Channel Interference

The performance of your network is affected by the 802.11 interference caused by intruding radio signal that interrupts normal system operations.

In some cases, an intruding signal can originate in another 802.11 network; in other cases, non-802.11 radio energy can disrupt 802.11 communications. Common sources of non-802.11 interference include microwave ovens, wireless phones, and Bluetooth devices. Access points that are not part of the network can also cause interference. Interference decreases the signal-to-noise ratio (SNR) for data rates. For more information on SNR, refer to Acceptable Voice Quality.

Interference can affect any 802.11 transmission and is not specific to the Vocera system. However, because Vocera is a voice application, interference will be noticed more on Vocera than a data application. Vocera recommends the use of a spectrum analyzer or similar third-party tool to identify and eliminate sources of possible RF interference.

Under the 802.11b/g standard, a transmission on one channel can interfere with transmissions as far as four channels away. 802.11b/g signal on channel 1 can cause channel with a transmission on channels 2, 3, 4, or 5. The channel interference concerning Vocera system are adjacent-channel interference and co-channel interference.

Adjacent-Channel Interference

Adjacent-channel interference is the interference caused by power from a signal in an adjacent channel. In this case, every client and access point on overlapping channels talk over each other. If the radio channels in nearby access points are not separated from each other by five channels, an adjacent channel interference occurs. You can mitigate this issue by separating the access point by five channels.

In the United States, you must use channels 1, 6, and 11. There is flexibility for channel selection in an 802.11b/g network in Europe, where channels 1 through 13 is available. You should assign specific non-interfering channels to your access points, rather than relying on settings such as least congested channel that allow access points to select a channel dynamically.

Note: The Vocera system locale determines which wireless channels are supported on Vocera badges. When you install the software, you specify the locale in the Country field. For more information on working with locales, refer to the appendix in the Vocera Voice Server Installation Guide.

Co-Channel Interference

Co-channel interference is a crosstalk between two radio transmitters using the same frequency. Co-channel interference occurs when access points on the same channel competes for time to talk. When this situation occurs, multiple access points can transmit at the same time on the same channel, corrupting packets on both channels, and causing transmission delays.

Note the details of the areas where co-channel interference occurs instead of creating coverage gaps to avoid it. Test these areas thoroughly and keep track of user complaints. Badge usage patterns can determine whether it is sufficient to manage these areas or if you need to change the access points.

You can mitigate some co-channel interference problems by using directional antennas. In some situations, these antennas provide better performance than omnidirectional antennas because you can use them to fine-tune coverage areas.

For a network to provide continuous coverage over a large area, access points must be placed fairly close. Considering that only three non-interfering channels are available for use in an 802.11b/g network, it is quite possible that the location of some access points will cause co-channel interference.

Vocera recommends moving to 5 GHz (if your network is designed for it) to mitigating CCI.