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Difficulty: Very hardWireless Network Deployment and Standards

A network architect is designing a high-density wireless LAN for a enterprise campus using 802.11ac Wave 2 access points (APs). To achieve maximum throughput per client, the architect intends to configure wide 80 MHz channels in the 5 GHz spectrum. However, a post-installation site survey reveals that radar equipment is actively operating in the UNII-2 and UNII-2 Extended frequency ranges (channels 52 through 144). If Dynamic Frequency Selection (DFS) events are frequently triggered across these bands, which of the following operational impacts will most significantly compromise the wireless design?

  1. The APs will be forced to vacate the DFS channels upon detecting radar, reducing the available non-overlapping 80 MHz channels to as few as two in UNII-1 and UNII-3, which dramatically increases co-channel interference.Answer
  2. B
    The APs will automatically fall back to using adjacent 2.4 GHz channels 2, 3, and 4 to sustain 80 MHz channel bonding without experiencing radar disruption.
  3. C
    The wireless controller will disable 802.1Q VLAN trunking and strip 802.1p class of service tags on switch interfaces whenever radar pulses are logged.
  4. D
    The access points will force all associated clients to re-authenticate using WPA3-Personal pre-shared keys to bypass mandatory channel availability check timers.

Answer

The APs will be forced to vacate the DFS channels upon detecting radar, reducing the available non-overlapping 80 MHz channels to as few as two in UNII-1 and UNII-3, which dramatically increases co-channel interference.
In the 5 GHz band, configuring 80 MHz channel bonding consumes four 20 MHz channels per bonded group. There are only 6 non-overlapping 80 MHz channels available across the entire 5 GHz spectrum. When radar activity triggers Dynamic Frequency Selection (DFS) in the UNII-2 and UNII-2 Extended bands (which contain 4 of those 6 bonded channels), the access points are forced to evacuate those bands. This leaves only two 80 MHz channels available (one in UNII-1 and one in UNII-3) for the entire enterprise deployment, resulting in severe co-channel interference (CCI) across adjacent access points.

Step-by-Step Solution

1
Analyze the available 5 GHz channel spectrum and 80 MHz channel bonding requirements.
The 5 GHz spectrum contains 24 non-overlapping 20 MHz channels. Bonding four adjacent 20 MHz channels into an 80 MHz channel yields 6 total available 80 MHz channels across UNII-1, UNII-2, UNII-2E, and UNII-3.
Calculating total channel availability determines how density and channel width interact.
2
Evaluate the impact of active radar signals on UNII-2 and UNII-2 Extended channels.
FCC and ETSI regulatory rules require APs to employ Dynamic Frequency Selection (DFS). When radar is detected in UNII-2 (channels 52-64) and UNII-2E (channels 100-144), APs must immediately cease transmission and change channels.
DFS compliance forces APs out of the middle 5 GHz bands when radar bursts are detected.
3
Assess the resulting channel capacity and co-channel interference (CCI) in non-DFS channels.
Excluding UNII-2 and UNII-2E leaves only UNII-1 (channels 36, 40, 44, 48 -> one 80 MHz block) and UNII-3 (channels 149, 153, 157, 161 -> one 80 MHz block). Deploying a multi-AP campus on only two 80 MHz channels causes extensive co-channel interference.
Re-using only two 80 MHz channels across multiple enterprise access points causes severe channel overlap and performance degradation.

Key Concept

5 GHz Channel Bonding and DFS Radar Constraints
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Topics

Wireless Network Deployment and Standards

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