Five Facts About 6 GHz Wi-Fi

Two pieces of important news hit the wire recently: the 6 GHz band was approved for Wi-Fi, and alcohol sales have skyrocketed. 

Combine the two, and what do we get? A five-pack of Coors Light Cold Hard Facts about Wi-Fi in the 6 GHz frequency band.

Editor's note: This is a relatively long post, but 6 GHz Wi-Fi is an undoubtably robust topic. If you have questions after reading this blog, comment below or contact Ben via email or Twitter using the information below, and Sniff Wi-Fi will address those answers in a future post.

We may be knee-deep in a global pandemic, but that doesn't mean your humble blogger can't brew up a post on the topic of 6 GHz Wi-Fi.

Wi-Fi professionals will soon have unlicensed frequency available in the 6 GHz band. Here, then, are five facts to know about Wi-Fi in the soon-to-be-available 6 GHz frequency band.

Fact #1: The 6 GHz band adds 1,200 MHz of spectrum for Wi-Fi

The Federal Communications Commission (FCC) is the federal (meaning all of the USA) regulatory organization that sets communications rules in the United States. On April 2, 2020, the FCC released a document indicating proposed rules for allowing unlicensed wireless communications in the 6 GHz frequency band.

The rules apply to Wi-Fi channels that sit between the frequencies of 5.925 GHz and 7.125 GHz. Add that all up, and it comes to 1,200 MHz (or 1.2 GHz) of total frequency spectrum. Channel numbers start at 1 and go up to 233, with available channels spaced 20 MHz (which is four numbers) apart from one another, just like in the pre-existing frequency bands used by Wi-Fi, 2.4 GHz and 5 GHz.

6 GHz unlicensed frequency covers four separate bands. All four operate under the umbrella of unlicensed national information and infrastructure (U-NII) rules. Two of the U-NII bands are allocated for indoor-only use, the other two are indoor-outdoor. (More on the rules for indoor-only and indoor-outdoor later.)

The four U-NII bands of 6 GHz unlicensed frequency are set up like so:

• U-NII-5 (indoor-outdoor): channels 1 to 97, ranging from 5.925 GHz to 6.425 GHz
• U-NII-6 (indoor-only): channels 101 to 117, ranging from 6.425 GHz to 6.525 GHz
• U-NII-7 (indoor-outdoor): channels 121 to 185, ranging from 6.525 GHz to 6.875 GHz
• U-NII-8 (indoor-only): channels 189 to 233, ranging from 6.875 to 7.125 GHz

Wi-Fi channel bonding is set up using the same structure as the 5 GHz frequency band. Each channel has a dedicated group of channel partners for creating 40 MHz, 80 MHz or 160 MHz wide channels. (One difference between 5 GHz and 6 GHz is that 6 GHz uses a fully bonded 160 MHz wide channel, while 5 GHz uses 80+80 MHz channel bonding. Fully bonded channels offer higher speeds, but also require a broader range of consecutive frequency space.) For example, channel 1 only bonds with channel 5, when making a 40 MHz wide channel. Channels 1 & 5 only bond with channels 9 & 13 when creating an 80 MHz wide channel. Channels 1/5/9/13 only bond with channels 17/21/25/29 when creating a 160 MHz wide channel.

Rick Murphy of Wireless Training Solutions put together an elegant chart showing the channel numbering and groupings for 6 GHz Wi-Fi.


Fact #2: Rules for indoor-outdoor 6 GHz channels are mostly the same as the rules for 5 GHz

APs and clients using U-NII-5 or U-NII-7 channels must abide by indoor-outdoor rules for the 6 GHz band.

Conducted power (refered to as "transmit power" by most Wi-Fi professionals) and equivalent isotropically radiated power (EIRP) rules for 5 GHz Wi-Fi and 6 GHz indoor-outdoor Wi-Fi are identical.

• Maximum conducted power for APs: 30 dBm (1,000 mW)
• Maximum EIRP for APs: 36 dBm
• Maximum conducted power for clients: 24 dBm (250 mW)
• Maximum EIRP for clients: 30 dBm

There are two important differences between rules for 5 GHz APs and 6 GHz indoor-outdoor APs: Automatic Frequency Coordination (AFC) and listen-before-talk (LBT) rules.

