Most modern spectrum analyzer solutions offer some form of automatic device classification, although many are rudimentary and can produce false positives. You can train yourself to identify interfering devices much faster and more accurately than any automatic tool simply by familiarizing yourself with transmitter modulation patterns. Here are a few items to test with a spectrum analyzer:
- Neighboring APs occupying an overlapping channel
- Cordless Phones
- Microwave Ovens
- Wireless Audio transmitters
- Video Transmitters (Security Cameras, IP cameras)
- PIR motion detectors
- WLAN download
- WLAN upload
Common Shapes: Wi-Fi - 802.11b
This is the most common shape you are likely to see when troubleshooting Wi-Fi in the 2.4 GHz band. APs tend to transmit beacons at the lowest common rate of all Wi-Fi devices, or the most basic modulation, BPSK. This image shows APs on channels 1, 6, and 11.
Common Shapes: Wi-Fi 802.11g ERP-OFDM
Identify 802.11g ERP-OFDM by looking for a flat top. The flat top of OFDM signatures are under 20 MHz wide. This is the same signature for 802.11a OFDM.
Common Shapes: Wi-Fi - 802.11n 2.4 GHz 40-MHz ERP-OFDM
802.11n OFDM APs with data rates of 300mbps use channel bonding, which appears as two 802.11g ERP-OFDM signatures bonded together.
The width of an 802.11n AP transmitter will not always be 40 MHz. In most cases, 40 MHz transmissions appear in bursts when an AP’s throughput exceeds the capabilities of 802.11g ERP-OFDM.
You can see the bursts in the waterfall view very easily. 802.11a/g/n typically shows abrupt edges, due to the dBm drop on each side of the signatures.
Common Shapes: Adjacent Channel Interference
Your Wi-Fi may be on the same channel as other wireless APs. There are 11 channels in 2.4 GHz, but only three that don’t overlap (1, 6 and 11 in the U.S.). When APs are placed on overlapping channels they must wait for neighboring APs to stop transmitting before they can.
Common Shapes: Microwave Ovens
Microwave Ovens operate in the 2.4 GHz range, typically creating a shape similar to a mountain slope in the Density View. Since microwaves are usually used from 1-5 minutes, it helps to adjust the timespan to about two minutes.
Interference occurs when the microwave oven transmits on the same frequencies as the Wi-Fi channel. In this image, you can see how the mountain-shaped microwave oven covers the curvature of the Wi-Fi on channel 11.
Microwave amplitude levels vary depending on their distance from the spectrum analyzer. Experiment with a microwave oven and varying locations.
You can also use the Waterfall View to help you identify microwave interference, because it shows how long a device is active, and will often show a distinctive “comma” shape when a microwave is used.
Common Shapes: Motion Sensors
Motion sensors tend to transmit within very narrow frequency ranges. Sometimes a building will have motion sensors in each room. If this is the case, verify that they are not in the 2.4 GHz range by walking close to each sensor and watching the corresponding amplitude levels in the Waterfall View.
Common Shapes: Audio Video Transmitter
Wireless security cameras generally create three spikes in a 20-30 MHz wide frequency range. They constantly transmit and rarely change channels. Look for three adjacent vertical lines in the Waterfall View.
Common Shapes: Cordless Phones
Not all cordless phones create the same pattern; some are very narrow and use the 2.4 GHz frequency band, while others scan for the clearest channel and vary in the frequencies used.
One of the easiest ways to identify cordless phone interference is by using the Waterfall View to search for a unique series of vertical lines. When a cordless phone changes channels, it will appear as a break in the waterfall.
Some cordless phones may hop across the entire spectrum similar to Bluetooth.
Common Shapes: Bluetooth
Bluetooth hops across the 2.4 GHz 1,600 times a second, which is a form of frequency-hopping spread spectrum (FHSS) modulation.
Because frequency-hopping transmitters broadcast across large frequency ranges, they may cause WLAN degradation, though it is unlikely that Bluetooth will cause severe interference.