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Appropriate channels for use on a wireless network
Published Date : 10 Feb 2006   Last Updated : 28 Apr 2010   Content Ref: TEC597527  





Cause

Although 802.11a wireless access points will automatically select the most appropriate channel, 802.11b and 802.11g wireless access points need to be configured or re-configured manually if they are near each other.


Procedure


Overview of wireless networking standards:

802.11a 802.11g 802.11b
Radio frequency 5GHz 2.4GHz 2.4GHz

Nominal data rate
(Fall back speeds at distance)

54Mbit/s
(48,36,24,18,12,6)

54Mbit/s
(48,36, 24,18,12,9,6
then 5.5,2,1)

11Mbit/s
(5.5,2,1)
Theoretical max throughput
(Throughput shared by clients)
24Mbit/s
24Mbit/s
(6Mbit/s with 11b)
6Mbit/s
Number of non-overlapping channels
(Channel numbers)
8
(36, 40, 44, 48,
52, 56, 60 and 64)
3
(1, 6 and 11)
3
(1, 6 and 11)

Note: 802.11n is at the draft approval stage and is yet to be ratified fully. Please see the 'Other useful links' section at the end of this article for details of RM's recommendations about the adoption of Draft 802.11n devices.

802.11b and 802.11g

As we can see from the above chart, 802.11b and 802.11g operate on the 2.4GHz radio frequency. In Europe the spectrum is divided into 13 overlapping channels whose centre frequencies are 5 megahertz (MHz) apart. Channels with less than 25MHz separation are considered to overlap and interfere with each other, resulting in a reduction in the available bandwidth and in some instances, unreliable wireless connectivity.

The wireless signal on the 2.4GHz radio frequency has a range of 120m in an open environment and 30m in an office environment. Therefore, wireless access points configured with the same channel number should be at least these distances apart.

Note: These figures can be reduced by physical obstacles and interference and so it may not always be possible to obtain maximum range from a wireless access point. With this in mind, the above ranges should be used as a guide to the potential maximum range of a wireless access point.

The three non-overlapping channels for 802.11b and 802.11g are 1, 7 and 13 in Europe. However, due the widespread use of North American specification equipment (which only supports up to channel 11) the recommended channels are 1, 6 and 11.

Channels should be separated by at least 25MHz. It is recommended that if a number of rooms positioned next to each other require wireless access points to be installed, then the channel numbers should rotate from room to room, eg 6, 11, 1, 6, 11 and so on. If the access point is still too close to an access point on the same channel you should consider reducing the power setting on each access point or deactivating the radio on the access points in alternate rooms.

Ultimately, you are fundamentally limited to a maximum of three 802.11b/g APs in any one area.

802.11a

802.11a operates on the 5GHz radio frequency and provides a total of 8 non-overlapping channels. The considerations required in 802.11a are the same for 802.11b and 802.11g. That is, if a wireless access point with the channel is in close proximity to another wireless access point with the same channel, they will interfere with each other, causing a loss of throughput.

However 802.11a products need to be configured for automatic channel selection. Some access points use a fixed algorithm to select the channel so avoid starting them at the same time, this will allow one to choose the channel using the algorithm and the next one to detect the first before selecting its channel.

The wireless signal on the 5GHz radio frequency gives a range of 30m in an open environment and 10m in an office environment. As with 802.11b and 802.11g, wireless access points configured to use 802.11a should be at least these distances apart.

Note: These figures can be reduced by physical obstacles and interference and so it may not always be possible to obtain maximum range from a wireless access point. With this in mind, the above ranges should be used as a guide to the potential maximum range of a wireless access point.

The 802.11a standard is inherently more suitable for the type of high density deployments that are common in schools and colleges. The combination of the shorter range and greater number of channels allow a high density of access points to be co-located in any given area. RM recommend this as the preferred technology, and where tri-mode equipment is available it should be configured for 802.11a use.



Possible Issues

Interference

In addition to physical obstacles, there are a number of other sources of interference that could have an adverse affect on the performance of a wireless network.

For wireless networks using the 2.4 GHz radio frequency, sources of interference could include the following:

  • Microwave ovens.
  • Bluetooth enabled devices.
  • 2.4GHz wireless phones.
  • Other nearby wireless networks.
  • Electrical devices such as power lines.
  • Large metal cabinets (for instance fume cupboards in science labs)
  • Excessive metal pipe work.
  • Any other devices that operate in the 2.4 GHz bandwidth.

Tips for avoiding interference and poor signal quality

  • Change your wireless channel - If you feel that your wireless network may be suffering from interference, change to a different wireless channel.
  • Analyse the potential for interference - Before purchasing any wireless equipment, RM recommend that you arrange for a wireless survey to be completed.
  • Provide adequate wireless coverage - RM recommend that one wireless access point is installed for every eight-ten wireless devices e.g. workstations, notebooks or tablet PCs.
  • Deploy 802.11a - 802.11a operates on the 5GHz radio frequency and so is less susceptible to interference than wireless networks operating on the 2.4GHz frequency.
  • Physical location - Position your wireless access point in a central location, high up in the room to obtain maximum coverage and avoid physical obstacles.


Other Useful Articles

Introduction to wireless networks (DWN243055)

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Document Keywords: overlapping, band, 802.11a, 802.11b, 802.11g, best practice, access point, setup, interference, 802.11x, 802.11


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