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245U-E Wireless Ethernet Modem User Manual
Rev Version 2.24
Figure 22 5-GHz Channels
Dynamic Frequency Selection
Because of the push within the 802.11a market to open up new spectrum for unlicensed radio, a
mechanism called Dynamic Frequency Selection (DFS) was developed so that the 802.11 Wi-Fi could coexist
with existing military and telecommunication radar systems. Access points with 5-GHz radios comply with
regulations that require radio devices to use DFS, which can detect radar signals and avoid interfering with
them by automatically scanning and then selecting another channel or band.
When DFS is enabled, the access point (master device) goes through the following steps:
1. The master device that initiates communications selects a channel and monitors that channel for
potential radar interference for a minimum listening time of 60 seconds (channel availability check
time). No transmissions can occur during this period.
2. If interference is detected, the system selects another channel and repeats the channel availability
check on the new channel (the original channel is added to a list of channels with radar).
3. Once a channel has been selected and passes the channel availability check, the network starts to use
that channel.
4. While using the channel, the network’s master device continuously monitors for potential interference
from a radar source (this is referred to as “in-service monitoring”). If interference is detected, the
network master device issues commands to all other in-network devices to cease transmissions. The
channel is added to the list of channels with radar.
5. The master device then selects a new channel (one that is not on the radar list).
6. A channel that has been flagged as containing a radar signal (either by a channel availability check
or by in-service monitoring) is subject to a 30-minute non-occupancy period, during which time the
channel cannot be used by the device in order to protect scanning radars. The channel on the radar list
will be purged once the non-occupancy period has elapsed for that channel.
