How WiFi Works
Written by Harry Fairhead   
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Channels

The most popular wireless networking standards use the 2.4GHz band and they divide it up in the same way and use one of the spread spectrum methods to share it.

Exactly how this is done can help understand how to set up a network and what can go wrong. The 2.4GHz band is divided into 14 channels separated by 5Mhz. Only 11 channels are available in the US and 13 in the UK with Japan allowing all 14.

When a wireless network is established you set the channel it will operate on. The channels may only be separated by 5MHz but the spread spectrum uses 25MHz centred on each channel.

 

channel

The channel spacing is smaller than the range of frequencies used!

 

What this means is that if two wireless networks are close, in the same building say, and set to channels 1 and 2 they will both be trying to use the overlapping frequencies. Without spread spectrum techniques this would be a disaster with neither network able to operate. However, in many cases the two can share the frequency range but with a reduction in data rate.

 

To avoid this problem wireless networks that are within each other’s range should optimally be set to non-overlapping channels, e.g. channel 1, 6 and 11. By repeating the pattern you can cover large areas with non-interfering wireless networks.

 

overlap
Non-overlapping channels give trouble free coverage

 

overlap2

By repeating the pattern channel 1, 6 and 11 you can cover any area without interference.

 

Ad-hoc and infrastructure modes

As well as the radio link part of wireless networking there is also the problem of how to integrate a wireless network with a wired network. There are two basic modes of operation – ad-hoc and structured. A group of computers that can communicate using a wireless link are called a Basic Service Set, or BSS.

An ad-hoc network also known as an Independent Basic Service Set (IBSS) is the simplest because all of the machines communicate with one another in a peer-to-peer or workgroup network. When two or more IBSS operate as a single workgroup the result is called an Extended Service Set, ESS.

The alternative is Infrastructure mode, which makes use of special Access Point (AP) wireless device. This is a stand-alone box that only needs a network connection and not a PC to operate. The AP acts as a master and controls all transmission within a BSS. It has management algorithms and transmits special control packets to its clients to make best use of the available bandwidth. The AP also connects to a wired network and acts as a wireless/wired bridge passing data packets in both directions. Two APs never talk to each other via the wireless link and always transfer data between themselves via the wired network.

As well as allowing wireless connected machines to integrate with a wired network, an AP also provides extra facilities such as broadcasting the network identifier, the Service Set Identifier (SSI), allowing users to discover that a network is available. An AP also enables “roaming”. That is, if a user moves around from one BSS to another, as long as the access points are using the same SSI then the user will be automatically handed over as one AP goes out of range and another comes into range.

Infrastructure mode is so much better than ad-hoc that some wireless networking cards can act as APs, even though they aren’t standalone and need a PC to operate.

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Last Updated ( Thursday, 21 July 2011 )
 
 

   
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