Last Post: December 17, 2007:
I don't understand why there is difference in the number of channels of separation required between DSSS and HR/DSSS, given they use the same channel structure. Why is DSSS 5 and HR/DSSS 6?
Hi Sacim -
It basically boils down to what the standards say. Having said that, no one can definitively state why the IEEE board for 802.11 chose 30mhz between center channels as the original safe distance for non-interference for overlapping / adjacent cell deployments. My guess is it had something to do with the math behind it all.
Read "184.108.40.206 Number of operating channels" in the original 802.11 standard and "220.127.116.11 Number of operating channels" in the 802.11b standard. Basically it looks that they found a tighter separation between the channels could be used while still maintaining an acceptable level of non-interference. You can download them for free here: http://standards.ieee.org/getieee802/802.11.html
Perhaps it was a typo, but you have your numbers backwards in the original post. DSSS requires 30mhz separation while HR/DSSS requires 25mhz separation, or as you put it, 6 and 5 channels respectively.
For a visual, check http://proquest.safaribooksonline.com/0596100523/wireless802dot112-CHP-12 and search for "Figure 12-7"
Hi Brett, thanks for providing an explanation. I will check out the links you provided.
The CWNA book, 4th ed. page 148, review answer 1 says:
"Six channels of separation are required for HR/DSSS and 5 channels are required for DSSS systems."
Who's got it backwards?
Well I would have to defer that to one of the CWNP admins for the official response.
I couldn't find the errata for the 4th edition of the CWNA book when doing a quick search, so I'm not sure if it is a typo or not. I think my math is correct in that 30mhz (original standard) is 6 channels for DSSS and 25mhz wide (802.11b standard)is 5 channels for HR/DSSS but I could be wrong. :?
Channel 1 is at 2.412 for channel center frequency, a 30mhz spread puts the next channel at chanel 7 or 2.442 for its channel center - a spread of 6 channels. The original 802.11 standard documents this on page 216. If I had to put money on it, I'm going with what I'm reading from the standards.