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AP3915i sending packets at MCS9 with two spatial streams in 20MHz

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TitleAP3915i sending packets at MCS9 with two spatial streams in 20MHz
Symptoms
AP3915i sending packets at MCS9 with two spatial streams in 20MHz.
Environment
  • ExtremeWireless appliances.
  • ExtremeWireless Access Points AP3915.
  • Firmware 10.41.xx, 10.51.xx.
Cause
There are 10 excluded mcs rates in 11ac:
Invalid 802.11ac MCS values
20 MHz 80 MHz 160 MHz
MCS 6 n/a 3, 7 SS n/a
MCS 9 1,2,4,5,7,8 SS 6 SS 3 SS

For the 802.11ac transmitter data flow, the number of coded bits per subcarrier must be an integer value for each BCC encoder, otherwise the MCS is "invalid". This limitation is added only for BCC encoder, but LDPC has the same MCS exclusions to make the set of allowable data rates equivalent between BCC and LDPC in 11ac standard even LDPC actually does not have this inflexibility for coded bits. The reason is LDPC is optional in 11ac. and BCC is mandatory.

For LDPC only encoder like 11ax, it has no such limitation and it has zero invalid MCS values (LDPC is pretty much mandatory in 11ax).

QCA 11ac implementation chooses to exclude above invalid MCS values only if ldpc is NOT enabled for the radio. If ldpc is enabled, all MCS values are supported by radio.

If the client is OK for these rates, then LDPC enabled on the radio will allow these MCS rates.

You can turn off these rates by disabling LDPC, then these rates will not be used in transmission.
Resolution
  • Interpretation in the 802.11ac specification.
  • If end clients are having problems, please disable LDPC.
Additional notes

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