The capabilities of infrastructure WiFi reliably precede the capabilities of the devices that use it, including laptops and phones. A previous major standard for WiFi, 802.11ac, included mechanics for Multi-User MIMO, or MU-MIMO. It provided a way to send data to two clients at once by adjusting power on multiple antennas. The signal that reached first client would be canceled for the other, and vice versa; one transmission, two different interpretations.

The access point that can craft a MU-MIMO transmission is a functionally a supercomputer. The transmission is the product of matrix calculations that factor in gains and the constructive/destructive interferences experienced at each client. MU-MIMO is uber-cool, except that there are still very few clients for it (five years later), and the opportunity to employ it comes only once-in-a-while.

Access points you would buy today are based on next standard, 802.11ax or WiFi 6. MU-MIMO is still part of the mix, but there is a much more interesting multi-user capability in the standard, called Orthogonal Frequency-Division Multiple Access (OFDMA). It works by sharing sub-carriers in a transmission between multiple client devices.

What are sub-carriers? At WiFi’s higher modulation rates, transmitted data are conveyed in multiple, bonded streams which are transmitted at neighboring frequencies. These are reassembled on receipt. Sub-carriers are orthogonal, meaning that the transmission of one does not interfere with the transmission of another. Sub-carriers provide a way to slice WiFI bandwidth into resilient pieces of modest width. Narrower bands can be demodulated and bonded more easily than if the whole channel were taken altogether at once. Fatal interference within a sub-carrier doesn’t necessarily ruin the transmission.

In WiFi 6, the sub-carriers can be shared so that some are destined for this client; some are for that client. This means that in one transmission, an access point can talk to multiple clients. That would be significant enough, but the real performance benefit comes from the elimination of overhead.

To make a single transmission, a modern access point has to perform channel assessment (to see if the air is busy). It has to insert guard bands (dead air) to allow for response turnaround. And, an access point has to contend with overlapped transmission, back-off and retry. The overhead time associated with acquiring the channel can be much greater than the data transmission window. This makes the air-time efficiency of a very fast access point with typical client data be about 10%. That’s low! By combining the data for multiple clients on multiple subcarriers, the efficiency can increase dramatically. The same amount of channel acquisition time can be shared among multiple users. The problem, as ever, is that there are few clients for OFDMA as of yet.

-Kevin Dowd

The Death of the Heat Map

Fifteen years ago, when organizations were just beginning to experiment with 802.11 wireless networks, the WiFi heat map was considered a good, splotchy plan to show where wireless would be available, and how good it might be. Wifi can travel pretty far under the right conditions. Back then, we built networks for coverage, not necessarily capacity. So any signal was a good signal.

A few years later, we began building WiFi networks for capacity. The object was/is to provide good connections to a community of users across the whole campus. For a good experience, one needs to make an association with an access point that is nearby. Generally, the closer the AP, the better the signal and thus the higher the negotiated data rate. In short: nearby AP good; far-away AP bad.

Consider this, though: if every client is going to be near an AP, then every AP is probably going to be near other APs. That means that there will be overlapped WiFi. Here is an example of what we find in the air is a typical busy campus. This data, in fact, is associated with the heat map above:

The list shows that from the location where the measurement was taken, the client could hear 28 unique access points and 43 radios! Moreover, many of them were on same channels. There were, in fact, so many APs in this space that the amount of bandwidth available for users was being cannibalized by WiFi management frames. This was particularly true in the 2.4 GHz band, where slow beacons transmitted by every radio, every 100 ms could consume the lion’s share of the channel in a kind of death of a thousand cuts. So, in this case, pretty heat map; bad WiFi.

Dense WiFi networking depends on lots of APs running at low power, near their clients. The WiFi infrastructure can play many roles in encouraging clients to choose AP associations wisely. And the latest current WiFi standard, 802.11ax, provides extra features for managing overlapped APs. WiFi has advanced to reach blazing speeds and high densities in the last fifteen years, but the heat map doesn’t tell you much more than it did back then.

• Kevin Dowd

BSS Coloring in WiFi 6

If you’ve ever used a two-way radio or walkie-talkie, you’ve probably had the experience where the person you’re listening to gets “stepped on” by somebody else’s transmission. You may have also noticed times when the person you’re listening to “steps on” someone else’s transmission, overpowering it.

WiFi networks have long dealt with the same issues, avoiding the “stepped on” transmission by performing channel assessments and signaling intent to use a channel. This coordination and cooperation even happens between WiFi networks that otherwise have no connection to one another. If the barber shop runs an AP on channel 53 and the tire store also runs on channel 53, the two are going to share the channel. The tire store AP has its clients; the barber ship AP has its clients. Their access points and clients will each listen for the transmissions on channel 53, yielding access if the power of any transmission is above a modest -82 dbm.

In each case, the AP and its clients form a Basic Service Set, or BSS. To make better use of shared channels, WiFi 6 (802.11ax), introduces the notion of BSS coloring. The ‘color’ is a small integer in the transmission preamble. For the sake of our discussion, lets say that the integers actually correspond to colors; the BSS of the AP and all of the WiFi clients in the barber shop is blue; the tire store BSS is red. BSS color makes it immediately possible for all WiFi devices to tell whether a transmission is meant for the tire store or the barber shop.

BSS coloring facilitates “stepping on” another BSS’s traffic. If the AP in the tire store (red) wishes to transmit while a device in the barber shop is talking (blue), it can make a decision to broadcast over the ongoing transmission, even if the power is as high as -62 dbm. The reason this will work is that each BSS and its clients are in proximity to one another and the interference caused by its neighbors is dynamically judged to be low enough to permit the simultaneous transmission to succeed. One channel; two transmissions.

The benefit of BSS Coloring is that we can build denser WiFi networks with more channel overlap. BSS Coloring is one of the powerful new capabilities in WiFi 6.

• Kevin Dowd