802.11ax’s Achilles Heel

While there are some technical reasons to be excited about the soon forthcoming 802.11ax (hereafter, just 11ax) technology, it has one big Achilles Heel that must be considered by any customer who is going to upgrade their 802.11ac Wave-1 or Wave-2 infrastructure to 11ax.

 >> >> >> Legacy Client Devices << << <<

Yep, that’s it. Legacy client devices will be the bane of 11ax. The purpose of the 11ax PHY is improved efficiency, but 100% of the features in the 11ax amendment improve client performance for 11ax client devices. No thought is given to 11ac, 11n, or previous PHYs, save for backwards compatibility.

An 11ax AP, upon winning a TxOP, can send data downlink to up to 9 clients on a 20MHz channel (18 on 40MHz, and 37 on 80MHz) at a time, and equally as cool, it can “trigger” up to 9, 18, or 37 clients to transmit uplink, for the same amount of time each. In this way, the AP is dynamically subdividing channels into 26-tone, 52-tone, 106-tone, or 242-tone Resource Units (RUs) within a 20MHz channel.  Very cool.

The problem? All legacy clients’ transmissions (management frames, control frames, data frames, and MCS data frames sent with OFDM or HT-OFDM) all use the entire channel for every transmission. Many of these frames are very small (under 256B), so the efficiency of these transmissions is terrible. That’s the very reason 11ax now exists.

Can We Remedy This Weakness?

The keys to getting as much efficiency gain from 11ax clients as possible, are:

  • Replace legacy clients with 11ax clients as soon as possible
  • Greenfield (separate them from legacy PHYs) as many of the 11ax clients as possible using techniques such as D5G with 11v support

If you want to learn more about 802.11ax, you can check out my on-demand 11ax deep dive webinar here or come see me speak at length on this topic at Extreme Networks’ 2018 User Conference, April 17-20, 2018, at the Westin Kierland Resort, Scottsdale, AZ.