AFC is a database of existing networks using licensed frequencies in the 6 GHz band. That's LICENSED, not unlicensed. Licensed 6 GHz networks could be some kind of site-to-site wireless link, mobile network or perhaps satellite link. Licensed frequencies are owned, and thus cannot be compromised by unlicensed devices operating on the same frequencies.

AFC compliant Wi-Fi 6E APs will have to search the AFC database and blacklist any channels that are used by nearby licensed frequency deployments. As of two years ago, Wi-Fi lobbyists were pushing the FCC to eschew AFC requirements over as much of the 6 GHz band as possible. My understanding is that Wi-Fi vendors are now amenable to creating products that use AFC (although I am not certain).

Listen-before-talk (LBT) is a rule Wi-Fi lobbyists were hoping the FCC would implement across as much of the 6 GHz band as possible. Wi-Fi networks naturally use LBT, in the form of the 802.11 Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) protocols. CSMA/CA requires Wi-Fi devices to go through a process called Arbitration before transmitting any frames (aka packets). Arbitration involves staying quiet until the AP's channel is clear. Listen (Arbitration) before Talk (frames). LBT is required on 5 GHz Wi-Fi channels, under the current interpretation of FCC rules.

Service provider networks do not use LBT. Their network infrastructure equipment is designed to operate in licensed frequency bands, which can be owned by the service provider. Since the service provider owns the license, there is no need to waste time "listening". It's talk, talk, talk, talk, talk.


By requiring AFC and eschewing LBT requirements for the indoor-outdoor channels of the 6 GHz band, the FCC has made those channels less friendly to Wi-Fi. Channels 1 to 97 and 121 to 185 may end up being available in Wi-Fi 6E APs and clients. It is possible that APs and/or clients might avoid those channels. Time will tell.

Fact #3: Rules for indoor-only 6 GHz channels are quite different than 5 GHz channel rules

APs and clients using U-NII-6 or U-NII-8 channels must abide by indoor-only rules for the 6 GHz band.

There are three physical AP rules for indoor-only 6 GHz channels: APs may not be weatherproof. No external antennas are allowed. Battery-powered APs are forbidden. I don't expect any of these rules to be a problem for enterprise AP vendors or residential wireless router/modem vendors.

The radio power rules for indoor-only 6 GHz channels could be a problem.

For indoor-only 6 GHz channels, the FCC limits radios and antennas by power spectral density (PSD) rather than EIRP.

• EIRP (dBm) = conducted power (dBm) + antenna gain (dBi) + MIMO offset (dB)
• PSD (dBm/MHz) = EIRP (dBm) - 10 * log [channel width, in MHz] (dB)

The limits are:

• Maximum PSD for APs: 5 dBm/MHz
• Maximum PSD for clients: -1 dBm/MHz

This information leads to our next 6 GHz Wi-Fi fact...

Fact #4: Indoor-only 6 GHz Wi-Fi is intended to be used by APs configured for wide channels

Indoor-only 6 GHz bands, which include channels 101 to 117 and 185 to 233, have radio power limits covered by PSD, rather than EIRP.

Many Wi-Fi professionals are, for the moment, unfamiliar with PSD. I certainly was, before I began researching 6 GHz Wi-Fi rules. The key to tying radio power limits to PSD instead of EIRP is that it creates a bias towards wider channels.

For example...

If you do the math on 20 MHz wide channels, you'll find that indoor-only 6 GHz channels have the following limits:

• Maximum EIRP for APs on a 20 MHz wide indoor-only channel: 18 dBm
• Maximum EIRP for clients on a 20 MHz wide indoor-only channel: 12 dBm

Seems OK, until you do the math for conducted power. If we take a typical, 4-stream multiple input/multiple output (MIMO) AP with a 3 dBi internal antenna, the the limit becomes:

• Maximum conducted power for APs on a 20 MHz wide indoor-only channel: 9 dBm

9 dBm is a very low conducted power. It's only about 8 mW. Most client devices rarely use a conducted power as low as 8 mW, if at all. If APs are limited to a MAXIMUM conducted power of 8 mW, it could cause all sort of performance issues for modern Wi-Fi clients.

But...

