Airtime Fairness is a control you might come across buried deep in the Advanced Wireless settings of your router or, more likely, not. Here's how TR-398 introduces the test:
Wi-Fi signal transmission can be seen as a multicast process since the STAs involved share the transmission medium. Air interface becomes a rare resource when dense connections or high throughput requests exist. Channel condition determines the MCS selection, therefore affecting the data throughput. In general, long distance to travel or obstacle penetration leads to larger attenuation, which makes the data rate in a low level. Occupying excessive air time of STA with small MCS will be unfair to the STAs with large MCS (here, assuming the QoS requirement is similar) when the air resources have already run out.
Written like only a committee could... To state it more clearly, Airtime Fairness generally describes techniques that try to ensure that slower devices don't slow down faster devices. Implementations vary, but the general idea is to adjust an AP's airtime scheduler to give more airtime to faster devices. This is illustrated below.
Airtime Fairness explained
(graphic courtesy of TP-Link)
TR-398's Airtime Fairness test (6.2.3) runs three scenarios shown below.
Test 1 has two two-stream STAs at equal strong-signal distance. Test 2 moves the second STA to a "medium" distance that BBF equates to adding 38 dB path loss in 2.4 GHz and 32 dB in 5 GHz. The STAs are configured as 802.11n devices for 2.4 GHz and 11ac devices for 5 GHz for these tests.
Test 3 puts both STAs back at strong signal distance, but changes the second STA to a single-stream "legacy" device (802.11g for 2.4 GHz, 802.11a for 5 GHz). Although TR-398 specs the "legacy" device as 802.11a/b/g, I configured the octoScope Pal for 11g for the 2.4 GHz test.
Each test runs traffic simultaneously to both STAs and measures throughput to each. As is TR-398's standard method, tests are run separately on both bands.
Airtime Fairness test
The Airtime Fairness tests use Variation and Total throughput benchmarks to judge performance. Basically, the Variation tests check how far the STA1 reference STA's throughput varies from the ideal case of two equal-capability STAs. The reference STA's throughput is allowed to change +/- 5% compared to the mean of the Test 1 throughput for both devices, i.e. Mean (STA2_throughput_1, STA1_throughput_1). When the STA is "moved" (Test 2) or when a "legacy" STA is used (Test 3), STA1's throughput can change only +/- 15% in 2.4 GHz and +/- 25% in 5 GHz .
The Total Throughput tests check throughput totals against set throughput limits of 80, 54 and 50 Mbps for 2.4 GHz and 475, 280 and 230 Mbps for 5 GHz for tests 1,2 and 3, respectively.
The 2.4 GHz results in Table 2 show the RAX80 failing both 5% limit tests and each product failing one or more of the 15% limit tests.
|Linksys LAPAC1200||NETGEAR R7800||NETGEAR RAX80|
|5% limits||49.8 | 55.1||53.7 | 59.3||56.9 | 62.9|
|15% limits||44.6 | 60.3||48 | 65||50.9 | 68.9|
|Variation (Mbps)||STA1_throughput_1||Pass [53.7]||Pass [56.5]||Fail [55.7]|
|STA2_throughput_1||Pass [51.2]||Pass [56.5]||Fail [64.2]|
|STA1_throughput_2||Fail [86.2]||Pass [57.7]||Fail [110.2]|
|STA1_throughput_3||Fail ||Fail [80.7]||Pass [51.8]|
|Pass [104.9]||Pass ||Pass [119.9]|
|Pass [97.1]||Pass [113.5]||Pass [117.47]|
|Pass [96.2]||Pass [87.23]||Pass [63.8]|
Table 2: Airtime Fairness Test result summary - 2.4 GHz
I used octoScope's Expert Analysis tool to generate some plots to gain insight into the failures. Sharp-eyed readers will notice the 40 second test time instead of the 120 seconds TR-398 specifies for most of its tests. I've been using shorter test times while developing the scripts to speed things along.
The Test 1 plot below shows a wider gap between the RAX80's two throughput lines, which accounts for its failures on the first two +/- 5% limit tests. Note these are borderline failures and +/- 5% variation is a pretty tight given the nature of the Wi-Fi beast.
Airtime Fairness - 2.4 GHz "Test 1" reference
The 5 GHz results in Table 3 show all three products failed the 25% variation test limits when comparing the "legacy" STA's throughput to the mean of STA1 and 2's throughputs. The NETGEAR RAX80 also fails the total throughput test for the total of STA1 and "legacy" STA 3...by a lot.
|Linksys LAPAC1200||NETGEAR R7800||NETGEAR RAX80|
|5% limits||271.2 | 299.8||319.6 | 353.3||299 | 330.4|
|25% limits||214.1 | 356.9||252.3 | 420.6||236 | 393.4|
|Variation (Mbps)||STA1_throughput_1||Pass [289.9]||Pass [333.7]||Pass [309.4]|
|STA2_throughput_1||Pass [281.1]||Pass [339.2]||Pass [320.0]|
|STA1_throughput_2||Pass [331.4]||Pass [333.6]||Pass |
|STA1_throughput_3||Fail [384.3]||Fail [539.67]||Fail [16.7]|
|Pass ||Pass [673.9]||Pass [629.4]|
|Pass [534.5]||Pass [675.6]||Pass |
|Pass [391.4]||Pass [544.7]||Fail [37.9]|
Table 3: Airtime Fairness Test result summary - 5 GHz
Here's the STA1 and 2 5 GHz reference plot. All three products have a good amount of variation, but not enough to fail the 5% limit tests.
Airtime Fairness - 5 GHz "Test 1" reference
This plot shows the three products for the 2.4 GHz "legacy" test run. "Pair 2" is the "legacy" STA in each run and it appears the faster STAs are not being held back by the slower ones.
Airtime Fairness - 2.4 GHz reference + "legacy"
The 5 GHz "legacy" plot is different. The Linksys and NETGEAR R7800 properly allocate more bandwidth to the faster 802.11ac STAs, but the NETGEAR RAX80 does not. Both the 11ac and 11a STAs are down in the mud. This is why the RAX80 fails the STA1_throughput_3 variation and STA1_throughput_3+ STA3_throughput_3 total throughput tests.
Airtime Fairness - 5 GHz reference + "legacy"
One flaw I found in this benchmark is there are no tests that verify that the second station in Tests 2 and 3 has non-zero throughput. This is not captured in the total throughput tests. During test debug, I found the second STA in these tests could become disassociated or have essentially 0 throughput and the test could still pass.