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8×8:8 versus 4×4:4 – More is Not Better!

David Coleman Director, Product Marketing Published 12 Feb 2020

Lately, the Wi-Fi 6 technology question I have been getting asked the most often is, “Which is better? 8×8:8 APs or 4×4:4 APs?” Several WLAN vendors are selling Wi-Fi 6 APs that have a 2.4 GHz radio that is 4×4:4 and a 5 GHz radio that is 8×8:8. These WLAN vendors are forging marketing pitches for 8×8:8. Eight must be better than four — right?

First of all, the PoE power requirements of 8×8:8 APs are substantial. In some cases, these 8×8:8 APs will require 31 watts of power or more, which means that even PoE Plus power will not be sufficient. While some of the 8×8:8 APs can use the 25 Watts provided by 802.11at (PoE Plus), there is often some downgrade functionality such as loss of USB, BLE radio, and so on. Regardless, these very expensive 8×8:8 APs will also be an enormous drain on any PoE power budget.

Theoretically, an 8×8:8 AP could modulate data on all eight radio chains to a single client, which would result in some substantially high data rates. The problem is that there will never be any 8×8:8 mobile client devices due to the drain on battery life. Figure 1 depicts Wi-Fi client information in the Network 360 monitoring view of ExtremeCloud™ IQ. We look at the probe request and association requests to determine the Maximum Client Capabilities of your Wi-Fi client population. As you can see in Figure 1, we live in a world where the bulk of Wi-Fi client devices are 2×2:2. In the cloud, we can then compare the client capabilities to both the real-time and historical operations of your client population. Depending on existing conditions, the 2×2:2 clients will often downgrade to 1×1:1 communication.

Figure 1

So, is there any advantage of an 8×8:8 AP over a 4×4:4 AP? Yes, throughput gains due to MU- MIMO functionality. An 8×8:8 AP could modulate two independent streams of data each to four Wi-Fi 6 2×2:2 clients that support downlink MU-MIMO. Also, an 8×8:8 AP could transmit downlink a single and unique modulated stream of data to each of eight Wi-Fi 6 clients, simultaneously. While this sounds good in theory, MU- MIMO requires spatial diversity. Wi-Fi 6 clients support MU-MIMO; however, the majority of modern-day enterprise deployments of Wi-Fi involve a high density of users and devices that is not conducive for MU-MIMO conditions. To function, MU-MIMO requires spatial diversity. In other words, there must be a sizable physical distance between all the clients, as well as the AP, as shown in Figure 2.

Figure 2

A good use case for MU-MIMO is an outdoor point-to-multipoint (PtMP) bridge link. The spatial diversity required for MU-MIMO exists in this type of outdoor deployment. But once again, almost all indoor WLANs are high-density environments because there are so many users and devices. Most users want to connect to an enterprise WLAN with as many as three or four Wi-Fi devices. Furthermore, high-density environments consist of multiple areas where roaming is also a top priority. MU-MIMO rarely works well with mobile clients, and the required spatial diversity simply does not exist within the bulk of indoor enterprise Wi-Fi high-density deployments.

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The significant gains realized by Wi-Fi 6 are a result of OFDMA, a multi-user technology that holds the most promise for efficiency improvements. You might hear some marketing hype that 8×8:8 APs will support more clients than a 4×4:4 AP. That is a complete falsehood; regardless of the number of radio chains and regardless of stream count, all Wi-Fi 6 APs will support the same number of OFDMA clients during a transmission opportunity (TXOP).

Although the theoretical MU-MIMO gains sound enticing, the reality is that an 8×8:8 AP offers no real advantage over a less expensive 4×4:4 Wi-Fi 6 access point. I will also make the argument that a Wi-Fi 6 AP with dual 5 GHz 4×4:4 radios will offer greater capacity capabilities than a Wi-Fi 6 AP with a fixed 2.4 GHz 4×4:4 radio and a fixed 5 GHz 8×8:8 radio.

Extreme Networks offers a software-selectable radio (SSR) along with a fixed 5 GHz radio within many models of dual-frequency Wi-Fi 6 APs. The radio that has SSR functionality can operate as either a 2.4 GHz or a 5 GHz radio. As a result, a dual-radio AP can either offer 2.4 GHz/5 GHz coverage or offer coverage on two different 5 GHz channels, as shown in Figure 3. The whole point behind dual 5 GHz coverage is to provide more capacity for clients. Offering double the 5 GHz coverage using 4×4:4 Wi-Fi 6 radios together with Wi-Fi 6 OFDMA mechanisms holds enormous capacity potential.

Figure 3

Portions of this blog were excerpted from the free E-Book, “Wi-Fi 6 for Dummies.”

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