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.On Nov 23, 2015, at 6:19 PM, Chuck Lukaszewski [mailto:clukaszewski@arubanetworks.com] wrote:
Great to see all the work on this. Peter, love your attention to detail. Keep it coming!
I have run many many of these spatial reuse type tests. Both of Peterās test runs clearly suggest that his cells are too isolated from one another. The combined goodput should be marginally higher than a single cell. Instead, heās almost getting full reuse in both V1 and V2 test reports.
Rickās results are more consistent with what we see here when the cells are properly separated (e.g. just inside one anotherās preamble ranges). BTW ā a quick way to test this is to see whether STA2 can pass traffic to AP1, and vice versa. If the cells are truly overlapping then this will be possible at MCS0. If you canāt pass traffic then the cells are independent collision domains.
Please note that there are some subtle but important differences between Peter and Rickās test designs at the radio level.
– Peterās choice of channel 44 limits significantly limits EIRP, might be better to test on Ch 108 or 108+. Suggest not to use a channel on the band edge as for most vendors these are slightly reduced EIRP from inside channels due to spurious emissions issues.
– By comparison, Rickās test uses 2.4GHz and HT20. since his company is USA based I assume this is with full 36dBm allowed EIRP (or whatever the tested devices are capable of). As opposed to Peter using HT40 with ETSI limit of 23dBm.
– Now you could say this doesnāt matter since its RX power on far side that matters for this test. I nominally agree. I think the key difference is the HT20 vs. HT40 in these two tests. But it is possible that reduced launch power is resulting in different channel fading in the two tests.
Also, I note that there is some difference between Peterās two test runs.
– V2 shows -50 and -61 respectively inside each cell, whereas V1 shows -56 and -50. So V2 has a 10dB AP-STA SNR delta as compared with V1 which has a 6dB delta. This likely explains the reduced goodput in the AP2 cell.
Some general observations to questions raised over the last few days.
ā¢ 40 vs 20.
o Yes this will make a difference for a test of this type due to the discussion we are having on the other thread about reduced SNRs.
o Peterās configuration is right at the limit of coverage for HT40 ā here is the table from 7131 data sheet (attached). Itās very possible that the total goodput being measured is additive because the cells are outside one anotherās collision radius.
o HT20 improves sensitivity to -93. Suggest if anyone is going to put more time into this to rerun with 20, or alternatively close up the physical distance to improve SINRs by 3-4dB for good measure.
o Alternatively, move to channel 100 and go to max EIRP. Might buy you another 6dB.
ā¢ Up vs. down
o Definitely a different test.
o This one will be tricky because the 4965 probably has lower EIRP than the AP.
o Best way to test this is fire up a soft AP on the two laptops and check RSSI to see if there are any big RX power deltas.
o 4965 RX sensitivity may also be worse than the 7131. Iām not aware that the Intel specs are public, so not sure what these values are.
ā¢ Separate iperf servers
o I think this is a NOOP at the throughput levels we are talking about here (11n 2SS)
o However, given that Peter is using distance and structural loss to achieve the signal levels, itās probably simpler from a cabling perspective to have 2 servers.
o We generally used to use a single IxChariot server for this type of test, but in my lab building I have a common cabling so itās easy.
o I will say that for 11ac 3SS VHT80 testing we have gone to separate IxChariot wired endpoints since a single cell can generate 850Mbps+ TCP/UDP goodput.
It is expected that data frames will not be decodable on the far side, as their payloads require high SNR. Only the preambles will make it across.
Something else I didnāt talk about in person but is relevant. With 11n HT, a lot of vendors do not do RTS/CTS. Not sure about these specific products. So depending on exact timing of when BSS1 sends preamble for a data frame, BSS2 could miss it if it is already TXing. Itās quite possible to see a slight increase in 2 BSS test as compared with 1 BSS test for this reason, especially when operating at low SINR levels which then allow each BSS to get through.
If you enable RTS/CTS with 11n, or if you test with 11ac, you will get a much clearer result. The RTS at 6Mbps rate will fully clear both BSS at both L1 (L-SIG) and L2 (NAV) levels.
-cl
On Nov 29, 2015, at 5:23 PM, Chuck Lukaszewski
Been trying to locate something, finally found it staring me in the face.
Figure 5.8 from the Perahia book that Devin sent around captures this nicely. Look at the line with the ā*ā for 20MHz MCS0.
At 4 dB SNR, MCS0 will produce a packet error 15% of the time. To drop the PER to ~1% which is the usual target for a given modulation you have to be at 7dB.
