802.11b Interference with Digi 24XStream Radios

Digi 24XStream radio modems operate in the 2.4GHz ISM frequency band. The 2.4GHz band is also widely used by other wireless systems such as WiFi networks (802.11b/g), Bluetooth and cordless phones, etc.

The FCC and international regulations specify how the different systems may utilize the frequencies in an attempt to allow various systems to operate in the same area with minimal interference.

In practice whenever two separate systems attempt to use the same frequencies there will be some interference. The severity of the interference will be directly related to several factors including: relative transmit powers, modulation types, frequencies used, and proximity to interfering radios.

Because wireless computer networks are so common, here are some guidelines for minimizing interference when installing Digi 24XStream radio modems in the presence of 802.11b or 802.11g equipment.

802.11b Frequency Overview

The 802.11b/g technologies utilize Direct Sequence Spread Spectrum (DSSS) technology to transfer up to 45Mbps data over distances of up to 300ft indoors and several miles outdoors.

802.11b/g systems have a total of fourteen channels available for communication. Only 11 are allowed in the USA under current FCC regulations. Each channel occupies 25MHz of bandwidth and is spaced 5 MHz apart resulting overlapping of adjacent channels.

When multiple access points are operating in the same vicinity, it is possible to minimize interference by using 3 non-overlapping channels - 1, 6 and 11.

24XStream Frequency Overview

Digi 24XStream radios use Frequency Hopping Spread Spectrum (FHSS) technology to minimize interference. The 24XStream products operate using frequencies between 2.45GHz and 2.46GHz.

802.11b and 24XStream Interoperability

FHSS technology typically has a higher peak power than DSSS and can operate reliably in the presence of DSSS interference though the range may be reduced depending on the relative signal strengths.

If the two systems are operating at exactly the same frequencies, the 24XStream needs the desired signal to be slightly stronger at the receiver than an interfering signal to have the system work reliably. If they are not on exactly the same frequencies then the 24XStream can actually block interference that is many times stronger than the desired signal and still operate reliably.

Note in the diagram that 802.11b channels 7 through 12 all use the exact frequencies used by the 24XStream radios. For the best performance from the 24XStream radio, configure any 802.11b networks to use channels 1, 2, 3, 4, 5, 6, 13 or 14 where applicable.

Frequency Table:

Relevant Files

• Frequency usage and power levels for world-wide 802.11 networks