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Few APs with Broadcom chipset on 2.4GHz support 256 QAM. Please any one can advice me how 256 QAM will help to get more throughput/datarate on 2.4GHz.Do we need special client to get benefit of 256QAM on 2.4GHz.

Are these2.4GHZ  chipset on 11n or 11ac ? If they are on 11n  then how 256 QAM supported by them?

http://www.broadcom.com/products/Wireless-LAN/802.11-Wireless-LAN-Solutions/BCM4335

Looks like 100% proprietary:

'

• Broadcom's TurboQAM™ technology implements the highest data-rate 256-QAM mode in 2.4 GHz and enables devices equipped with the BCM4335 to deliver 10 percent faster throughputs than 802.11n speeds when communicating with other 5G WiFi devices.

'

So, between radio devices both based on this chip in particular.

Definitely proprietary.    And only single stream - it's really meant for small devices, and definitely not AP's.

Several manufacturers have had "turbo" modes over the years, but as far as I know their impact was minimal.   I even remember D-Link having a turbo mode at one time - until the WFA made them remove it as not being "certified".

It's not going to buy you much in 2.4 GHz because of the higher SNR requirements of 256-QAM.   The wider 40 MHz channels will also make you less popular in the neighborhood.

I suppose if you are really close to the AP you might enjoy a 10 % increase in speed.   I wonder if it honors the 40-MHz Intolerant bits from another AP.   If it does, you may never see the 40 MHz improvements anyway.

I don't remember if the WFA 802.11n/ac certifications require the 40 MHz Intolerant feature to be followed.   I'll have to check.

Update. The WFA does check for proper behavior with 802.11n, should a device see another device's 40 MHz Intolerant flag enabled.  That is, it enforces 20 MHz wide transmissions.   Naturally it does not require another device to set that flag to begin with.   There is no 802.11ac intolerant test.

BTW, you might think that the WFA tests against all of the items in the IEEE standard, but there are many cases that are ignored.  Despite the claim of testing to the standard, the main purpose of their tests is to insure inter-operability - not necessarily adherence to the standard.

Thanks Howard.

So this Broadcom chipset is 11ac on 2.4GHz  and supports 256 QAM modulation (100% proprietary)…..As 11ac doesn’t support “40 MHz Intolerant bit” concept so that clients can enjoy 40MHz data rate in 2.4GHz.

Please correct me if I am wrong.

By definition 802.11ac is only on 5 GHz.   Probably the most you can say is that it's OFDM.   Closer to 802.11n.

It may or may not support 40 MHz intolerant signalling, but I am betting it will.

I would expect that we won't see many devices capable of these rates.  

I think the key thing to remember about this chipset is that it is everything wireless:

• 5 GHz 802.11ac
• 2.4 GHz 802.11n standard
• 2.4 GHz 256-QAM modulation in standard channel widths (I'm not sure if it allows 40 or more than 40 in 2.4, the basic pages is vague)
• FM receiver
• Bluetooth

When you see all this integrated, think consumer devices or non-consumer devices without full chip utilization.

TurboQAM - like Howard said - is reminiscent of the old days with Turbo-G, and the Buffalo Wireless 125 High Speed Mode and others.

Now, I think it's fair to say that the 256 QAM implemented may be "standard", it's just implemented in the non-standard band. Therefore, if it is according to standard, technically other vendors could match it as there's no secret sauce. But, I wouldn't hold my breath for anything that's not IEEE and WFA.

Tom