Understanding Wi-Fi Speed and How 6 GHz Compares
Originally Posted: June 1st, 2021
Last Edited: February 12th, 2022
Understanding Wi-Fi Speed and How 6 GHz Compares
Wi-Fi Speed Components
Before we talk about the nature of 6 GHz Wi-Fi, it’s helpful to understand the components of Wi-Fi connections and how they interact to determine performance. The Nighthawk RAXE500 claims 10,800 Mbps of throughput, but where does that number come from? Why are the numbers what they are, and why don’t I get 10,800 Mbps on my speed tests, dang it!?
The Netgear Nighthawk RAXE500.
Start With 11,000 Mbps
2.4 GHz: 4x4, up to 1,200 Mbps with 40 MHz Channels
5 GHz: 4x4, up to 4,800 Mbps with 160 MHz Channels
6 GHz: 4x4, up to 4,800 Mbps with 160 MHz Channels
1,200 Mbps + 4,800 Mbps + 4,800 Mbps = 10,800 Mbps.
Go Down to One Band
Wi-Fi connections only happen on a single band, so you’re only able to access one band at a time. If you use 5 GHz or 6 GHz, you’re down to 4,800 Mbps. This is using 160 MHz channels, and 4 spatial streams.
Limit MIMO to 2x2
MIMO (Multiple Input, Multiple Output) is a direct capacity multiplier, and it multiplies capacity using the same spectrum. While most high-end Wi-Fi 6 access points support 4x4:4 MIMO, the vast majority of client devices top out at 2 spatial streams. Battery operated Wi-Fi clients like your smartphone or laptop are almost all 2x2:2 devices.
Going from 4 streams to 2 streams cuts our maximum link rate from 4,800 Mbps to 2,400 Mbps, if using a 160 MHz channel.
If Using 5 GHz, set Channel Width to 80 MHz
Using 160 MHz channels in 5 GHz requires the use of DFS, and not all devices support DFS operation. 80 MHz channels are much more realistic option for 5 GHz, limiting maximum link rates to 1,200 Mbps.
With Wi-Fi 6E, you get access to 6 or 7 more 160 MHz channels, and don’t need to use AFC or DFS if operating indoors. Range is less though, since 6 GHz attenuates faster, wider channels increase background interference, and 6 GHz indoor low-power AP transmit power is limited. For more details, see the Device Class and EIRP Limit section of Wi-Fi 6E Progress and Reality.
Set Modulation/Coding to 256-QAM or Lower
The maximum link rate requires 1024-QAM modulation, and a very high signal-to-noise ratio (SNR). The highest data rates are only possible in the best situations, with an AP nearby and limited interference on the channel. A more realistic modulation is 256-QAM or 64-QAM, resulting in a maximum link rate in the range of 600-900 Mbps for 80 MHz 2x2, or 1,200-1,800 Mbps for 160 MHz 2x2.
TCP/IP Overhead
In all networks wired or wireless, there’s around a 5% overhead in TCP/IP connections. That 5% comes from all the data that’s required to setup the connection and address the packets and frames being exchanged. With standard frame sizes, a wired 1 Gbps connection tops out around 940-950 Mbps of TCP throughput. Jumbo frames can help a bit, but come with their own issues. See Wikipedia for more details.
Beacons and Management Traffic
Beacon frames are how an AP advertises networks to client devices. In order to ensure that all devices in range are able to understand them, access points send out management traffic such as beacon frames at the lowest supported data rates. This expands the range of the broadcasts, but also acts as a speed bump, consuming precious airtime. The amount of management traffic increases with additional SSIDs, and features such as beamforming.
6 GHz changes a few things here. With so many channels to pick from, devices take longer to scan through all available channels and pick an AP to join. This is solved by using Preferred Scanning Channels (PSCs). Extreme networks has a good overview of how AP discovery works in 6 GHz. You can limit the impact of management traffic by restricting minimum data rates. That’s usually only necessary in dense multi-AP networks, where small cell sizes and careful channel planning are important.
