During a wireless penetration test you find four 2.4 GHz access points in neighboring offices that have been manually set to channels 2, 3, 5, and 7 in an attempt to increase capacity. All client devices support 802.11b/g/n. From a wireless-networking standpoint, what is the main technical issue with using these channels together?
Channels 2, 3, 5, and 7 all overlap in the 2.4 GHz spectrum, so the access points will create significant co-channel and adjacent-channel interference instead of increasing capacity.
Channels 2 through 7 require Dynamic Frequency Selection (DFS), so legacy 802.11b/g clients will be forced to disconnect.
Channels 2-7 reside only in the 5 GHz band; configuring 2.4 GHz access points on them prevents 2.4 GHz clients from associating.
Channels 2-7 operate using frequency-hopping spread spectrum (FHSS), while 802.11b/g/n clients expect direct-sequence or OFDM, leading to incompatibility.
In the 2.4 GHz band, each 20 MHz-wide 802.11b/g/n channel overlaps heavily with adjacent channels except for three that are spaced far enough apart to avoid mutual interference. In North American and most worldwide regulatory domains, only channels 1, 6, and 11 are non-overlapping. Channels 2, 3, 5, and 7 all share significant portions of frequency space; placing adjacent access points on them causes co-channel and adjacent-channel interference, reducing throughput and increasing retransmissions. Dynamic Frequency Selection (DFS) applies only to portions of the 5 GHz band, not to 2.4 GHz channels. All 802.11b/g/n PHYs in 2.4 GHz use DSSS or OFDM, not FHSS, and channels 2-7 are valid 2.4 GHz channels, so client compatibility is unaffected. Therefore, the critical flaw is that the selected channels overlap rather than remaining non-overlapping.
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What are non-overlapping channels in the 2.4 GHz spectrum and why are they important?
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What is co-channel and adjacent-channel interference in wireless networking?
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What is Dynamic Frequency Selection (DFS), and does it apply to the 2.4 GHz band?
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Why does channel overlap occur in the 2.4 GHz band?
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What are the differences between DSSS and OFDM in 802.11b/g/n?
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Why are channels 1, 6, and 11 considered non-overlapping in wireless networks?
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