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How to solve Radio Frequency Interference issues

Core Issue

Radio interference is affecting the wireless network.

Resolution

RFI involves the presence of unwanted interfering RF signals that disrupt the original data signals from wireless devices. RFI in a wireless network can lead to adverse effects, for example, intermittent connectivity loss, poor throughput, and low data rates. There are different types of RFI that can occur in a wireless network environment, and one must take these RFI types into consideration before implementing wireless networks. RFI types include narrowband RFI, all-band RFI, and RFI due to adverse weather conditions.

Many factors impair the successful transmission or reception of a radio signal. The most common issues are radio interference, electromagnetic interference, cable problems, and antenna problems. Interference can come from a number of sources, including 2.4 GHz cordless phones, improperly shielded microwave ovens, and wireless equipment manufactured by other companies. Police radar, electrical motors, and moving metal parts of machinery can cause interference, too.

A spectrum analyzer is the best tool to determine the presence of any activity on your frequency. The Carrier Busy test available in the Test menus of Cisco Aironet bridges functions as a substitute for this item.

Radio Frequency Interference

Radio Frequency Interference (RFI) involves the presence of unwanted interfering RF signals that disrupt the original data signals from wireless devices. RFI in a wireless network can lead to adverse effects, for example, intermittent connectivity loss, poor throughput, and low data rates. There are different types of RFI that can occur in a wireless network environment, and you must tale these RFI types into consideration before you implement wireless networks. RFI types include narrowband RFI, all-band RFI, and RFI due to adverse weather conditions.

Narrowband RFI—Narrowband signals, depending on the frequency and signal strength, can intermittently interrupt or even disrupt RF signals from a spreadspectrum device, such as a wireless bridge. The best way to overcome narrowband RFI is to identify the source of the RF signal. You can use Spectrum analyzers to identify the source of the RF signal.

Spectrum analyzers are devices that you can use to identify and measure the strength of interfering RF signals. When you identify the source, you can either remove the source to eliminate RFI, or shield the source properly. Narrowband signals do not disrupt original data RF signals (from a wireless bridge) across the entire RF band. Therefore, you can also choose an alternate channel for the bridge where no narrowband RF interference occurs. For example, if unwanted RF signals disrupt one channel, say channel 11, you can configure the wireless bridge to use another channel, say channel 3, where there is no narrowband RFI.

All-band RFI—As the name suggests, all-band interference involves any unwanted RF signal that interferes with the data RF signal across the entire RF band. All-band RFI can be defined as the interference that covers the whole spectrum that the radio uses. The entire RF band does not point to the ISM band alone. The RF band covers any band of frequencies that the wireless bridges use.

A possible source of all-band interference that you can find commonly is a microwave oven. When all-band interference is present, the best possible solution is to use a different technology, for example, move from 802.11b to 802.11a (which uses the 5Ghz band). Also, the whole spectrum that the radio uses is 83.5 MHz in FHSS (the whole ISM band), while for DSSS it is only 20 MHz (one of the sub-bands). The chances of an interference that covers a range of 20 MHz are greater than the chances of an interference that covers 83.5 MHz. If you cannot change technologies, try to find and eliminate the source of the all-band interference. However, this solution can be difficult, because you have to analyze the entire spectrum to track the source of the interference.

RFI Due to Adverse Weather Conditions—Severely adverse weather conditions, for example, extreme wind, fog, or smog can affect the performance of wireless bridges, and lead to intermittent connectivity issues. In these situations, you can use a radome to protect an antenna from the environmental effects. Antennas that do not have radome protection are vulnerable to environmental effects, and can cause degradation to the performance of the bridges. A common problem that can occur if you do not use the radome is the one due to rain. Raindrops can accumulate on the antenna and affect performance. Radomes also protect an antenna from falling objects, such as ice that falls from an overhead tree. With the Cisco Outdoor Bridge Range Calculation Utility, you can choose your climate and terrain, and the program compensates for any degradation in weather.

CRC, PLCP errors

CRC errors and PLCP errors can occur due to Radio Frequency interference. The more radios a cell has (APs, Bridges or Clients), the more are the chances of the occurrence of these errors. A cell means a single channel (for example, channel 1) or a channel that overlaps the channel. Radio interfaces are half duplex. Therefore, radio interfaces are just like collision messages on Ethernet. Here are some reasons for the occurrence of CRC errors:

Packet collisions that occur due to a dense population of client adapters

Overlapping access point coverage on a channel

High multipath conditions due to bounced signals

Presence of other 2.4-GHz signals from devices like microwave ovens and wireless handset phones

Wireless is a more open medium than wired networks, and is subject to environmental effects. The radio waves bounce off surrounding objects, which can create a weaker or broken signal. This happens with cell phones, FM radios, and other wireless devices. The more 802.11 radios and clients are in a cell area, higher is the contention level and the potential for retries and CRC errors. The same applies to wired segments.

CRC and PLCP (Physical Layer Control Protocol) errors are normal when traffic flows through the AP. You do not need to consider these errors to be an issue unless the number of errors is very large. Here are some parameters you must check if there is a large number of CRC errors:

Line of Sight (LOS)—Check the LOS between the transmitter and the receiver, and ensure that the LOS is clear.

Radio Interference—Use a channel that has lower radio interference

Antennas and Cables—Ensure that the antennas and cables are appropriate for the distance of the radio link.

Cisco recommends a site survey in order to minimize these errors. Refer to Performing a Site Survey for more information on Site Survey

For more information, refer to these documents:

Problem Type

Interference issues ( EMI / RF issues)

Products

Access point

Bridge

WLAN adapters (wireless card) / ACU (Aironet Client Utility)

Base Station

Workgroup bridges

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