One thing New: AP Discovery Strategies for 6GHz Wi-Fi – Half 2

Half 2 of the 4-part Wi-Fi 6E Sequence

In Half 1 (One thing Outdated) we checked out fundamental modifications to the bodily layer supplied by wave 1 of 801.11ax, how these modifications can have an effect on efficiency, and the way OFDMA permits the optimum use of the 6GHz spectrum. On this second article, we’ll discover “one thing new:” the challenges of discovery in 6GHz, new strategies used for fixing this, and the way these new strategies open 6GHz for a lot of completely different use circumstances.

Is There Anyone Out There?

In earlier generations, Wi-Fi purchasers would scan channels and ship unsolicited probe requests to find entry factors (APs). Scanning channels could be a well timed course of as beacons are solely broadcast each 1024us so the shopper should dwell lengthy sufficient to detect the beacon.  At 6GHz that is 1024us x 59 channels (there are 59 20MHz channels within the new 6GHz spectrum) which is over 6 seconds.  For the shopper, this loss in time represents a disruption in communication. Creating insupportable latency in voice and misplaced alternative to a whole bunch of megabytes of information each time the shopper decides to scan. Moreover, the earlier course of can be to ship unsolicited probe requests (wildcard requests) to see how APs would reply. Now, bear in mind, that is all a contention-based medium, so these probe requests and responses on each channel for each shopper create a major quantity of interference and on the very least, inefficient use of the spectrum.

Through the years the IEEE has launched measures to handle these roaming challenges. 802.11k was launched to supply purchasers with a listing of neighboring APs, 802.11v was launched to supply a advisable AP candidate, and 802.11r was launched to scale back the roaming time for 802.1x purchasers. Not all purchasers and infrastructure assist these measures so whereas they helped, they didn’t get rid of the necessity for purchasers to ship unsolicited probes.

Whereas these IEEE updates are nonetheless accessible for 6GHz, the technique for AP discovery essentially modifications. To begin with, unsolicited probe requests are not allowed (with one restricted exception we’ll talk about shortly).

Three New Strategies to Enhance AP Discovery

Since we now have already established scanning channels at 6GHz is just not allowed, there are three new strategies launched in Wi-Fi 6E for locating AP candidates.

The first technique (and the one which purchasers usually reply to finest) known as Lowered Neighbor Report (RNR). Since most, if not all, purchasers may have legacy band functionality, there may be an Info Ingredient (IE) embedded within the legacy band beacons that checklist the 6GHz SSID(s) which are accessible on the serving AP. The shopper first scans the 5GHz or 2.4GHz channels and appears for this RNR ingredient.  The RNR report comprises details about the 6GHz channel, SSID, BSSID, a bit of data on the AP, and the allowed energy ranges (Energy Spectral Density). This successfully makes the two.4GHz and 5GHz channels a management channel for the 6GHz.  Purchasers can then ship a directed probe request to these channels which are realized within the RNR to find out which 6GHz AP to affix.  You will need to notice there might be a number of 6GHz SSIDs included within the RNR and they don’t have to match the legacy SSIDs.

The data contained in an RNR is similar to the knowledge supplied within the beforehand launched 802.11v [1]motion body. The RNR beneath is from a 5GHz beacon and is promoting two SSIDs on the 6GHz channel quantity 5.  The legacy 802.11v motion report beneath exhibits comparable data to the RNR however the elementary distinction is twofold:

• That is an motion body not a part of the beacon just like the RNR. It’s a request-response kind transaction. An RNR is broadcast within the legacy band beacons.
• The data within the 802.11v motion body comprises details about different APs on the identical frequency band. The RNR solely lists SSIDs broadcasted from the 6GHz band (completely different frequency band) as this identical AP.

What if the AP is barely broadcasting 6GHz? That is an unlikely situation, however nonetheless a possible one.  First, scanning might be lowered by limiting the variety of channels to be scanned.  That is known as Most well-liked Scanning Channels (PSC). The PSCs are the first channels (20MHz subchannel) of the 80MHz channels.  This works nicely since 80MHz will usually be the popular bandwidth to function for causes beforehand mentioned partially 1 of this weblog sequence.  If nonetheless, decrease bandwidth channels are used with out RNR or further assist from the strategies beneath, it could be very straightforward for a shopper to overlook this channel which needs to be a consideration when utilizing PSC with narrower band channels.

There are two mutually unique choices to additional improve the AP discovery wherein the AP will broadcast messages an extra 4 occasions between the beacons or about each 20ms (configurable from 5ms to 25ms). The primary technique known as Quick Preliminary Hyperlink Setup (FILS) and relies on a earlier customary of 802.11ai.  This can be a very light-weight message (someplace round 100 bytes as in comparison with a beacon which is 500+ bytes).  The second technique known as “Broadcast Probe Response” or “Unsolicited Probe Response” (UPR).  Like FILS, this commercial will likely be broadcast at a better price than the beacon.  Nevertheless, the UPR broadcasts every part within the probe response so whereas it provides the shopper with extra data, it’s a bit heavier within the quantity of information transmitted repeatedly.

Teamwork Makes the Discovery Dream Work

So how do these 4 strategies work collectively? First, if there are legacy band SSIDs transmitted on the AP the expectation is that the RNR will do the work of discovering the 6GHz channel, and no different technique is required.  Within the case the place solely 6GHz is broadcast from the AP the most probably situation can be using PSC with both FILS or UPR.  Discover UPR and FILS are unique choices, you’ll be able to solely use one or the opposite. Early testing of shopper units has seen some points with 6GHz standalone APs not being found with solely PSC and it’s wanted to have FILS (or UPR) enabled to help a shopper in discovering the AP.  This may increasingly change over time however for the early implementations, deploying 6GHz with solely 80MHz channels and PSC enabled is an efficient choice.  This enables the first channel to match the PSC channels. As well as, enabling FILS can present additional help for discovery with minimal impression on efficiency.

In Half 3 “One thing Borrowed” we’re going to take a deeper look into the channel construction of 6 GHz, what it has in widespread with legacy bands, what has modified, and what to be careful for.

[1] 802.11v which was outmoded by the roll up of 802.11-2012

[2] IEEE P802.11ax™/D8.0 pp 26.17.2.3.3

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