5.9GHz Wi-Fi 6: That Exciting UNII-4

5.9GHz Wi-Fi 6: That Exciting UNII-4

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With the release of the Synology RT6600ax and the upcoming Asus GT-AX11000 Pro and ZenWiFi Pro XT12, you might have many questions about the 5.9GHz Wi-Fi 6 band, that “final frontier” of the standard, or more precisely, of the 5GHz spectrum.

Or you might have never heard of it. In either case, you’re reading the right post. I’ll explain what 5.9GHz Wi-Fi is and set realistic expectations based on conversations with vendors and my real-world experiences.

Dong’s note: I first published this post on March 17, 2022, and updated it on August 20 to add up-to-date information.

Not a trick question: What do this well-used measuring tape and Wi-Fi have in common?

So, what is the 5.9GHz Wi-Fi 6 band anyway?

Wi-Fi first started with the 2.4GHz frequency band, and with the original move from single band to dual-band back in 2009, we have since also had the 5GHz band.

As you might have noted, 2.4 is just a portion between 2 and 3, while 5 is a whole number. In other words, 5GHz is supposed to be the entire band.

Or is it?

The initial three UNII groups

In reality, things are much more complicated. From the get-go, Wi-Fi has never had the entire 5GHz band for itself — far from it — nor will it ever.

Like all frequency bands, the 5GHz spectrum is divided into smaller portions from 5.1GHz to 5.9GHz.

You can visualize Wi-Fi airspace by putting a measuring tape on a floor. Before it hits that 6-meter mark, the surface must encompass the entire 5-meter section, including many small sub-sections called millimeters.

In fact, the sub-portions are so small they are often conveyed in MHz — 1GHz = 1000MHz. And for better management, in the US, these are divided into four frequency range groups, not-so-aptly called Unlicensed National Information Infrastructure or UNII.

For Wi-Fi-related applications, the following is the ballpark (not 100% accurate) breakdown of these groups — the MHz values are rounded:

  • UNII-1 ranges from 5170MHz to 5250MHz
  • UNII-2: 5250MHz to 5330MHz
  • UNII-2e (extended): 5490MHz to 5730MHz
  • UNII-3: 5735MHz to 5835MHz
  • UNII-4: 5850MHz to 5925MHz

You will note that there are gaps in the spectrum between these groups. Those are areas of spectrum permanently dedicated to non-Wi-Fi applications. For example, the gap between 5330MH and 5490MHz is used for Doppler RADAR at all times.

On top of that, both UNII-2 and UNII2e are part of the Dynamic Frequency Selection (DFS) shared between other RADAR applications and Wi-Fi, with the former always having the priority.

The 5GHz Wi-Fi channels and their positions on the spectrum.
The original 5GHz Wi-Fi channels and their positions on the band’s traditional Wi-Fi-related spectrum

The chart above shows how the first three U-NII groups apply to 5GHz Wi-Fi, and the channels end at the 5835MHz mark. Until late 2021, only UNII-1, UNII-2/e, and UNII-3 were available to Wi-Fi.

And that brings us to the fourth UNII group, which includes the 5.9GHz portion.

The controversial UNII-4 spectrum

To understand the significance of the 5.9GHz band, we first need to know how Wi-Fi works in terms of speed.

In a nutshell, the smallest portions of the airspace, called channels, are 20MHz wide. But you can add contiguous ones to increase the width and, therefore, the bandwidth. So two 20MHz channels make a 40MHz one, and two 40MHz create an 80MHz channel.

Back to the measuring tape analogy, you can combine multiple millimeters into a centimeter and multiple centimeters into a decimeter.

Essentially, it’s as basic as putting adjacent sections of a surface together to create a large single continuous area.

