Adjacent Wifi Channel Throughput Testing

What is the effect of wifi clients on adjacent channels when performing over-the-air wifi throughput testing?

We decided to investigate this question to help show why it happens and also to determine if the physical proximity of the wifi NICs could also be to blame.

After running the tests described below, we found that co-channel interference happens with Access Points using adjacent channels and with LANforge using physically adjacent wifi NICs. LANforge test results show better on-air throughput performance when the PCI-E NICs are physically separated by different systems rather than having the wifi NICs side by side in a single platform.

The test setup consists of two LANforge systems, a CT523c and a CT523b and two identical test APs. The CT523c, labeled LF1, has two 4×4 wifi NICs with clients wlan0 and wlan2. The CT523b, labeled LF2, has two 4×4 wifi NICs, but only one client, wlan2, is in use. The test APs are each setup with a single unique SSID using open authentication and are physically placed about 1m from each LANforge system. Each AP is also using its specified channel at 80MHz channel width.

Each test iteration is a 30 second download test, then 30 second upload test for each individual wifi client and then again for combinations of clients for download and upload.

First test: AP-A on ch100 and AP-B on ch116 – here is what it looked like on inSSIDer:

The baseline results of each individual client show that the test setup has ideal conditions with all clients reporting similar results:

The combination tests show that the clients’ throughput results suffer from the adjacent channel interference:

If we look at a spectrum analyzer while testing, we can see that the overlap at 5.57GHz between channels 112 and 116 is the reason for the interference.

The next test is a repeat of the first but on channels 36 and 52 which are also adjacent to each other.

Test 2: AP-A on ch36 and AP-B on ch52

Test 2 results:

As we move AP-B over to channels further away from AP-A, the combination tests show that the clients experience less interference and therefore show better throughput results:

Test 3: AP-A on ch36 and AP-B on ch100

Test 3 results:

Test 4: AP-A on ch36 and AP-B on ch149

Test 4 results:

Another important conclusion is that the throughput results for a single active NIC in each LANforge system is better than having both NICs active in a single LANforge system. We can show the difference by comparing the total on-air throughput.

In Test 1, channels 100 and 116:
The all-in-one on-air download throughput is 371Mbps + 674Mbps = 1045Mbps.
The separated on-air download throughput is 467Mbps + 963Mbps = 1430Mbps.
1430/1045 = 1.37 (separating the NICs shows a 37% increase).

In Test 2, channels 36 and 52:
The all-in-one on-air download throughput is 351Mbps + 747Mbps = 1098Mbps.
The separated on-air download throughput is 487Mbps = 974Mbps = 1461Mbps.
1461/1098 = 1.33 (separating the NICs shows a 33% increase).

In Test 3, channels 36 and 100:
The all-in-one on-air download throughput is 948Mbps + 966Mbps = 1914Mbps.
The separated on-air download throughput is 982Mbps + 974Mbps = 1956Mbps.
1956/1914 = 1.02 (separating the NICs shows a 2% increase).

In Test 4, channels 36 and 149:
The all-in-one on-air download throughput is 982Mbps + 980Mbps = 1962Mbps.
The separated on-air download throughput is 981Mbps + 967Mbps = 1948Mbps.
1962/1948 = 1.007 (separating the NICs shows a 0.7% decrease).

We hope that this post helps remind you that wifi is subject to RF interference from a variety of sources including active clients on a neighboring channel as well as the active wifi NIC in the next PCI-E slot.

If you would like more info on using LANforge for wireless testing, please contact us at or call +1-360-380-1618.