Why A is better than B/G for Voice over WLAN

Question

Hello all,
Now this post has some questions and statements in that you guys know all about and would like your valued advise and corrections/confirmations on the points.
Why A is better than B/G for voice:
B/G is congested, ie BT, Microwaves. Lots of other WLANs etc etc etc - FACT!
More non-overlapping channels - FACT!
The scatter and reflection on A is better than B/G. The idea being, the higher the frequency, the better the scatter. Would this be correct?
Higher frequencies produce better penetration results through walls, I have read. Or is it worse. Cisco say worse in the document "2.4 GHz waveform of 802.11b and 802.11g can pass through many walls. The 5 GHz waveform of 802.11a has approximately half the tendency for a given power to transmit suitable amounts of energy through walls because of its higher frequency" Ref: enterprise mobility guide. I am a little confused on this ?
A travels thru water and other moisture better than B/G, thus damp walls, wet walls, open areas (lets say between two buildings) would handle A better than B/G?
The Fresnel Zone (clearance required of an obstacle without degrading signal) is better on the A band due to a small wavelength size. Would this be a major factor for an RF design engineer? Any more technical "dummies guide" to this subject would be good?
OFDM handles signalling shortcomings like multipath fade better that previous modulations techniques (I know that G 11Mbps and above uses this also). Anyone have an English explanation to why this is? Also, please look at the graphic attached that shows the enterprise mobility guide in chaper 3, the table. What is the difference between the modulation and transmission type as many people refer to DSSS and OFDM as "modulation" I am a little confuse by this table.
Can anyone give me any more reasons and facts, and clarify any of the points above so when someone comes to me and says, "ahh, just roll out voice on B/G" I can say well this is why not?
Many thx indeed,
Ken

Answer 1

You have some good questions. The main reason why people are moving voice to 802.11a is the fact that there is more congestion in the 2.4GhZ spectrum. For example in a downtown area building. There are so many wifi devices out in the 2.4Ghz along with all the other interference that you get on that spectrum. 802.11a, the 5Ghz is a shorter wavelength, so the more it is susceptible to attenuation. The lower the frequency the better the signal can travel through various materials. Look at cell phones.... they are in the 800mhz or lower spectrum and they have greater distances than higher frequencies. Another reason is that if you have guest access, you have to look at devices you need supported. There might be legacy 802.11b clients or newer clients that only support 802.11b or 802.11g. So now the 5Ghz is open which give you more bandwidth for voice.
Water is an RF killer no matter if you use 2.4Ghz or 5Ghz.