Apr 02, 2012First it was used in laptops, and then in mobile phones and tablet PCs, but now, Wi-Fi technology is everywhere, in all types of systems that connect to each other to create an Internet of Things. More than a billion Wi-Fi devices were shipped during the past year, and the rate of growth is increasing. A large number of these devices will power machine-to-machine (M2M) communications and real-time location systems (RTLS).
The 802.11 WLAN standard has defined operation in the 2.4 GHz and 5 GHz ISM bands. Additional bands will be added in the future, but these two bands are what are in use today. Most devices currently in operation employ the 2.4 GHz band, for a variety of reasons—a greater likelihood of finding a hotspot or infrastructure access point, lower cost, longer range and lower power consumption. But as the 2.4 GHz band sees increasing amounts of traffic, the quality of a user's experience in connectivity begins to drop. The most visible effects seen include slower connections, a higher number of drop-outs and longer connection times.
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This traffic affects battery-operated M2M and RTLS tags significantly in a different way. To understand this, let us consider the traffic profiles of these applications. These apps send or receive small amounts of data at long intervals—say, a few hundred bytes every few minutes. The IEEE 802.11 WLAN signal is complex, and during the time that information is being transmitted or received, the device draws several hundred milliwatts of power. If that level were sustained, it would drain the battery in a matter of hours. But the standard enables the device to sleep while remaining connected to the network, so developers put in modes of operation that include a very-low-power sleep state between bursts of activity. Tags and M2M devices are configured differently for various applications, and systems engineers attempt to estimate a battery's lifespan based on a device's operational profile. Now, when a channel is crowded with traffic, a device, upon awakening, must wait for a chance to transmit, adhering to the CSMA/CA protocol of 802.11. The more traffic on air, the longer the device must wait, during which time it burns energy.
Higher traffic also results in a greater number of collisions—each of which wastes energy. Even moderate traffic reduces a tag's expected battery lifespan by more than 45 percent. This is a sufficiently important reason for M2M devices and RTLS solutions to move to the 5 GHz band—where there are 24 channels from which to choose, compared with the three available in the 2.4 GHz band in the United States. (Although there are 11 channels in the U.S. 2.4 GHz band, adjacent channels overlap, thus interfering with each other. Consequently, the use of only three channels—1, 6 and 11—is recommended.)
Apart from the battery-life issue, deployment in the 5 GHz band also helps to reduce user density, and provides better quality of service for throughput- or latency-sensitive applications. Ultimately, operation in the 5 GHz band would significantly accelerate Wi-Fi's adoption for M2M communications and RFID.
Venkat Mattela is the CEO of Redpine Signals. Headquartered in San Jose, Calif., Redpine is a fabless semiconductor and wireless system solutions firm focused on innovative, ultra-low-power and high-performance products for next-generation wireless applications.
