Can 40 MHz and 80 MHz channels co-exist in a Wi-Fi 6E world? - Part 2

This is the second of two blogs where I discuss future channel reuse design concerns in the exciting world of 6 GHz Wi-Fi.

In the first blog, I discussed the various 802.11 physical carrier sense methods and the implications when channel bonding is used in the 5 GHz frequency band. Multiple 20 MHz channels can be bonded together to create larger 40 MHz or even 80 MHz channels. To summarize, the implementation of physical carrier sense is different between primary and secondary channels. Mismatched primary and secondary channels step on each other and result in collisions and data corruption. Throughput and performance are negatively impacted. The technical term for this problem is sometimes referred to as primary/secondary overlapping basic service set (OBSS) interference. Bottom line, when configuring channel reuse patterns in the 5 GHz frequency band, one must ensure that you have a consistent selection of the primary channels to avoid this problem. But what about 6 GHz?

If you do not know already, Wi-Fi 6E has recently brought 802.11 wireless technology to the 6 GHz frequency band for the first time. Wi-Fi 6E and the availability of the 6 GHz spectrum is truly a gamechanger. And because of the new 6 GHz power spectral density rules, it is widely expected that 80 MHz channel reuse patterns to be commonly used in the enterprise.

In a previous blog, I discussed the alignment of both preferred scanning channels (PSC) and primary channels in the 6 GHz frequency band. The PSC channels must also serve as the primary channels when channel bonding is used for 40, 80, or 160 MHz channels. Please note that the positioning of the PSC channels is the second 20 MHz of a bonded channel. If enabled, the PSC channels also function as the primary channel for half of the 40 MHz channels and all 80 MHz channels, as shown in Figure 1. Please note that half of the 40 MHz channels do not align with the 80 MHz channels.

Figure 1 - Preferred scanning channels and primary channels

I started to ask myself, can primary/secondary OBSS interference problems still occur in 6 GHz? If the primary channels do not align, will we see the same performance degradation?

So, I enlisted the help of my friend, Karl Benedict, a senior solutions architect at Extreme Networks. Karl cranked up his home lab and did some 6 GHz Wi-Fi throughput testing with a variety of 2x2:2 Wi-Fi 6E clients. I asked Karl to test what would happen in a 6 GHz environment where both 80 MHz and 40 MHz channels were deployed. Figure 2 displays some baseline throughput numbers when only an 80 MHz channel (7) is deployed. An average of 600 Mbps using a Wi-Fi 6E smartphone is achieved with the 80 MHz channel. Additionally, Figure 3 displays some baseline throughput numbers for a 40 MHz channel (3).

Figure 2 - 80 MHz baseline traffic with no 40 MHz traffic

Figure 3 - 40 MHz baseline traffic with no 80 MHz traffic

So, Karl then ran a test with two nearby APs, one transmitting on channel 7 (80 MHz) and one on channel 3 (40 MHz). The APs and their connected clients share the same frequency space and, therefore, must contend for the medium. But the good news is that the primary channels are aligned, as shown in Figures 4 and 5. The numbers look lower, but remember, they are now sharing the frequency space, and the numbers add up to just under an average of 600 Mbps.

Figure 4 - 80 MHz traffic with 40 MHz traffic (aligned primary & PSC)

Figure 5 - 40 MHz traffic with 80 MHz traffic (aligned primary & PSC)

So far, so good. But what if primary channels do not align? Unfortunately, this can happen because half of the available 40 MHz channels do not align with the primary channels used in 80 MHz. So, Karl ran another test with two nearby APs, one transmitting on channel 7 (80 MHz) and one on channel 11 (40 MHz). As you can see in Figures 6 and 7, the primary channel of channel 7 (80 MHz) does not match the primary channel of channel 11 (40 MHz). The mismatched primary and secondary channels step on each other and result in collisions and data corruption. The total aggregate throughput is now about 225 Mbps which is almost two-thirds less than when primary channels are aligned.

Figure 6 - 80 MHz traffic with 40 MHz traffic (non-aligned primary)

Figure 7 - 40 MHz traffic with 80 MHz traffic (non-aligned primary)

Always remember that the bonded primary and secondary channels are used together only for any data frame transmissions between capable APs and clients. All 802.11 management and control frames are transmitted only on the primary channel. If there is a mismatch of primary channels, expect the performance hit, as seen in Figures 6 and 7.

So, what are the real-world implications of this? Although we expect many 80 MHz deployments in the enterprise, what if a neighboring business used a 40 MHz channel reuse plan? Half of the APs the neighboring business deploy can potentially step on your 80 MHz channel reuse pattern. Be aware that this might impact your 6 GHz network. Additionally, some businesses may use a mix of 40 MHz and 80 MHz channels in 6 GHz. If different channel sizes are used, you might want to consider not using the non-aligned 40 MHz channels.

Also, be aware that nearby consumer-grade Wi-Fi 6E APs can also create OBSS problems. The default setting in most home Wi-Fi 6E APs will be 160 MHz channels. And I have already seen many of these consumer devices not use the optimal PSC and primary channel settings. As shown in Figure 8, expect many occurrences of primary/secondary mismatches in consumer deployments.

Figure 8 - 160 MHz channels: non-aligned primaries

There is an optional Wi-Fi 6/6E technology called preamble puncturing that could address these issues. But currently, it is not widely adopted. Also, I expect some enterprise vendors to start making claims of dynamic bandwidth operation (DBO) using their radio resource management (RRM) protocols. My advice… be very skeptical.

Bottom line, when configuring 6 GHz channel reuse patterns, ensure a consistent selection of the primary channels. Good design and planning will proactively ensure optimal performance.

The 6 GHz frequency band is the future of Wi-Fi for the next ten years and beyond. And the future is bright.