Look what happens when 80 MHz wide channels are used:

• Maximum EIRP for APs on a 80 MHz wide indoor-only channel: 24 dBm
• Maximum EIRP for clients on a 80 MHz wide indoor-only channel: 18 dBm

And, therefore:

• Maximum conducted power for APs on a 20 MHz wide indoor-only channel: 15 dBm

15 dBm is a totally acceptable conducted power level for enterprise-class APs supporting modern Wi-Fi client devices. It is equal to approximately 30 mW. With rare exceptions, modern client devices are capabile of using conducted power levels at or close to 30 mW.

The bottom line is, the FCC created PSD limits for indoor-only 6 GHz channels which allow APs using channel widths of at least 80 MHz to function normally, but that restrict the performance of APs which use channel widths of 20 or 40 MHz.

Fact #5: 6 GHz Wi-Fi is for Wi-Fi 6 (802.11ax), not Wi-Fi 1/2/3/4/5 (802.11b/a/g/n/ac)

For starters, there is no standard for Wi-Fi 1 (802.11b), 2 (.11a), 3 (.11g), 4 (.11n) or 5 (.11ac) in the 6 GHz frequency band. There is a standard for Wi-Fi 6 (802.11ax) APs and clients in the 6 GHz band, and it is called Wi-Fi 6E.

History has taught us that Wi-Fi AP and client vendors will often make products that operate on the edges of standards, or in some cases fully outside standards.

Your humble blogger would not be surprised if, for example, Wi-Fi 5 (and perhaps even Wi-Fi 4) devices that support 5 GHz channels will some day get a software upgrade that allows for 6 GHz channel support. Perhaps there is a way; perhaps there are also limitations. I'm not sure.

Whether non-standard Wi-Fi 5 (and perhaps Wi-Fi 4) upgrades for 6 GHz frequencies happen, there is a mitigating factor: FCC rules for 6 GHz Wi-Fi channels are designed for wide channels, and the radio technology for Wi-Fi 5 (and Wi-Fi 4) isn't.

Wi-Fi 5 (and Wi-Fi 4) supports orthogonal frequency division multiplexing (OFDM). (Wi-Fi 6 also supports OFDM, as a backwards compatibility measure and a way for clients to opt-out of AP control [which is a topic for a future blog post!].) OFDM is a technology that often struggles for stability when 80 MHz wide channels are used, because OFDM APs and clients must always use the entire channel width when transmitting or receiving frames (aka packets).

Wi-Fi 6, on the other hand, supports orthogonal frequency division multiple access (OFDMA).  OFDMA is flexible, in terms of channel widths. APs can choose to use the entire channel when transmitting or receiving wireless frames, or divide up channels between individual client devices by assigning different subchannels -- called resource units (RUs) -- to each client. This flexibility allows for super fast speeds when fewer clients are active on a given AP, and better stability when a high number of clients are active.

Wi-Fi lobbyists and FCC commissioners likely felt good about designing 6 GHz unlicensed rules to suit channel widths of 80 MHz and up (see: Fact #4) because Wi-Fi 6 is the only standard certified for the 6 GHz band, and Wi-Fi 6 includes OFDMA.

Unfortunately (or fortunately, if you are a vendor or integrator), enterprise Wi-Fi deployments with Wi-Fi 5 (or Wi-Fi 4) APs will almost certainly need to do a rip-out and re-mount upgrade in order to be able to take advantage of the additional channels of 6 GHz.


In summary, it can be fairly said that the FCC's unlicensed rules for the 6 GHz band are very helpful, while perhaps falling short of being a silver bullet. If regulations for the indoor-outdoor channels of UNII-5 and UNII-7 allow for upgraded, consistent Wi-Fi performance, then 6 GHz Wi-Fi will paint an even rosier picture than your humble blogger anticipates. If not, we still get the extra indoor-only channels of UNII-6 and UNII-8, and the accompanying improvements that come from greater spectrum availability.
 
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Ben Miller works as a Wi-Fi contractor, with a background in pre- and post-installation consulting, technical and marketing writing, and instruction for vendor-neutral and vendor-specific Wi-Fi training. You can contact Ben via email, or follow him on Twitter and Twitch, using the contact information below.

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