Wi-Fi 6 is the first standard that supports the 160MHz channel width. I wrote about the standard in great detail in this post, but the gist is:

  • Wi-Fi 6 needs 160MHz channel width to deliver top performance.
  • Since 160MHz is wide, within the first three UNII groups, there is enough space for only two 160MHz channels.
  • Both of these 160MHz channels encompass DFS air space. As a result, the router might have a brief disconnection when RADAR signals are present. To avoid that, many Wi-Fi 6 broadcasters (routers, access points) might not use the 160MHz channel width and opt for the narrower but more reliable 80MHz, which cut the standard’s ceiling speed in half.

And that brings us to the UNII4 portion of the 5GHz band, often referred to as the 5.9GHz band.

For decades, this portion has been controversial because it was reserved for the auto industry, which has never used it — it’s a long story.

Wi-Fi air space is regulated and varies from one region to another. Information in this post is based on US regulations and applicable only to the United States.

The use of UNII-4 might not be available in other parts of the world though the concept of Wi-Fi bands and channels are generally the same worldwide.

Wi-Fi advocates fought long and hard for this final airspace of the 5GHz band, and, finally, in late 2020, FCC approved it for Wi-Fi use and then made it available for hardware vendors in late 2021.

The 5.9GHz portion of 5GHz Wi-Fi 6 Band
The 5GHz Wi-Fi 6 band with the newly available UNII-4 channels, including the 5.9GHz section

The table above shows how the addition of the UNII-4 group completes the 5GHz band for Wi-Fi use. (Again, the borders dividing these groups are flexible.)

Specifically, it extends the band’s tail with four more 20MHz channels, including 169, 173, 177, and 181. Combining existing channels with the first three makes a third 160MHz channel possible on this band.

Most importantly, this third 160MHz channel is the only 5GHz band that does not use DFS. In other words, it’s the only “clean” high-bandwidth channel that can deliver Wi-Fi 6’s top speed and reliability, even when used near RADAR stations.

That said, the 5.9GHz might have the best of both worlds: Fast Wi-Fi 6 speeds (up to 4800Mbps in the current top 4×4 specs) and long range.

To be clear, without this portion, existing Wi-Fi 6 hardware can already deliver up to 4800Mbps of bandwidth on a single band. But in most cases, you can only expect half of that due to DFS restrictions. The 5.9GHz portion allows hardware to consistently archive this theoretical bandwidth regardless of the environment.

And that’s how this new portion can be exciting, which brings us to the endearing questions relating to hardware.

Synology RT6600ax Wi Fi 6 Router 5
The RT6600ax is the first Wi-Fi 6 router from Synology and one of the first to support the 5.9GHz band.

5.9GHz Wi-Fi 6 band’s hardware: Will UNII-4 work with existing equipment?

Regarding hardware, the first question is, will existing Wi-Fi 6 broadcasters (routers and access points) support the new 5.9GHz (the UNII-4 portion) via firmware update?

The answer to this question has shifted since I first wrote on this topic in March 2022.

On the broadcasting (routers/access points) side: It depends

The first time around, I asked multiple hardware vendors this question, and the answer was consistently a no.

Most vendors told me they might need to get an existing broadcaster re-certified, which could complicate things. But things change. By mid-2022, it’s clear that UNII-4 can come to existing broadcasters via firmware upgrades.

The Asus ZenWiFi XT8 is the first router that gets UNII-4 via firmware update. Note the extra channels made available by the latest firmware.

The Asus ZenWiFi XT8 is the first router I know that got this treatment, starting with firmware version

Still, whether or not your particular Wi-Fi 6 router will get it depends on the vendor. Some might think it’s better to make consumers buy new hardware instead.

By the way, if a broadcaster (router/access) supports UNII-4, the use of this portion is turned off by default. Users must turn it on manually and should only do so when their clients support the 5.9GHz portion.

For compatibility reasons, UNII-4, so far, has been (or planned to be) available only in traditional Tri-band broadcasters where it’s safe to use this portion on the upper-channel band since we can use the lower-channel band for unsupported clients.

