Question 1
ACME corporation has decided to setup wireless (IEEE 802.11) network in it's sales branch at Tokyo and found that channels 1, 6, 9,11 are in use by the neighboring offices. Which is the best channel they can use?
I disagree with the suggested answer D and agree with the community consensus of A. In 2.4 GHz 802.11 networking, channels are 22 MHz wide and spaced 5 MHz apart, meaning a channel overlaps with those ±4 channels away. To minimize Adjacent Channel Interference (ACI), one must select the channel with the least spectral overlap with the occupied channels (1, 6, 9, 11). Channel 4 is the most distant from the occupied centers in the largest available spectral gap.
Reason
Option A is correct because the existing channels are 1, 6, 9, and 11. The gaps between these are 1-6 (4 channels free), 6-9 (2 channels free), and 9-11 (1 channel free). The largest gap is between 1 and 6. Within this gap, Channel 4 provides the maximum possible buffer from both Channel 1 and Channel 6, resulting in lower interference than any other choice provided.
Why the other options are not as suitable
- Option B is incorrect because Channel 5 is immediately adjacent to the occupied Channel 6, leading to significant signal overlap and interference.
- Option C is incorrect because Channel 10 sits directly between occupied Channels 9 and 11, causing Adjacent Channel Interference from both sides simultaneously.
- Option D is incorrect because Channel 2 is immediately adjacent to the occupied Channel 1, which would result in heavy overlap and poor signal quality compared to the more centered Channel 4.