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300-425 · Question #279

During a wireless design, why should DFS (Dynamic Frequency Selection) channels be used cautiously?

The correct answer is C. They may be subject to radar interference and APs must vacate the channel. DFS channels in the 5 GHz band are shared with radar systems, and regulatory requirements force APs to vacate the channel upon radar detection, potentially causing brief service disruptions.

Wireless Design

Question

During a wireless design, why should DFS (Dynamic Frequency Selection) channels be used cautiously?

Options

  • AThey provide less throughput.
  • BThey can cause interference with Bluetooth devices.
  • CThey may be subject to radar interference and APs must vacate the channel.
  • DThey are not supported on 802.11ac.

How the community answered

(40 responses)
  • A
    5% (2)
  • B
    3% (1)
  • C
    93% (37)

Why each option

DFS channels in the 5 GHz band are shared with radar systems, and regulatory requirements force APs to vacate the channel upon radar detection, potentially causing brief service disruptions.

AThey provide less throughput.

DFS channels operate in the 5 GHz band and support the same maximum throughput as non-DFS 5 GHz channels - throughput is determined by channel width and protocol capabilities, not DFS designation.

BThey can cause interference with Bluetooth devices.

Bluetooth operates in the 2.4 GHz ISM band, entirely separate from the 5 GHz spectrum where DFS channels reside, so Bluetooth cannot interfere with or be affected by DFS channel operation.

CThey may be subject to radar interference and APs must vacate the channel.Correct

DFS channels in the 5 GHz band (UNII-2 and UNII-2 Extended) are regulated to coexist with military and weather radar systems. When an AP detects a radar signal on a DFS channel, it must cease transmitting within 10 seconds and switch to a clear channel, causing a temporary connectivity interruption while clients reassociate on the new channel.

DThey are not supported on 802.11ac.

802.11ac operates exclusively in the 5 GHz band and fully supports DFS channels, including UNII-2 and UNII-2 Extended frequencies, as defined in the 802.11ac specification.

Concept tested: DFS channel radar detection and mandatory channel evacuation

Source: https://www.cisco.com/c/en/us/support/docs/wireless-mobility/wireless-lan-wlan/212920-dynamic-frequency-selection.html

Topics

#DFS channels#radar interference#5 GHz channel planning#dynamic frequency selection

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