802.11ac & 5GHz: The Emperors New Clothes? - Part 1

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The WiFi industry has been buzzing with excitement around the recently ratified 802.11ac standard. The promise of higher speeds, lower battery usage for mobile devices and the enforced move to the higher-capacity 5GHz band is enough to put a smile on the face of even the most curmudgeonly members of the WiFi fraternity.

I've been giving some serious thought recently to what the best approaches might be in terms of designing and deploying 802.11ac networks. There are obviously challenges as we move through the transition from legacy standards to the shiny new 802.11ac standard: 
  • new cabling requirements for higher uplink speeds to 802.11ac APs
  • Increased power requirements for our 802.11ac APs
  • accommodating the mix of new and legacy clients
  • figuring out exactly how we plan our channels for the brave new world of 802.11ac

The 802.11ac standard mandates that access points and clients using the new standard will only be supported on the 5GHz band, which is great news...right? We can still use our legacy 802.11g and 802.11n standards on good old 2.4GHz, but 802.11ac is 5GHz only. 

The 5GHz band offers us far more channels that the noisy cesspit which is the 2.4GHz band. It provides around 20+ channels (depending on where you are in the world), compared to the pitiful 3 useable channels of the 2.4GHz band. 5Ghz is currently used by fewer (non-WiFi) devices, so has less noise and interference than it's noisy cousin down on 2.4GHz. As we have more channels to play with on 5GHz, we can also start bonding them together to get 40MHz (double-width) and 80MHz (quad-width) channels to get even faster throughput for our 802.11ac clients.

On the face of it, we are in easy-street on 5GHz compared to 2.4GHz. Faster speeds (for faster transfers and better spectral efficiency), together with lower noise and more channels for easier planning and higher capacity.

But, not so fast. 5GHz has a few little 'gotchas'' that maybe the marketing boys didn't tell you about...

DFS

When the 802.11 standards were created and the 2.4GHz and 5GHz bands were allocated for use by WiFi traffic, there was some difficulty surrounding existing services that already used the 5GHz band. Particularly in Europe, there was the issue of weather and airport radar systems that already used parts of the 5GHz band. Though this was initially confined to Europe, it is now a consideration in many areas of the world.

Unless protection mechanisms were put in place, there was a danger that a WiFi network on specific channels within the 5GHz band could interfere with these radar systems.  Therefore, it was decided to implement a protection system to guard against radar system interference: Dynamic Frequency Selection (DFS).

DFS ensures that if a WiFi network detects an RF signature that looks like a radar pulse on the channel on which an access point is operating, it will cease all transmissions on that channel and move to a new channel. This has a disruptive effect on associated wireless clients, who will also need to switch channels. This might cause a relatively short 'blip' for clients not carrying time-sensitive data (e.g. for simple web browsing, files transfers etc.). But, for latency sensitive traffic (e.g. voice & video), this channel switch will have significant effects, such as dropping established voice calls over WiFi.

To find out more about DFS, take a look at a great series of articles from Jennifer Minella at Network Computing.

Fortunately, the DFS mechanism does not apply to all channels within the 5GHz band. In many parts of the world, the first 4 channels of the 5GHz band for WiFi may operate without being subject to DFS (non-DFS channels). These are channels 36,40,44,48. This means we can confidently use these channels without any fear of service interruption due to radar detection events. (Note that in the USA, there are 9 non-DFS channels)

Unless you are near an obvious radar source (e.g. an airport), it may be difficult to determine if your network is going to affected by radar. Often, the first time this becomes apparent is when a wireless network has been deployed and radar blasts are reported by the wireless network. Due to the nature of the radar pulse width, radar systems tend not to detected during a wireless survey, even if using a spectrum analyser.

In addition to detection of genuine radar systems, there may also occasionally be local interference from non-radar systems which inadvertently trigger the DFS mechanism. This is due to the generation of a pulse of RF energy that looks similar to a radar pulse.

In the real world, reports from other wireless professionals I've spoken to suggest that actual radar events affecting WiFi networks tend to be rare. However, in very high density deployments, the incidence of false-positives from mis-behaving clients is not unusual.

In summary, although radar detection is infrequent, it may be unpredictable and may cause significant disruption when using a WiFi system on 5GHz near a radar source. The effects of DFS may be completely mitigated by using only non-DFS channels, but this will reduce our usable channels from our original 20+ channels down to just four (OK, maybe 9 if you're in the USA...).


In part 2 of this article series, we'll look at more restrictions impacting the use of the 5GHz band and 802.11ac.

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