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.
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 the floor. Before it hits that 6-meter mark, the surface must encompass the entire 5-meter section, including many small sub-sections called millimeters.
Substitute "meter" with "foot" and "millimeter" with "inch" if you use the inch-pound system. The units are different in values, but the idea remains the same.
The sub-portions are so small they are often conveyed in MHz -- 1GHz = 1000MHz. And for better management, these are divided into four frequency range groups in the US, 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
Note that the grouping is flexible. For example, depending on who you're talking to, the UNII-2e can be considered part of UNII-3 and UNII-3 part of UNII-4.
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 exclusively used for Doppler RADAR.
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 priority.
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 surface sections 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 on this website is generally based on US regulations and applicable to the United States.
The use of the UNII-4 portion, or even its definition, might not be the same or available in other parts of the world, but the concept of Wi-Fi bands and channels is applicable 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 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.
The last channel (181) might not be available to Wi-Fi though it's generally considered part of UNII-4.
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 4x4 specs) and long range.
To be clear, existing Wi-Fi 6 hardware can already deliver up to 4800Mbps of bandwidth on a single band without this portion. But in most cases, you can only expect half of that due to DFS restrictions. The 5.9GHz portion allows hardware to consistently achieve this theoretical bandwidth without using any DFS channel.
And that's how this new portion can be exciting, which brings us to the enduring questions relating to hardware.
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
In March, 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.
Still, whether or not your particular Wi-Fi 6 router will get UNII-4 depends on the vendor. Some might think it's better to make consumers buy new hardware instead.
UNII-4 in Tri-band vs Dual-Band broadcaster
If a broadcaster (router or access point) 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 5.9GHz.
The reason is that a band can only work on one channel at a time. Consequently, putting the 5.9GHz portion to use means the band will not support non-UNII-4 5GHz clients, which is currently the case for all existing clients. While that might change, it's safe to say there will always be 5GHz clients that don't support this portion.
So far, UNII-4 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 -- the lower-channel band is there for unsupported clients.
On a Dual-band Wi-Fi 6 (or 5) and Tri-band Wi-Fi 6E router, there's only one 5GHz band. Turning on UNII-4, in this case, will cause incompatibility.
And that makes sense.
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 Wi-Fi clients. We travel with our mobile devices, meaning the receiver should work with all Wi-Fi broadcasters worldwide. But it's ultimately the vendors who decide if or when it allows the hardware to connect via this portion.
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, that has gotten a new software driver to handle the 5.9GHz portion 5GHz band.
Even Intel's latest Wi-Fi driver software, designed for Windows 11 22H2, doesn't include the support for UNII-4 in the hardware.
In my experience with existing UNII-4-enabled routers -- including the Synology RT6600ax, ZenWiFi XT8, and ZenWiFi Pro XT12 --, when any of the newly added UNII-4 channels (169, 173, or 177) is used, no existing client can connect to the band anymore.
While that might change with Wi-Fi 6 adapters, it's safe to say many existing 5GHz clients (of older Wi-Fi standards) will never support the UNII-4 portion.
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
Fronthaul is the Wi-Fi signals broadcast outward for clients or the network ports for wired devices. It's what we generally expect from a Wi-Fi broadcaster.
Backhaul (a.k.a backbone,) on the other hand, is the link between one satellite Wi-Fi broadcaster and another, which can be the network's primary router, a switch, or another satellite unit.
This link works behind the scenes to keep the hardware units together as a system. It also determines the ceiling bandwidth (and speed) of all devices connected to the particular broadcaster.
The connection type, a Wi-Fi band or a network port, used for the backhaul is often called the uplink. A Wi-Fi broadcaster might use one of its bands (2.4GHz, 5GHz, or 6GHz) or a network port for the uplink.
When a Wi-Fi band handles backhaul and fronthaul simultaneously, only half its bandwidth is available to either end. From the perspective of a connected client, that phenomenon is called signal loss.
A Wi-Fi connection between two direct parties occurs in a single band, using one fixed channel, at any given time. This principle applies to all existing Wi-Fi standards, up to Wi-Fi 6E.
When a Wi-Fi band functions solely for backhauling, it's called the dedicated backhaul. Often that means no other band will do this job, though that depends on the hardware.
In a mesh system, only traditional Tri-band hardware -- those with an additional 5GHz band -- can have a dedicated backhaul band without ostracizing clients of the same band.
Generally, it's best to use network cables for backhauling -- wired backhauling. And that's an advantage of mesh hardware with network ports. In this case, a satellite broadcaster can use its entire Wi-Fi bandwidth for front-hauling.
In networking, network cables are 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 off 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.
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. And that remains until all existing 5GHz clients support this portion of the spectrum which is highly unlikely.
In any case, until supported clients are available, 5.9GHz Wi-Fi has zero impact in standalone routers or mesh systems with wired backhauling.
In March this year, 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, if ever.
But I guess starting in 2023, the use of the 5.9GHz band will be relatively ubiquitous -- at least in mesh systems. I'd hope so, anyway.