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Understanding Wireless Interference


Wi-Fi interference is a common and troublesome issue. The lack of wires that makes WLAN so attractive is also the feature that makes other consumer devices capable of causing Wi-Fi interference. Your WLAN network might be working fine one day and sluggish the next day, without you having made any network changes, all due to interference.

This topic describes:

What Causes Wireless Radio Frequency Interference?

Because the air is shared by all transmitters, transmissions by any device at the same frequency as an access point's radio can cause interference. Because 802.11 wireless networks operate in unlicensed bands used by many technologies, such as microwave ovens, video surveillance cameras, cordless phones, they are subject to interference. In addition, wireless access points sharing the same channel might interfere with each other. The effect of interference is highly dependent on the strength of the transmission and the distance from the interferer. Access points closest to and on the same channel as an interferer will be affected more than those that are further away.

These are some common causes of wireless interference:

  • Leaving the channel number on each radio set to the default value can result in high interference among the radios because too many radios are sharing the bandwidth on one channel.

  • Hidden nodes in a wireless network referring to nodes that are out of range of other nodes or a collection of nodes. A hidden node can generate a high number of cyclic redundancy check (CRC) code errors.

  • Co-channel interference or adjacent channel interference can result from setting radios to bands that have overlapping channels. The channels might not all be in use by your network—neighboring company signals can also cause interference.

  • Some non-network devices, such as microwave ovens, car alarms, cordless phones, or wireless video cameras can interfere with wireless channels. Most often, these devices are using the 2.4-GHz frequency.

  • Bad electrical connections can cause broad RF spectrum emissions.

  • RF jamming is a deliberate attempt to disrupt the network with a powerful signal.

Effects of Interference Seen by Clients

Your network clients might notice the results of interference before you do. They might complain of network slowdown, but not of data loss. This slowdown might not be immediately obvious with low capacity data transmission because, if interference is intermittent, packets eventually get through. Therefore, there is no packet loss, just retransmissions that take time. Another possibility is that some devices, such as microwaves, reduce throughput without blocking it entirely. Complaints will increase when more users log in, increasing data capacity until data loss occurs, or when Voice Over IP calls are placed. VoIP requires significant bandwidth because resending voice is not an option—the result is dropped or jittery voice transmission.

You Can Monitor RF Interference with Network Director

Network Director includes a Monitor Mode that displays the compiled RF data gathered by scanning the mobility domain of your network. There are two monitors that indicate network interference for access points and radios:

  • AP Interference Sources—On this chart, interference sources for each radio or each access point (either one can be selected) are sorted into categories such as Microwave Oven, Phone FHSS, and Continuous Wave. The occurrences of each source are tracked and added—the sum appears on a bar chart. The categories with the tallest bars are those causing the most interference on this radio or access point, but the sum of all interference is what really matters. For more information, see Monitoring RF Interference Sources on Wireless Devices.

  • Radio Interference Sources—The pie chart for radio interference is also sorted into categories such as Microwave Oven, Phone FHSS, and Continuous Wave. In addition, a list of details are provided about each source of interference—time last seen, transmitter ID, listener MAC address of the access point that found the interference, channel, RSSI, duty cycle, and percentage of compliance to the common information model (CIM). For more information, see Monitoring RF Interference Sources on One Radio.

  • RF Interference For Radios on One Access Point—The bar chart that displays each radio on an access point lets you compare the interference experienced on two radios when the access point has dual radios. With single radio access points, you see the same data as you do for the radio interference pie chart, but in bar chart format. For more information, see Monitoring RF Interference Sources For Radios on One Access Point.

What Is RF Jamming?

RF jamming is a DoS attack. The goal of RF jamming is to take down an entire WLAN by overwhelming the radio environment with high-power noise. A symptom of an RF jamming attack is excessive interference. If an access point radio detects excessive interference on a channel, and channel auto-tuning is enabled, the radio changes to a different channel. The radio continues to scan on an active data channel and on other channels and reports the results to the controller.

Jamming occurs at the physical layer of the network, saturating the channel or band with noise and making it difficult or impossible for a receiving radio to detect a real transmission. Think of jamming as trying to hear someone talking as a siren goes off. The increased noise floor results in a poor signal-to-noise ratio (SNR), usually detected by the clients as poor signal quality. Jamming can also be detected by an access point, which then triggers dynamic temporary tuning of the channel if automatic channeling is enabled. However, selecting a different channel does not always stop jamming. An experienced attacker will often use all available channels in the attack.