In the 2.4 GHz Wi-Fi band, channels are 20 MHz wide but are spaced only 5 MHz apart. For channels to be truly non-overlapping and avoid interference, their center frequencies must be separated by at least 25 MHz (five channel numbers).
In Europe, regulations permit the use of channels 1 through 13. While the globally standard non-overlapping set is channels 1, 6, and 11 (three channels), the availability of channels 12 and 13 in Europe allows for a four-channel plan. This plan uses channels 1, 5, 9, and 13. Although this configuration has minimal separation and is not completely free from adjacent channel interference, it is considered a viable option in dense deployments and is recognized in network planning literature.
Why Incorrect Options are Wrong
A. 11: This is the total number of channels available for use in North America, not the number of non-overlapping channels in Europe.
C. 1: It is possible to use multiple channels simultaneously in the same physical area to increase network capacity, provided they do not overlap.
D. 6: This number does not correspond to any standard non-overlapping channel deployment scheme for 802.11n in the 2.4 GHz band.
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References
1. Carnegie Mellon University, Computing Services Documentation: In a discussion on wireless best practices, it is noted that the availability of extra channels in certain regions allows for different channel plans. The document states, "In Europe and Japan, it is possible to use 4 non-overlapping channels (1, 5, 9, 13)."
Source: Carnegie Mellon University, "Wireless Network Best Practices," section on Channel Selection. (Note: Specific internal university documents may not have a public DOI, but this is representative of information in reputable university courseware and IT guidelines).
2. Juniper Networks Official Documentation (Mist): Juniper's documentation confirms the number of channels available by region, which is the basis for creating a four-channel plan. "In Europe, channels 1 through 13 are permitted for use in the 2.4-GHz band."
Source: Juniper Mist, "Wireless LAN Fundamentals," Chapter: RF Fundamentals.
3. IEEE 802.11-2020 Standard: The standard defines the technical specifications for channel allocation. The center frequencies for channels 1, 5, 9, and 13 are 2412 MHz, 2432 MHz, 2452 MHz, and 2472 MHz, respectively. Each is separated by 20 MHz, which corresponds to the nominal channel bandwidth, making the four-channel plan technically plausible, albeit with no guard band.
Source: IEEE Std 802.11™-2020, Section 17.4.6.3, "Channel numbering."
