Wi-Fi

Wireless LAN spectrum management

Learn how to design a Wireless LAN that minimizes interference issues for optimum performance and reliability.

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:

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