On the client’s side: It also depends

On the receiving end, the question is will existing Wi-Fi clients support the 5.9GHz band via driver update? On this front, the answer is also it depends.

Regulations are generally less restrictive on the client-side. We travel with our mobile devices, meaning the receiver should work with all Wi-Fi broadcasters worldwide. But it’s ultimately the hardware vendors who decide if or when it allows the hardware to connect via this portion.

Asus ZenWiFi XT8 UNII 4 Default Selection
On a supported broadcaster, the UNII-4 portion is generally not in use by default to avoid incompatibility.

And as I’m updating this post in late August 2022, there’s no Wi-Fi 6 client I know of, including the popular Intel AX2xx chips, which have gotten a new software driver to handle the 5.9GHz portion 5GHz band.

Specifically, when I set a supported router, be it the Synology RT6600ax or the ZenWiFi XT8, to work in any of the newly added UNII-4 channels (169, 173, or 177), the client will not see the Wi-Fi network or can’t connect to it.

But that might change.

Wireless mesh backhauling gets a sizable boost

Up to now, the addition of UNII-4 has proven in my testing to be meaningful only when you use a traditional Tri-band mesh system in a fully wireless configuration.

In this case, the dedicated backhaul has the option of a clean 160MHz channel. While the performance is the same, the reliability of that performance grade improves significantly. And that was generally my experience with the Synology RT6600ax.

Backhaul vs fronthaul

A Wi-Fi connection between two direct devices occurs in a single band, using a fixed channel, at any given time. (That’s always been the case before Wi-Fi 7, which might work differently.)

Generally, when you use multiple Wi-Fi broadcasters, like in the case of a mesh network, there are two types of connections: fronthaul and backhaul.

Fronthaul is the Wi-Fi signal a mesh hub broadcasts outward for clients or its network ports for wired devices. That’s what we generally expect from a Wi-Fi broadcaster.

On the other hand, backhaul, a.k.a backbone, is the link between one broadcasting hub and another, be it the main router or another satellite hub.

This link works behind the scene to keep the hardware units together as a system. It also determines the ceiling bandwidth (and speed) of all devices connected to a satellite hub.

Dual-WAN: Where the distinction between bandwidth vs speed is clear

When a Wi-Fi band handles backhaul and fronthaul simultaneously, only half of its bandwidth is available to either end. From the perspective of a connected client, that phenomenon is called signal loss.

When a band functions solely for backhauling, it’s called a dedicated backhaul band. In a mesh system, only traditional Tri-band hardware with an additional 5GHz band can have a dedicated backhaul band.

Generally, it’s best to use a network cable for backhauling — wired backhaul. And that’s an advantage of mesh hardware with network ports. In this case, a hub can use its entire Wi-Fi bandwidth for front-hauling.

In networking, using network cables is always much better than wireless in speed and reliability.

My guess is Tri-band mesh-ready broadcasters will continue to be the first to get UNII-4, either via firmware updates — like the ZenWiFi XT8) — or right of the bat, which is what you can expect from the ZenWiFi Pro XT12, at least in the US version.

And that’s a good thing.

5.9GHz band: The takeaway

The availability of the 5.9GHz band for Wi-Fi use is a natural progression. This UNII-4 portion should have been open to Wi-Fi years ago. Better late than never, this final portion helps complete Wi-Fi 6.

But, so far, its adoption has proved fragmented. For now, this new stretch of spectrum applies meaningfully only to the backhauling of a wireless mesh system.

Until supported clients are available, 5.9GHz Wi-Fi has zero impact in standalone routers or mesh systems with wired backhauling.

In March, I predicted there would be a long window when you can get a UNII-4-ready router without any supporting client. And now we’re still within that period without knowing when it will end.

But I guess by the end of 2022 or earlier 2023, the use of the 5.9GHz band will be relatively ubiquitous. I’d hope so anyway.

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