Bluetooth Devices and AFH - WLAN FoundationsBy CWNP On 08/01/2012 - 48 Comments
In the WLAN Foundations series, I will be covering the foundational concepts that longtime WLAN engineers have placed in the deepest recesses of their minds and think little about these days. The WLAN Foundation series blog posts will be short and to the point. In this post, I explain Bluetooth AFH.
As a wireless administrator and engineer, it is important to know that Bluetooth devices, since they use the 2.4 GHz ISM band, can cause some level of interference to Wi-Fi equipment operating in the same band. DSSS, HR/DSSS, ERP and HT devices all support the 2.4 GHz ISM band (though enterprise implementations of the HT PHY should use the 5 GHz U-NII bands for clients that support them).
There is good news about Bluetooth devices, since they are so popular today. The newer Bluetooth standards (starting at 1.2) provide a feature known as adaptive frequency hopping (AFH) that can help to prevent or diminish this interference problem. AFH actually works on the Bluetooth devices and a simplification of how it works follows:
When a Bluetooth device, using version 1.2 or higher of the Bluetooth standards, detects interference on a frequency being used in the hopping sequence, it removes that frequency from the hopping sequence. Since data is sent and received on the same frequency during a hop, the communicating devices can determine that the data transfer failed because of interference on that frequency (assuming the other frequencies in the hopping sequence have been working fine). In these situations, the Bluetooth devices do not know what caused the interference, which is acceptable since we only need them to stop using the frequency regardless of the interference source.
AFH makes Bluetooth devices more compatible with Wi-Fi equipment so that the benefit is two-fold: both the Bluetooth devices and the Wi-Fi equipment can co-exist without significant interference since one of them (the Bluetooth devices) is looking both ways before crossing the street – or in this case, after crossing the street. It might not be a bad idea to provide a list of authorized Bluetooth devices for your users. With this list, they can bring in hands-free telephone headsets, mice, keyboards and any other devices they desire; but you can be sure that the devices they are selecting use AFH. If a device doesn’t use AFH, it doesn’t make it on the list. The good news is that most new Bluetooth devices take advantage of AFH today.
Of course, you can resolve this problem quickly by moving everyone to the 5 GHz spectrum; but with all the tablets and mobile devices that are 2.4 GHz-only, that may be awhile off still.
Frames are food, Tom