Tuesday, May 7, 2024

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In Half 1 (One thing Outdated) we checked out fundamental modifications to the bodily layer supplied by wave 1 of 801.11ax, how these modifications can have an effect on efficiency, and the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 allows 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 instances.

Is There Anyone Out There?

In earlier generations, Wi-Fi shoppers 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 consumer 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 consumer, this loss in time represents a disruption in communication. Creating insupportable latency in voice and misplaced alternative to a whole bunch of megabytes of knowledge each time the consumer 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 consumer 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 shoppers with a listing of neighboring APs, 802.11v was launched to supply a advisable AP candidate, and 802.11r was launched to cut back the roaming time for 802.1x shoppers. Not all shoppers and infrastructure help these measures so whereas they helped, they didn’t eradicate the necessity for shoppers to ship unsolicited probes.

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

Three New Strategies to Enhance AP Discovery

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

The first methodology (and the one which shoppers sometimes reply to greatest) known as Diminished Neighbor Report (RNR). Since most, if not all, shoppers could have legacy band functionality, there may be an Data Aspect (IE) embedded within the legacy band beacons that record the 6GHz SSID(s) which are out there on the serving AP. The consumer first scans the 5GHz or 2.4GHz channels and appears for this RNR factor.  The RNR report accommodates 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 discovered within the RNR to find out which 6GHz AP to affix.  You will need to observe there may be a number of 6GHz SSIDs included within the RNR and they don’t have to match the legacy SSIDs.

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

  • That is an motion body not a part of the beacon just like the RNR. It’s a request-response sort transaction. An RNR is broadcast within the legacy band beacons.
  • The knowledge within the 802.11v motion body accommodates 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.
RNR on 5GHz beacon
Determine 1: RNR on 5GHz beacon
802.11v Action Frame
Determine 2: 802.11v Motion Body

What if the AP is barely broadcasting 6GHz? That is an unlikely situation, however nonetheless a possible one.  First, scanning may be diminished by limiting the variety of channels to be scanned.  That is known as Most popular 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 partly 1 of this weblog sequence.  If nevertheless, decrease bandwidth channels are used with out RNR or further help from the strategies under, it could be very simple for a consumer to overlook this channel which ought to be a consideration when utilizing PSC with narrower band channels.

Preferred Scanning Channels (red)[2]
Determine 3: Most popular Scanning Channels (pink)[2]

There are two mutually unique choices to additional improve the AP discovery through which the AP will broadcast messages an extra 4 instances between the beacons or about each 20ms (configurable from 5ms to 25ms). The primary methodology known as Quick Preliminary Hyperlink Setup (FILS) and is predicated 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 methodology known as “Broadcast Probe Response” or “Unsolicited Probe Response” (UPR).  Like FILS, this commercial will probably be broadcast at a better charge than the beacon.  Nevertheless, the UPR broadcasts all the pieces within the probe response so whereas it provides the consumer with extra info, it’s a bit heavier within the quantity of knowledge 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 methodology is required.  Within the case the place solely 6GHz is broadcast from the AP the more than likely state of affairs can be using PSC with both FILS or UPR.  Discover UPR and FILS are unique choices, you may solely use one or the opposite. Early testing of consumer gadgets 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 consumer in discovering the AP.  This may occasionally change over time however for the early implementations, deploying 6GHz with solely 80MHz channels and PSC enabled is an efficient possibility.  This permits the first channel to match the PSC channels. As well as, enabling FILS can present additional help for discovery with minimal influence 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




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