IPERF; Packet loss over adjacent wifi channels

Question

I have two iperf clients and servers. Both on 5GHz. Both are on 20MHz channel. One iperf client is sending data over channel 36 and the other one is sending data over channel 40. While sending data on both channels simultaneously there is a huge loss while there is almost no loss when we send data through either of the iperf clients. What are the possible reasons for this scenario? And how to correct them?

Note: There are no other wifi devices working on these channels. Experiment is set up in a room so range wont be an issue. UDP is being used.

iperf server command: iperf -s -u -w 320k

iperf client command: iperf -c 192.168.XX.XXX -u -w 320k -t 10 -i 1 -b 300M

Answer 1

There will be packet drops if rts/cts is on. 1st access point will keep on sending data while 2nd access point waits for the 1st to send all data. 2nd will drop packets as it is waiting to send data. Although the channels are different but access points will detect channel interference.

You should consider having channel 36 and 44 to solve the problem.

There may be other reasons for these.

Answer 2

In theory, CH36 (5180 MHz) shouldn't interfere with CH40 (5200 MHz) if you are using 20MHz channel in both APs. So interference shouldn't be a reason for your iPerf results.

Also, the RTS/CTS option can usually be turned off by most APs (through Web admin page). It shouldn't affect iPerf results as well because RTS/CTS is normally used to prevent collisions for small frames - whereas iPerf normally sends large frames.

It's not entirely clear about the network topology you are testing. Are the two APs completely independent or connected? Where are the iPerf servers - are they connected over WiFi as well or via Eth?

Finally I doubt the "-w" option you are using. You are testing UDP, but "-w" is for specifying TCP windows size.