There has been lots of discussion in blogs and on Twitter about Co-Channel Interference (CCI) between APs and adjusting data rates. However, I haven’t seen much about Client Induced CCI (CI-CCI). Devin Akin briefly mentioned it in his blog about adjusting data rates and GT Hill brought it up with the idea of the “Hidden AP“, but other than that I wasn’t able to find a great deal of info about CI-CCI. So over the next two posts I want to illustrate that there is more to CCI than just APs, and what we can do about it.
In this first post I want to make sure everyone is on the same page around AP induced CCI, that is CCI caused by APs.
Co-Channel Interference is where 2 (or more) APs share the same RF spectrum. In other words, when there at least two APs that are using the same channel and where they can “hear” each other so that the effect is that their cells “join”. Like in the image below where the APs are still able decode the Preamble and PLCP header of the other AP. If you want to read about the data rates used by the preamble and PLCP header read this.
Now that we have cleared up what CCI is, how do we solve it? The basics are that you move APs further apart. But there are many ways to increase the “effective distance” between the APs:
- Physically make the distance between the APs further
- Increase the attenuation between the APs
- Reduce AP Tx power
- Rx sensitive tuning (CSR, RX-SOP)
- Narrower channels
- Directional antennas
Let’s look at those one by one.
Moving APs further apart
Physical distance is most obvious method. However, this can be difficult given how far away the data rates used by the preamble and PLCP header can be decoded by today’s APs. The physical properties of the facility may also make that impossible.
Increase attenuation between the APs
It may be possible to relocate the APs to locations that allow the natural attenuation (walls and other obstacles) of the facility to make the APs to appear further apart by attenuating the signal and therefore not be able to “hear” each other.
Reduce AP Tx power
Turning down Tx power results in a weaker signal at the other AP, which may be below the decodable SNR for that particular data rate at the other AP. But there is a point in which you can go to far. If the AP Tx power is turned down to far to reduce CCI it could reduce the ability of clients on the edge of the cell to achieve acceptable throughout due to a drop in MCS caused by the lower SNR. In a lightly used network, this drop in throughput might not be outweighed by the gains from the reduced CCI. Best practice is to make sure you have a balanced link between the AP and Clients. Each client device type typically uses different Tx power, so obtaining a balanced link between the AP and all clients is not likely. Most designs are done by matching the AP Tx power with the weakest client. However, even at low Tx power, today’s APs can decode the preamble and PLCP header at a large distance.
Rx Sensitivity Tuning
Some vendors have features (RX-SOP, CSR, etc) that make the radio deaf beyond a certain threshold. This could reduce the effects of CCI as the radio will not ever enter the 802.11 state machine as the radio will drop the frame due to the threshold limit and such be able to transmit even in the presence of the another AP on the same channel. These types of features need to be used with care, as it can prevent the AP from hearing necessary frames if the thresholds are not set correctly.
Moving to narrower channel widths allows for more separation in the contention domain (and therefore more channel reuse) as there as more separate RF domains to choose from. However, even with lots of channels, in scenarios like VHD there still aren’t enough channels to eliminate CCI, and even environments like outdoor yards and large open plan offices can also make this challenging. With the 2.4GHz band it is highly unlikely that you will be able to completely remove CCI, due to the limited channel availability.
The use of directional antennas allow for shaping of the RF propagation. By directing the RF energy towards the users, it may also be directed away from neighbouring APs. But be careful as APs can also hear better in the direction of the antenna. This could lead to APs hearing another AP on the same channel even better, or at a greater distance.
So, you can see that there are lots of options when it comes to APs and reducing the impact of CCI, but there is no silver bullet to completely remove CCI in most environments. One big problem still remains – Clients. The next blog post will discuss Client Induced Co-Channel Interference.
Further Reading on Rx Sensitivity Tuning:
Aruba Cell Size Reduction: ArubaOS User Guide. Search for “Reduce Cell Size”
Cisco RX-SOP: No Strings Attached Show