The signal-to-interference ratio (SIR) measures the Wi-Fi signal in comparison to how much co-channel interference is present from other radio transmitters. NetSpot has two visualizations that show interference and both are available only to PRO users. First is the Signal-to-Interference Ratio visualization, and then there is the Troubleshooting: Overlapping Channels visualization. In both cases, poor SIR levels are considered to be below 30 dB and critical levels are below 10 dB.
If your survey shows areas of poor SIR, first check the Signal Level visualization to make sure your signal levels are OK in those areas. If they are, then you will want to work on minimizing interference. If they aren’t, then focus on raising your signal level.
Strategies to minimize co-channel interference:
- Change the channel selection. APs working in close proximity should never use overlapping channels. Consider the classic “honeycomb” AP placement, if possible. Note that in some 802.11n equipment, the position of the secondary channel (below or above the primary one) is a user-configurable option, which gives you an additional degree of freedom.
- If you experience low SIR values in the 2.4 GHz band, consider switching your APs to the 5 GHz band, where there are more non-overlapping channels from which to choose. If you use an 802.11n AP with 40 MHz bandwidth in the 2.4 GHz band, you have virtually no way of avoiding interference. If channel bonding is not used (i.e., a single 20 MHz channel), in the United States you only have three non-overlapping channels from which to choose: 1, 6, and 11. This is illustrated in the image above.
- In case your network has more than one Access Point, you might need to lower the transmitters’ capacities to keep interference at an acceptable level. This is a situation where higher capacity doesn’t mean a better connection. Usually, the optimum AP transmitting capacity is 5-10 dBm. This will ensure the best density of installed APs, minimize the load on each of the APs, and minimize interference between neighboring hotspots that might share the same channel. That doesn’t mean that a powerful AP is bad, but several powerful APs neighboring each other will interfere with each other. That’s why their capacities have to be lowered.
- Make sure your users don’t abuse your network with powerful client Wi-Fi adapters, as interference may also be provoked by more than one of these adapters being active in the same location. There’s no need for the client adapter to transmit at 50 mW, when the AP is transmitting at only 5 mW.