Ofcom Relax the Rules on the 5Ghz Band

By Michael Spalter
March 2021

About the author

Michael Spalter

Michael Spalter

Michael Spalter has been a networking technician for over 30 years and has been the CEO of DrayTek in the UK since the company’s formation in 1997. He has written and lectured extensively on networking topics. If you’ve an idea for a blog or a topic you’d like explored, please get in touch with us.

Ofcom Relax the Rules on the 5Ghz band

After campaigning by vendors and users, Ofcom announced two important decisions in July 2020 relating to the 5Ghz band; that was quite timely as vendors prepare to introduce more 802.11ax devices in that band and usage will therefore increase.

Dynamic Frequency Selection (DFS)

The allocated 5Ghz Wi-Fi channels are split into 4 groups: A-Lower, A-Upper, B and C.  Group C required a licence and was used for outdoor point-to-point long range links. Group A-Lower contains 4 non-overlapping channels (20Mhz wide each) and are supported by all 5Ghz Wi-Fi devices. Group A-Upper and Group B provide another 15 channels however, in order for a wireless router or access point (AP) to support those bands, it must support a feature called DFS.

The 5Ghz band is shared with some satellite and radar services. These take priority and in order to avoid interference, DFS must scan (listen to) the channel for 60 seconds and confirm that there are no priority services transmitting.

On three of the allocated channels (channels 120, 124 and 128) it's considered that radar use is likely so your device must monitor for radar and wait 10 minutes with no radar detected before it can consider transmitting. This means that when you turn your AP on, you have no Wi-Fi for the first 10 minutes and every 24 hours thereafter.

Once the channel has been confirmed as clear, it can be used but DFS must then continue to monitor the channel during usage for any priority activity and if detected, the Wi-Fi device must shut down transmissions within 10 seconds (the 'Channel Move Time'). The device can then select a different channel but connection with all other clients/devices will be temporarily lost and the DFS connection sequence must re-start.

DFS not only slows down the initial Wi-Fi initialisation time and forces loss of connectivity but it also mandates rechecking of the channel at least every 24 hours. During these checks, Wi-Fi is unavailable for 60 seconds again (the 'Channel Observation Time') or 10 minutes for channels 120-128.

DFS is not entirely reliable - it's subject to false positives (signals wrongly detected) and false negatives (signals not detected).  This is partly due to the 'simple' nature of radar signals compared to Wi-Fi signals.

The effect of a false positive is that your Wi-Fi device shuts down unnecessarily, causing interruption to your connection. If that keeps happening, your connectivity becomes unusable as, once DFS has detected radar, that channel cannot be retried for 30 minutes.  If there's a false negative, your device may transmit on a channel being used by radar, theoretically causing critical interference to it.  There's a more detailed article on this here ( http://wifinigel.blogspot.com/2018/05/the-5ghz-problem-for-wi-fi-networks-dfs.html )

The good news, however, is that in 2020, Ofcom in the UK announced that the requirement for DFS will be removed on more channels. Specifically, frequencies in the range of 5725-5850Mhz (5.8Ghz) which correspond to channels 149, 153, 157, 161 and 165 will be allowed for indoor unlicensed use in the UK, without requiring DFS checks before or during use. Until now, these Group C channels have only been permitted for limited outdoor use in the UK, with licensing granted by Ofcom.

This means that more products will support the full range of channels, whereas previously the development complexity and cost of adding DFS to products meant that many vendors did not support the DFS channels, causing more usage and therefore congestion on the lower ones. For users, not having to use DFS means that the connections will be more stable as DFS checking, bumping and monitoring will not be required.

It was suggested by some respondents to Ofcom that DFS be relaxed across more channels, however they believe that weather radar in the lower 5Ghz band was more susceptible to interference, so the DFS requirement on channels 52 to 64 and 100 to 140 remains in place.

Transmission Power Control (TPC)

All channels in the 5Ghz band are still required to operate TPC. As TPC was mandated along with DFS, it's often assumed that they are related, but they are not.   TPC limits the transmission power of a device to the minimum required for an adequate connection. The purpose of this is to reduce interference to adjacent neighbours, transmitting only enough power to maintain your connection reliably.  It's similar to avoiding talking too loudly in a restaurant - you only need to speak loud enough for your party to hear and avoid disturbing neighbouring tables.

The 6Ghz Band

Recognising the need for more bandwidth and ever-increasing demand for channels, Ofcom's also announced  their plans to open up a new band allocation at 6 GHz (5925-6425 MHz). The USA's FCC have announced similarly.  As this is a new development, it will be some time before chipset makers have mass production and hardware vendors can build products but it's a welcome development.

The 60Ghz Band

The 60Ghz band allocation was released by Ofcom in 2009.  Being ten times the frequency of current Wi-Fi, its range is even lower but its capacity is much larger, with throughput of up to 7Gb/s so it has applications in very short haul (line of sight) applications requiring very high throughput.  This includes wireless 4K/8K video between AV devices including TVs but also notably Virtual Reality headsets which currently have cumbersome cables.  Other applications of 60Ghz include wireless backhaul (wireless uplink of conventional wired or Wi-Fi connections).  60Ghz is also usable for 5G (cellular/mobile networks) which may not be a good thing as it means that the mainstream carriers might be using these unlicenced frequencies to complement their costly dedicated band allocations, though as range is very low, it would hopefully not penetrate into your own 60GHz usage area.

As always, I hope you find these blogs useful - please do share them using the links above, make comments below and let us know if you have any suggestions for new blog entries.


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