Attenuation – Friend or Foe
I recently attended a CWNA course taught by none other than Devin Akin, wireless guru and co-founder of CWNP. During the course I was reminded about how attenuation can become your best friend when building high density Wi-Fi networks.
During my time working for a WISP years ago we had a particular site that started to run into problems. This site had a six sector Motorola Canopy setup running on the ISM unlicensed 5Ghz band at the top of a hospital building. The site provided 360 degrees of coverage utilizing six 60 degree sector APs which had worked well for quite some time. These APs utilized a proprietary TDMA radio technology and were also GPS sync’d to allow for efficient channel reuse. However, the AP’s had the potential to hear other non Motorola Canopy GPS sync’d 5Ghz devices from any direction. One day the cluster of AP’s started to pick up numerous interference from other competitive WISP deployments in the area using the same 5Ghz band. Signal to noise ratio dropped and so did CPE performance. We came up with a solution to take each individual sector AP off the tripod at the center of the six story building and mount each of the 60 degree sectors (orange cylinders in picture) below the top edge of the outer building walls. Here’s a simple illustration:
This new setup allowed the building to provide attenuation from other 5Ghz interference (blue cylinders in picture). After a spectrum analysis was performed on each AP we verified that interference dropped significantly due to the building attenuation. SNR increased and CPE performance increased.
The Motorola Canopy hardware (now Cambium Networks) protocols in use are not using 802.11 protocols, however they use the same unlicensed frequency band and follow the same principles of RF propagation. In high density Wi-Fi deployments attenuation can become your best friend just like the hospital building became ours once we relocated the sector APs. Attenuation such as walls, wall thickness, and number of walls RF propagates through can help reduce co-channel interference between access points AND clients that are reusing the same channel space in high density Wi-Fi deployments.
An easy way we discussed identifying CCI/CCC during the CWNA course was to fire up your favorite wireless tool like Wifi Explorer Pro. Grab a laptop with a similar spec radio that your AP has (ex: your ap is 3×3, use a 3×3 client) and stand right underneath your AP. Here’s what a scan in my house looks like right next to my AP reading a -16 on channel 36.
Identify how many other APs your laptop can hear that are on the same channel of the AP your standing by. In the example above you can see that I can hear another AP using a primary channel of 36 at a -81. This AP along with other nearby clients could have the potential to cause co-channel contention. What you see may not be exactly what the AP hears as every radio has variations in receive sensitivity, but it will help to identify possible contention or interference.
We should no longer build Wi-Fi for maximum distance in enterprise environments like we did years and years ago, but we should now build for capacity and efficiency. So make sure you take advantage of those walls and other building obstacles when designing your next high capacity Wi-Fi network if needed.
Take a look at some of the following references to familiarize yourself with co-channel interference/contention:
If you have any comments or feedback, I’d love to hear from you.