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I sent the following question in, but was told to post it here.

I have a question about some information found in your CWNA study guide, second edition. Chapter 9, starting on page 347 and including figure 9.10 on page 348. Your picture showing the overlap of channels 1,6, and 11 is incorrect. The picture is showing overlap of the 3 channels, but channel 1's center frequency appear to be on channel 2, and channel 11's center frequency appears to be on channels 10. If the center frequencies fell in the correct position there would be no overlap. Channel 1 being centered at 2412 Mhz plus 11 Mhz (half of the 22 Mhz bandwidth) equals 2423. If you do the same coming backwards from channel 6 you get 2437 - 11 equals 2426 Mhz giving you a 3 Mhz separation.

Could you please clarify this for me.

Good catch ! I just looked at that and see the same thing. I went to pages 76,77,78 looked at Fig 3.4 and Fig 3.5 ,and Fig 3.6 and see your point. That is THEORECTICALLY what happens.

But what I think this page is talking about is that v/s REALITY. Go on to page 350 and Fig 9-11. The physical dynamics of locating APs close to each other bleeds and the center frequencies get pushed around. Read the summary on page 351 too.

This may clear it up.

Compughter, thanks for the reply and you are correct. I was so caught up in memorizing facts that I over looked the "theroy vs reality".

Peace

Hi Nezz of Durham:

The CWNA study guide v2 figure 9.10 does misalign the three "channels" with the markers on the horizontal axis. I had not noticed this before. If they were better aligned the three RF channel profile graphics would each have to be fatter to still show the overlap.

The standard does not say that each channel is 22 MHz wide. It says that channel centers are spaced at 5MHz intervals. It says that HR/DSSS (802.11b) PHYs deserve 5 channels of separation to not overlap, hence the USA popularity of channels 1 6 and 11, and that the older DSSS PHYs require 6 channels of separation to not overlap, hence the European advantage of channels 1 7 and 13.

The overlap is worse than this though. In a lab environment with stations close together frames transmitted on channel 1 have been received without error on channel 11.

Please correct page 346 5th line from bottom. Replace "22 MHz wide" with "spaced at 5MHz centers." And please run out and buy a copy of the much improved CWNA study guide v3 (as soon as released).

I hope this helps. Thanks. /criss