Sunday, September 27, 2015

The Limitations of Channel 144



Channel 144 (5.720 GHz) is a 20 MHz channel that is a part of the UNII-2e band.   It was added in March 2014 specifically as part of the official 802.11ac specification, so that the new Wi-Fi amendment would support an additional 80 MHz channel of 132-144 (see the figure above).   

While this additional 20 MHz channel is supported now in the 5 GHz band for Wi-Fi use, most legacy 802.11a and 802.11n clients don't actually support Channel 144 in their firmware.  These clients will support the 20 MHz channels 132, 136, 140 and (for 802.11n 5 GHz) the 40 MHz channel 132-136.   

Wi-Fi uses extended channels in order to be backwards-compatible to older generations.  One of the 20 MHz channels is defined as the primary.   The choices are upper and lower.   An upper extension uses the lowest channel in the primary and extends upwards to the neighboring channels.  Hence, for the 80 MHz channel 132-144, upper extension will use channel 132-136 for 40 MHz clients and channel 132 for 20 MHz clients.   By comparison, lower extension uses the highest channel as the primary and extends downwards, and thus would use 140-144 for 40 MHz clients and channel 144 for 20 MHz.   

For 80 MHz channels, technically the intermediate 20 MHz channels could be selected as the primary.  I have recommended to clients the following nomenclature (using Channel 132-144 as an example):
  • Upper / Upper Extension:  Channel 132 (20 MHz), Channel 132-136 (40 MHz)
  • Upper / Lower Extension:  Channel 136 (20 MHz), Channel 132-136 (40 MHz)
  • Lower / Upper Extension:  Channel 140 (20 MHz), Channel 140-144 (40 MHz)
  • Lower /Lower Extension:  Channel 144 (20 MHz), Channel 140-144 (40 MHz)
The terminology shall likely get even more confusing if we ever start using 160 MHz channels.

While there aren't a lot of legacy 802.11a clients, 802.11ac currently only has about 30% market penetration for 5 GHz client devices.  Hence, 40 MHz clients will be around for quite some time, and these clients will probably not support Channel 144.
  
What this means in practice is that you can ONLY use channel 144 as part of an 80 MHz channel, and must use the Upper Extension (or Upper / Upper or Upper / Lower), so that 40 MHz and 20 MHz legacy clients would avoid the use of Channel 144.

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