Here we have a “Saturday Night Test!”
We were talking about 4 channel plan with OFDM modulation in 2,4Ghz band and trying to understand whether that is a reality or not.
We all know that the non-overlapping channels in 2,4Ghz band are 1,6 and 11; right?
However, if the numbering is consistent and correct between 2,4Ghz and 5Ghz, there are 4 channel-numbers between non-overlapping 5G channels but why is there 5 channel-numbers between non-overlapping 2,4Ghz channels?
For example, you have channel 36 and plus 4, you have channel 40 and this is non-overlapping with each other. So, if that is OFDM and we also have OFDM in 2,4Ghz; then the same should apply right?
So, if we have ch1 and plus4, the channel 5 should NOT overlap with the channel 1, if we use the exact the same channel spacing math between 2,4Ghz and 5Ghz.
So, what is the reality here?
Channel 1 and Channel 5 (like ch36 and ch40). Are they non-overlapping or no?
Let’s test this.
We should be careful, because there should not be any DSSS transmissions there (due to 11b client Tx or due to RTS/CTS to “protect” them because DSSS ruins this test as this is 22Mhz wide, not 20Mhz wide like OFDM Tx)
So, to be sure about zero DSSS Tx around, I removed all DSSS rates and started with 12Mbps as the basic rate, Tx rate and beacon rate. Yes there could be DSSS Tx in the area that I run these tests but I checked the air and saw that no STA around was doing DSSS.
So, here is my ultra basic test setup:
One client downloads from an AP, another client downloads from another AP, all are in the same RF domain, meaning they do hear each other very well.
So, in this case, if AP1 and AP2 both are on the same channel, that is called CCI, right? So, they share the air.
If they are on ch1 and ch11, that means they don’t contend for the medium, so they can use their “own” air bandwidth, right?
If they are on ch1 and ch6, and if they are placed apart from each other (o couple of meters) they won’t need to go into contention again, so they can again use their own air, right? –because ch1 and ch6 do not overlap, we all know this very well.
What if ch1 and ch5? With a couple of meters away from each other, will they contend for the medium?
Let’s test this with pure OFDM Tx.
With the above test scenario, I run these tests
- iPerf between clients, AP1 is on ch11, AP2 is on ch1
- iPerf between clients, AP1 is on ch6, AP2 is on ch1
- iPerf between clients, AP1 is on ch5, AP2 is on ch1
- iPerf between clients, AP1 is on ch4, AP2 is on ch1
- iPerf between clients, AP1 is on ch3, AP2 is on ch1
- iPerf between clients, AP1 is on ch2, AP2 is on ch1
- iPerf between clients, AP1 is on ch1, AP2 is on ch1
(Clients were 2×2 11ac clients, connected to the 2,4Ghz 11n radios, 20Mhz channel, with MCS15, 144Mbps rate, SGI, 64 QAM 5/6)
Here are the results:
So, we can clearly see that the non-overlapping channels give the best overall throughput between clients.
For example, if the AP1 is on Ch11 and the AP2 is on Ch1, then the average throughput between clients are (completely non-controlled environment, with lots of interfering APs and Clients around) 85,64Mbps.
When we see the result between clients, when the AP1 is on Ch6 and AP2 is on Ch1, that’s another non-overlapping scenario. The result is 78,96Mbps between clients.
However, we can clearly see that, Ch5 and Ch1 does also NOT overlap with the results similar to the other scenarios, like 11-1 and 6-1.
Let’s get closer to the AP2 Ch1. When we configure AP1 in Ch4, things start to get bad with around %50 throughput loss, due to contention between APs.
That’s why Ch4-Ch1 contention drops the performance to 41Mbps and it stays the same even with Ch1-Ch1 CCI contention, in which the performance is around 42Mbps.
Here is the spectrum output while the tests were running:
The above part is the ch6 – ch1 test, and the below is where ch6 stays:
So, the channel 6 with OFDM modulation is there, starts with 2427Mhz and ends with 2447Mhz.
..but where is channel 5? Here it is:
Channel 5 starts with 2422Mhz. Does it interfere with channel 1. Where does channel 1 ends? Any overlap. No, they don’t overlap with OFDM, as you can see from the above waterfall output.
Here is the channel 1 and it ends at the exact point that ch5 starts, like 5Ghz OFDM channels.
So, ch1 ends at 2422Mhz and ch5 starts at 2422Mhz, so they DON’T overlap and we can prove this via simple throughput runs between clients with ch1-ch6 pairs vs ch1-ch5 pairs.