AC at 2.4 GHz?

[rotareneg]

Not sure if this is a bug in the wifi drivers (which I know are a closed box) or maybe just an UI glitch, but my phone, a Moto G Pure, gets reported by the router as connecting at AC rates on 2.4 GHz. Router is an RT-AC66U B1 running Merlin 386.5_2.

SSID: "xx_2.4GHz"
noise: -89 dBm    Channel: 6
BSSID: 04:D9:xx:xx:xx:xx    Capability: ESS ShortPre ShortSlot RRM
Supported Rates: [ 1(b) 2(b) 5.5(b) 6 9 11(b) 12 18 24 36 48 54 ]
VHT Capable:
    Chanspec: 2.4GHz channel 6 20MHz (0x1006)
    Primary channel: 6
    HT Capabilities:
    Supported HT MCS : 0-23
    VHT Capabilities:
    Supported VHT (tx) Rates:
        NSS: 1 MCS: 0-9
        NSS: 2 MCS: 0-9
        NSS: 3 MCS: 0-9
    Supported VHT (rx) Rates:
        NSS: 1 MCS: 0-9
        NSS: 2 MCS: 0-9
        NSS: 3 MCS: 0-9

Interference Level: Acceptable
Mode    : AP Only

Stations List                          
----------------------------------------
idx MAC               Associated Authorized   RSSI PHY PSM SGI STBC Tx rate Rx rate Connect Time
    D8:CF:xx:xx:xx:xx Yes        Yes        -38dBm ac  No  Yes Yes    86.7M   86.7M     00:32:53
    64:89:xx:xx:xx:xx Yes        Yes        -46dBm n   Yes Yes Yes       1M      1M     00:33:54

SSID: "xx_5GHz"
noise: -92 dBm    Channel: 149/80
BSSID: 04:D9:xx:xx:xx:xx    Capability: ESS RRM
Supported Rates: [ 6(b) 9 12(b) 18 24(b) 36 48 54 ]
VHT Capable:
    Chanspec: 5GHz channel 155 80MHz (0xe09b)
    Primary channel: 149
    HT Capabilities:
    Supported HT MCS : 0-23
    VHT Capabilities:
    Supported VHT (tx) Rates:
        NSS: 1 MCS: 0-9
        NSS: 2 MCS: 0-9
        NSS: 3 MCS: 0-9
    Supported VHT (rx) Rates:
        NSS: 1 MCS: 0-9
        NSS: 2 MCS: 0-9
        NSS: 3 MCS: 0-9

Interference Level: Acceptable
Mode    : AP Only

Stations List                          
----------------------------------------
idx MAC               Associated Authorized   RSSI PHY PSM SGI STBC Tx rate Rx rate Connect Time

I don't normally use 2.4 GHz, I was just checking something when I noticed this happening and was a bit confused since, as far as I know, AC is 5 GHz only.

 

[rotareneg]

Ah, I think I figured it out: TurboQAM was enabled on the 2.4 GHz Wireless - Professional settings on the router, so apparently the phone can use that on 2.4 GHz while my tablet, a Galaxy Tab A, SM-T510, doesn't.

 

[Yota]

Ah, I think I figured it out: TurboQAM was enabled on the 2.4 GHz Wireless - Professional settings on the router, so apparently the phone can use that on 2.4 GHz while my tablet, a Galaxy Tab A, SM-T510, doesn't.

I never really understood why TurboQAM existed, and I never saw it used in a phone/tablet. I do have wireless adapters from Asus and they support 256-QAM, but why don't I just use the faster 5 GHz?

TurboQAM is the introduction of 256-QAM of 5 GHz into 2.4 GHz, which means that any device that supports TurboQAM must support 5 GHz, but if it supports 5 GHz, why not use the 433 Mbps that 5 GHz can provide?

256-QAM means having to be very close to the router, so there is no 5 GHz signal attenuated to unusable within the range of 256-QAM.

Another point is that the BCM4360 radio chip used by the RT-AC66U_B1 supports 256-QAM, but in order to avoid the same rate as the more powerful RT-AC68U (AC1900), they have 256-QAM disabled (AC1750). So I guess their new version of the driver has lifted this restriction, so RT-AC66U_B1 is also an AC1900-level router now?

 

[Crimliar]

Okay, so maybe I need a primer on this! Previously a couple of Motorola phones here an E4 Plus and a G8 power (both now replaced) used to connect "AC" on 2.4GHz, along with a couple of Reolink Argus 3 (much further away) cams that would occasionally connect as AC but mostly as "N" clients.
I thought QAM had to do with a crafty way of encoding data in a pair of waves that 90 degrees out of phase, so I'm missing how this would only work on a 2.4GHz if the bandwidth is 40MHz? I was under the impression that the two waves are on the same frequency just phase-shifted by 90 degrees?

 

[rotareneg]

N normally uses up to 64-QAM, 256-QAM just uses smaller differences in phase and amplitude, meaning it's more sensitive to interference but can squeeze a bit more data into the same bandwidth.