes can be connected at a time and they must be relatively close together.
So which is the best?
After this brief explanation, it would seem that ZigBee would be used in most wireless sensor applications because it was built specifically for that purpose. Most of the time this is true. However, there are plenty of applications where Wi-Fi or Bluetooth are a better option. If the network has PCs, smartphones, tablets, and sensors that require high bandwidth, Wi-Fi is generally the best network. If the network is comprised of a battery-powered wireless sensor, then ZigBee is generally a better option. If your network is connecting seven or fewer personal devices, such as a headset to a cell phone, controllers for a Wii, or a wireless keyboard to your PC, then use Bluetooth. Like any rule of thumb, there are many exceptions, but it’s a good starting point.