Half-Duplex
OFDM-based Wi-Fi is half-duplex, meaning only one device can be transmitting at a time, and only in one direction. To make an analogy, Wi-Fi is a walkie talkie, not a phone call. Ethernet is full-duplex, and allows transmissions in both directions at the same time. Wi-Fi does not. This gets more complicated when considering OFDMA and MU-MIMO in Wi-Fi 6 networks. As a general principle it still applies, especially in networks with older devices hanging around.
Wi-Fi being half-duplex doesn’t mean that throughput is cut in half, but it does mean that Wi-Fi devices can’t multi-task. When downloading a large file, a client device has to take many short breaks to transmit TCP acknowledgement frames back to its AP, or to allow others to transmit. Wi-Fi devices can’t download and upload data at the same time, or talk when others are talking. MU-MIMO and OFDMA partially address this issue, but the real world impact usually doesn’t match up with the marketing claims.
Wi-Fi is a Shared Medium: Collisions and Re-Transmissions
In addition to being (mostly) half-duplex, Wi-Fi is a shared medium. When one device is transmitting on a channel, all other devices in range must wait their turn. If multiple devices transmit at the same time a collision can occur, causing the transmissions to be jumbled. When collisions occur, devices need to wait for a random length of time before re-transmitting. Coordinating the use of the shared medium and dealing with collisions consumes valuable airtime, resulting in lower effective throughput for everyone.
PHY Link Rate is an Estimate, and an Average
When you see a link rate of 1200 Mbps, that doesn’t mean every single frame gets sent at 1024-QAM modulation. Link rates are more like an average speed limit. Individual frames may get sent above or below the current link rate values as conditions on the channel change, or as transmissions fail.
In Summary
A 2x2 device on an 80 MHz channel can achieve a maximum link rate of 1200 Mbps, resulting in throughput around 800-900 Mbps in ideal conditions.
A 2x2 device on a 160 MHz channel can achieve a maximum link rate of 2400 Mbps, resulting in throughput around 1400-1600 Mbps in ideal conditions.
This isn’t even all of the factors. If you’re interested in reading more, the CWNP blog has a great list of sources of overhead in Wi-Fi.
How 6 GHz Compares
There’s nothing special added in 6 GHz to reduce latency, or increase speeds. Wi-Fi 6E uses the same PHY standard, MIMO, and modulation rates from Wi-Fi 6. The only thing new is the 6 GHz spectrum. An 80 MHz channel in 5 GHz is going to perform essentially the same as an 80 MHz channel in 6 GHz, with a few caveats:
Higher frequencies attenuate faster, so 6 GHz signals offer slightly less range than 5 GHz.
Indoor, low-power 6E devices like the RAXE500 are limited to a slightly lower EIRP in the 6 GHz band compared to the 2.4 GHz and 5 GHz bands. (24 vs. 30 dBm)
6 GHz outdoor operation is more complicated, and regular-power outdoor APs require the use of the new AFC system, which is similar to DFS in 5 GHz. Standard-power APs will need to report their location before being able to operate at their full power.
Indoor, low-power devices don’t need to worry about AFC or DFS. Combined with a big chunk of new spectrum, this makes 80MHz and 160 MHz channels more practical to use.
Maximum allowed transmit power in 6E increases with channel width. You’ll get the same 30 dBm maximum EIRP allowed in 5 GHz, but only with a 320 MHz wide channel. 320 MHz channels should be supported in Wi-Fi 7 (802.11be), but for now 6 GHz indoor range will be less than the maximum possible with 5 GHz.
160 MHz channels reduce maximum allowed EIRP by 3 dB
80 MHz channels reduce maximum allowed EIRP by 6 dB
40 MHz channels reduce maximum allowed EIRP by 9 dB
20 MHz channels reduce maximum allowed EIRP by 12 dB
6 GHz offers more bandwidth and less interference. 6 GHz allows for up to seven 160 MHz channels or fourteen 80 MHz channels, making them much more usable in the real world. Because of this, 6 GHz can be faster, if you’re near an AP using wide channels. 2.4 Ghz and 5 GHz still have advantages, such as longer range, better wall penetration, and legacy compatibility.
