Contents

  • Introduction to 802.11 radio spectrum
  • 2.4 GHz spectrum management
  • 5.3/5.8 GHz spectrum management

Introduction to 802.11 radio spectrum

In North America including the USA and Canada, two unlicensed radio bands
(technically 3 if you count 5.3 and 5.8 as 2 separate bands) are used for IEEE
802.11 wireless LAN transmissions.  The 2.4 GHz band is very narrow and only
supports 3 non-overlapping channels but unfortunately hosts the vast majority of
wireless LAN devices because of backward compatibility issues.  The wider 5
GHz bands which mostly go unused have an 8-channel block of spectrum in the 5.3
GHz range and a 4-channel block in the 5.8 GHz range for a total of 12
non-overlapping channels.

802.11 originally used the unlicensed
2.4 GHz band with the 802.11b standard, but later expanded to the 5.3 and 5.8 GHz
band with the 802.11a standard, which offered less congestion and higher speeds. 
802.11g was later added and it offered higher speeds and compatibility with
802.11b.  The only downside to 802.11g was that it also operated in the
congested 2.4 GHz band, and any 802.11b device on the same Access Point (AP) forced
all 802.11g devices to drop down into 802.11b mode, eliminating all the speed
benefits of 802.11g.  In 2006, the
IEEE 802.11n standards finally settled on a draft standard for MIMO (Multiple In
Multiple Out) transmission technology and it does not
dictate 2.4 or 5 GHz operation.  However the vast majority of commercial
pre-N
(no guarantee that “pre-N” is upgradeable to 802.11n when it comes out) products use only the 2.4 GHz band due to cost considerations.

The 5 GHz range is mostly used by high-end and
high-density enterprise-class wireless LANs in corporate or education
environments.  But even in those environments, a lot of the client devices
either only support 2.4 GHz or they were not configured to use the 5 GHz band. 
Not helping the situation is the fact that many devices like
Wireless VoIP phones and bar code scanners are only sold in 2.4 GHz
configurations.  Home-based Wireless LANs were ramping up 5 GHz usage in 2004
with dual-band 2.4/5 GHz for power-users, but that has taken a turn for the
worse since the arrival of “Pre-N” devices which are almost exclusively 2.4 GHz
because the additional radios used for MIMO already increased the price of the devices
considerably.  The result is a heavily congested 2.4 GHz band.

2.4 GHz spectrum management

With only 3 non-overlapping channels in the 2.4 GHz band, there isn’t much
flexibility in AP and antenna placement.  If we look at Figure A,
we can see how the 2.4 GHz spectrum is divided into three channels for 802.11b
and 802.11g.  However, there is a “cheat” for the 2.4 GHz spectrum in which four channels
are jammed closer together to minimize interference, but gain an extra channel;
this method is shown in Figure B.

Figure A:

Figure B:

Having that fourth channel available is useful because it offers more
flexibility.  You can also reserve the fourth channel for testing and lab
work.

In Figure C below, similar channel APs are kept to a maximum.  It is
divided into three colors with red, green, and blue APs in their
respective lighter-colored cells.  Since the cells have gaps in them, you
use the other channels to fill in the gaps.

AP placement is not the only thing that needs to be addressed in proper
spectrum management; cell sizes must be adjusted such that they don’t interfere
with the nearest AP on the same channel.  This is why in Figure C and D,
the circles belonging to an AP should never exceed the half-way point to the
nearest AP on the same channel.  These “circles,” however, are only
valid in free space and don’t account for obstructions and should only be used as
general guidelines.  Only a physical wireless survey can accurately predict coverage
cells; we will go in to site surveying in next week’s wireless article.  Also note that the lines represent borders for a certain dBi level
and not where the signal actually ends.  Wireless RF signals don’t actually
stop cold at a certain distance; they propagate forever and weaken by the square
of the distance from the signal source.  So these lines, for example, would represent – 80 dBi which is the
signal level considered to be the limit at which 802.11 devices cease to be
effective.

Figure C:

Note that when switch-based wireless LANs are used, intelligent switches that
control large numbers of APs will automatically manage power and
channel selection so that you simply need to place APs whereever more bandwidth
is needed.  This is the reason that anyone considering more than six APs should seriously consider using a switched architecture—because of the
ease of deployment and manageability.  It cuts out the majority of work in
deploying and designing a wireless LAN which offsets the extra cost in hardware.

5.3/5.8 GHz spectrum management

Most of the theories of 2.4 GHz management hold true for 5.3/5.8 GHz
management.  The only difference is that the 5 GHz signals don’t penetrate
walls as easily as 2.4 GHz signals and channel selection is much easier since
the number of channels is so abundant.  There are eight non-overlapping
channels in the 5.3 GHz range, and there are 4 in the 5.8 GHz range.  One thing to watch out for is that
cordless 5 GHz phones usually operate in the 5.8 GHz range, so it may be a good
idea to avoid using the 5.8 GHz band.

Since nearly every 802.11a AP has two radios to support 5 GHz and 2.4 GHz, Figure D shows a mixed
802.11 a/b/g environment.  It uses the same pattern for 802.11 b/g, using
three channels with 1, 6, and 11, but uses six channels for 802.11a with channels
36, 40, 44, 48, 52, and 56.  Cisco has an excellent

document on all spectrum allocation in various countries.  It also
contains useful information on radio power levels and gain.

Figure D: