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As I proceed in my CWNA studies, I read in a recent Bitcricket Whitepaper that 802.11b uses a slightly wider channel: 22Mhz.
Is this the only exception?
Or was the original 802.11 also 22Mhz?
Does 802.11g use only 20Mhz wide channels?
How can a 802.11b product and a 802.11g product, with different channel widths, coexist and work together?


thanks for your help in clearing this up!

If memory serves me... 22 Mhz is the original, used in DSSS modulation. The change to 20 Mhz comes with OFDM. So your "G"...and also "A" have the 20 Mhz wide channels you mentioned.

Lastly, when you mention G and B coexisting...I imagine you are referring to the backwards capability of G. In this case, G can make the change to DSSS and therefore 22 Mhz wide channels like used in B networks.

Your best bet is to check the standards as tedious as it can be...

Hope it helps! Good luck

Brett,
thank you for responding.
I had looked through the standards documents and found no reference to this.
I wonder where it is written....

First of all, thank you for all the knowledge I've learned from this forum. I would like to give this bit of knowledge about channel width. The answer to the .ll b/g channel width is in the final test of the Wireless# Mega Guide and it says that. "An 802.11b/g channel actually uses 22 MHz of carrier frequencies." and says again later, "22MHz of carrier frequencies are actually required for 802.11 DSSS and OFDM communications." Hope this helps.

The Cisco White Paper titled "802.11n: The Next Generation of Wireless Performance" states in the middle of page 6:

20- and 40-MHz Channels
The original 802.11 direct sequence radio and the 802.11b extension to the base standard use a
radio channel spacing that is 22-MHz wide. 802.11a and 802.11g use radio channel spacings that
are 20-MHz wide. Because 802.11g is an extension to 802.11b, 802.11g spaces its channels just
as 802.11b does, every 22-MHz.

Maybe I am getting confused by CISCO's terminology.

On the other hand the CWNA book states:

DSSS channels are 22Mhz wide

and

an IEEE 802.11g channel is 22Mhz wide

Can I therefore make the following conclusions?

802.11, 802.11b, 802.11g use 22Mhz wide channels
802.11a uses 20Mhz wide channels
802.11n uses 20/40 Mhz wide channels but also some 22Mhz channels when sending beacon frames in 2.4Ghz.

thanks

I've been doing some research on the net and found a Microsoft paper that claims that the channels are 25MHz apart! Lets stick to the CWNP material cause I think we both agree on 22MHZ. Also in my material I found that .lla is 20MHz width. Can't speak for .11n yet.

25 Mhz apart is ok, that is the spacing between center frequencies of non-overlapping channels. For example, Ch 1 on 2412 and ch. 6 on 2437 is 25 Mhz apart.

In my last post I said a Microsoft document said the channels were 25 MHz apart....what I meant to say was, they said the channel width is 25 MHz. Sorry for not being clear. I think you meant to say that the channel spacing is 5 MHz between center channels. eg channel 1 (2412) and channel 2 (2417).

I would appreciate it very much if the experts on this forum would give a definite answer to my channel width dilemma, as posted above.
thank you!

In reading the 11n standard (20 & 40MHz Channels), and in my review of 802.11, 11b, 11g and the respective modulation schemes used for each, the benefits of 20MHz channels come from OFDM, is what I've gathered.

That being said, when running 11G only networks where 22MHz channels aren't needed for backward compatibility, the full use of the slimmer channel can be used.

When mixed mode is used then OFDM has to support the lower modulation schemes (DSSS), and therefore use 22MHz channels.

When utilizing 11n technology and channel bonding, we are able to take two 20MHz OFDM channels and bond them for an aggregate 40MHz channel width.

I assume there is a channel inefficiency if supporting legacy clients (ie. 22 & 44MHz channels), but the newer OFDM algorithm, from what I've heard, is able to handle this process more efficiently.

There are many other efficiencies of 11n technology, and wikipedia is a great source, but the final reference is, as stated in the posts, within the standards bodies that create them...IEEE. And once there, deciphering the code within becomes 1/2 the fun. Best sources I am using now are 802.11 Handbook 2nd edition and 802.11 Network Monitor Unleashed. This is in addition to the vendor manuals (ahem...Cisco), user/admin guides, data sheets and excellent support/news sources like wifiplanet, weca.net, ieee.org, w2i.org, muniwireless.com, and MOST IMPORTANTLY - CWNP Learning Center - found under Learning Resources >Whitepapers.

That's what I've learned of this post so far, but I am no CWNE...at least not yet ;-)