Wireless networking is one of todays most used technologies to provide users with easy network and internet access. Just throw a few Access Points in your office, plug them into your network, set them up and you are good to go. At least, that’s what one likes to think. The IEEE 802.11 wireless lan standard allows radio communication in the 2.4, 5 and 60 GHz frequency bands. In this blog post, I will be discussing possible problems concerning the 5 GHz range.
So, you are running a 5 GHz Wi-Fi and keep experiencing dropping Network connections and packet errors? I once had the joy of looking into a problem just like this, that a friend of mine had to deal with: He worked on a Campus that rolled out new Wi-Fi infrastructure, covering both 2.4 and 5 GHz access points. The 2.4 worked like a charm, but the 5 kept experiencing said problems. Looking into his logfiles and mapping out the affected hardware, he learned two things:
1) The problem occurred mostly with access points in upper floors of several campus buildings.
2) When it happened, the problem started in the northernmost buildings, then within seconds, affected more and more buildings to the south.
Turns out: It looked like a radar – or at least a signal that was identified as radar by his equipment – was causing his connectivity problems. Let me explain what happened:
The IEEE 802.11a standard allocates the frequencies in the range of 5150 to 5725 MHz for wireless communication between computers and Wi-Fi hubs to facilitate networking. However, these frequencies are also used by other standards as well, like the C-band radar frequency range from 5250 to 5925 MHz. This can, and often will lead to degraded performance in the Wi-Fi network. More importantly, this can also cause problems where a Wi-Fi-Accesspoint can interfere with the radars ability to detect weak or false echoes. This is why the usage of these frequencies is highly regulated and why there is a legal distinction between primary and secondary frequency users. As you might guess, radar systems are considered primary users.
To understand how these 2 systems can interfere, let’s take a look at how a radar works. Basically, what a radar does, is sending out a sharply directed beam through a rotating antenna. When that beam scans across an object, it will be reflected back to the antenna (think of it like you are scanning through the night with a flashlight: If you light up an object, what you see is the reflected light thrown back at you). For this reason, a radar antenna consists of a transmitter and a receiver – the transmitter sending out the radio beam, and the receiver receiving the „echoes“ reflected from any objects. From an outside point of view, the rotating constant beam seems like a short impulse – everytime the beam crosses you, it will appear as a short and strong radio transmission. Now you have your Wi-Fi broadcasting at the higher end of the 5 GHz range, and you might have a powerful nearby radar in the same range, basically running over your Wi-Fi every few seconds. This might be annoying for you, but not only that: Since radar is a primary user, you – as a secondary user – are responsible for not interfering with the radars ability to operate correctly: When signals from your Wi-Fi network hit the radar, it might not be able to distinguish them from the radio waves it sent out. Instead, the radar might think the Wi-Fi’s signals are its signals reflecting back and therefore interprets them as solid objects.
This is, where IEEE 802.11h comes into play – adding DFS to the definition in 802.11a. DFS – aka Dynamic Frequency Selection – is mandatory for radio systems operating in the 5GHz band, meaning they have to be outfitted with means to identify and take action to avoid other radio transmissions that are considered primary-use. These primary-use devices are usually radar systems, used by industrial, federal, civilian and military organizations. Basically, DFS ensures that secondary users can use certain frequencies as long as they are not occupied by primaries: If hardware detects any primary activity on the channel, it will drop the connection and relocate itself to another frequency.
Now you might be thinking „well, this shouldn’t be a problem. I don’t see any radars around here!“. The problem is that DFS considers a distance of up to 35km/20mls „in very close proximity“. So, technically you are correct – to actually mess with your Wi-Fi, a radar would have to be pretty close to your location. However, since your equipment is required to detect any possible radar activity on its radio band (and because doing so is not as easy as you think it might be. I might do a blog post about the challenges of identifying radar signals), dropping Wi-Fi connections can be caused by actual radar, or more likely by random noise in the 5GHz range, wrongfully identified as a primary radar signal, causing your equipment to drop connections and trying to relocate to another channel (where, depending on the source of the identified „primary“, the problem might continue).
So is radar breaking your Wi-Fi? Technically and most likely, DFS is. So if you find DFS-related logfile entries, you now know what’s causing them.
Featured Image „The Atacama Compact Array“ CC-BY-4.0 by European Southern Observatory (ESO) – https://en.wikipedia.org/wiki/File:The_Atacama_Compact_Array.jpg