The Death of the Heat Map

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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.

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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:

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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

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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.

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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

Fun with DFS

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We have a customer who complained that every day, just before noon, users would lose their WiFi. The customer was located on a flight path for a nearby airport and military installation. As it turned out, an interim wireless firmware release changed their 5 GHz channel plan to include some Dynamic Frequency Selection (DFS) channels. These DFS channels are shared with aviation and weather radar with the proviso that if an access point detects radar on the same 5 GHz channel it serves, then it must abandon the channel. So for this customer, every day at the same time, an overhead flight knocked out part of their WiFi!

Of the twenty-two, 20 MHz, 5 GHz WiFi channels in the United States, thirteen of them are DFS channels. Because DFS channels are subject to abandonment, WiFi equipment ships with DFS channels disabled. Most WiFi systems apportion the remaining nine non-DFS channels between access points with limited contention, but there are situations where the DFS channels can solve big problems.

For example, we have a scholastic customer with some lightly built dormitories of wood-frame and gypsum. The dorms are loaded with APs. Additionally, the dorms are situated in an open space with many scholastic buildings and green space-WiFi around them. Standing next the dormitories, one can ‘hear’ forty radios. The air is busy! Adding more APs offers diminishing returns as the infrastructure competes with itself for channel access; the nine 5GHz channels are oversubscribed.

In another case, a customer had some new LED lighting installed over the summer. In the fall, they complained WiFi was intermittent on the 5 GHz band. We took measurements. The new lighting appeared to be blowing raspberries on the unlicensed 5 GHz spectrum; whatever the communication method, the LED lighting wasn’t speaking 802.11 protocols, so the WiFi infrastructure couldn’t work with it. The result was a bad WiFi experience.

In both cases, what we did was turn up some DFS channels. In the second case, we also turned down non-DFS channels. Here’s the method for enabling DFS: enable a few channels at a time. Doing it in pairs makes sense. Choose channels that can be bonded for 40 MHz. Then, watch the logs for a day or so. Look for channel abandonment events. If you don’t see any, move on to the next batch of channels. In the end, you will have an expanded collection of 5 GHz channels for your area and more available WiFi bandwidth.

We’re cowboys, here, by the way. In our office, all we use are DFS channels. So far, no deleterious effects!

-Kevin Dowd

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