Showing posts with label DFS. Show all posts
Showing posts with label DFS. Show all posts

Sunday, 20 May 2018

Updated White Paper on Licence-Exempt Spectrum in the 5GHz band for Wireless LANs in the UK

For the past few years, I've maintained a white paper on the use of the 5GHz spectrum for Wi-Fi networks here in the UK. As Wi-Fi text books tend to focus on the spectrum available in the USA, I put this document together to clarify how 5GHz spectrum may be used in the UK.

Following the release of a Voluntary National Specification document by Ofcom in August 2017 (VNS 2030/8/3), additional channels became available for use in the UK on 5GHz.

As we now have additional spectrum, it's time for an update to my white paper to detail the new spectrum that is available.

Prior to updating the white paper, I published a summary sheet that shows the new spectrum allocation. This can be obtained obtain from my previous blog article: UK 5GHz WLAN Spectrum Allocation (August 2017) (this is definitely one to print off and laminate).

I have now completed my updates to the white paper, which I am pleased to share with you now. Note that in addition to adding the new spectrum details, I have added a few other topics around the use of 5GHz and the unique challenges that it presents. I'd recommend taking a look at my description of how DFS operates in Appendix 2, and have a play with the Opera weather radar location tool shown in Appendix 3.

I hope you find the update useful and informative.

Links:





Tuesday, 27 January 2015

5GHz in the UK White Paper (Version 2)

[Note: The information in this white paper has been superseded. Check out my updated white paper: http://wifinigel.blogspot.co.uk/2018/05/updated-white-paper-on-license-exempt.html]

I decided it was time to update my white paper detailing the use of the 5GHz band here in the UK for wireless LANs.

I've tidied a few things up and added some information around 802.11ac channel planning within the constraints of UK 5GHz spectrum.

You can download the whitepaper from here:

  • PDF download
  • Google docs
  • Scribd

Tuesday, 13 May 2014

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

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.

Tuesday, 7 May 2013

5GHz - 3 Missing Channels in Europe

Last year, I put up a posting which highlighted the fact that here in the UK (and I suspect all of Europe) we often have 3 channels missing from our allocation of unlicensed channels in the 5GHz band.  Looking at many manufacturer data sheets, channels 120, 124 and 128 are often shown as not being supported. This is despite the fact that they are allocated for use by local regulatory bodies (OFCOM here in the UK).

I recently posted a question about this on a partner forum of a major WiFi vendor that I deal with and finally got a definitive answer on this. In this post, I'll share my findings.

The reason that these particular channels (120 - 128) receive special treatment is that they occupy frequencies that are used by weather radar systems. WiFi systems have to be very careful not to interfere with those systems during their normal operation. Therefore, WiFi equipment has some additional checks and tests imposed on it to make sure that it does not inadvertently cause any interference.

In the ETSI region (Europe), the standard EN 301 893 dictates that any channels operating in the frequency range 5.6GHz to 5.65GHz must wait an additional period of time before using a channel. For most DFS-affected channels, a WiFi device must wait for 60 seconds to verify that no radar is present before commencing operation. However, on the channels in the 5.6GHz to 5.65GHz range, the device (i.e. Access Point) must wait 10 minutes! The table below (taken from Annex D of the standard) details this requirement:


Due to this 10 minute wait period, it seems that many manufacturers have chosen to withdraw support for the channels affected, which are 120, 124 and 128. This makes perfect sense now - who would want an AP to be allocated to a channel and then wait for 10 minutes before it can use it..?

I'm pleased to have finally got to the bottom of this particular grey area, but it seems like bad news for WiFi usage in Europe generally. With the anticipated sharp uptake in 5GHz usage around the globe, as 802.11ac starts to roll out, the loss of 3 channels is quite a chunk of spectrum to lose.

In the UK we have 19 unlicensed channels to use on 5GHz for WiFi. Losing 3 of those channels is a 15% loss in spectrum. This is at a time when we really need to be increasing spectrum availability to cope with the additional channel bonding opportunities that  802.11ac provides to increase WiFi speeds.

Hmmm....let's hope some additional spectrum turns up in the near future.

References:
Update: I've now created a white paper which details 5GHz usage for WiFi in the UK. Find it here

Monday, 8 October 2012

The Missing Channels on 5GHz in the UK : 120, 124, 128

In my recent article : 'WiFi Channels On The 5GHz Band In The UK', I noted that although the 5GHz channels 120, 124 and 128 are unlicensed channels available for use by WiFi equipment in the UK, it appears that a few major WiFi equipment manufacturers do not allow their use (in the UK or EU).

I spoke with a major vendor representative today who advised me that the 3 channels are available for use, but that an update to the ETSI standard 301 893 v1.5.1 introduced some detection techniques for various military equipment used in the EU. However, many access points that were already manufactured (or using chip-sets that had already been manufactured) did not support the granularity of detection that is required for this equipment. So, it was decided to simply disable support for the affected channels.

Apparently, later APs which use an updated chip-set will not be subject to the same limitations (once a few firmware updates are sorted out).

I had a poke about in the standard to see if I could track down the offending addition, but there didn't seem to be a "what's new" or "change log" that accompanies the document. All I could find was the following note in the ETSI work program item that accompanies the standard:

"Include Staggered PRF radar test signals across the 5 250 MHz to 5 725 MHz band. Include narrow pulse widths for the radar test signals (0,8 ┬Ás) across the 5 250 MHz to 5 725 MHz band. Address noise calibration scan ("zero check") in the 5 600 MHz to 5 650 MHz band."

Perhaps the "zero check" scan that is referenced is the offending item that caused the issue - it certainly falls within the range of the channels that have been disabled.

Although this doesn't provide a comprehensive answer, it at least suggests why we have lost a few channels on 5GHz here in the EU (at least for the moment, anyhow).

UPDATE: I now have an answer on this! Check out my later article here.

Update: I've now created a white paper which details 5GHz usage for WiFi in the UK. Find it here