2. Wireless networking.

2.1. What about Wireless

Wireless is a new technology that can help you to connect computers at distance. It works with Wireless cards with a TX/RX inside at 2.4 GHz/5 GHz while the software interface is Ethernet-like, with an hardware address different for each card in the world. Typical transmit power is 10-20 mW till 100mW (see standard IEEE 802.11 and FCC/CEPT licenses).

2.2. What's the max distance between radio cards?

The most important thing in Wireless communications is the line of sight clear: you MUST SEE (with eyes or with a binocular) the antenna from the other end or you can have (at most) a little tree between them.

The distance depends on the antenna and (eventually amplifier) used: 2-300 meters with a omnidirectional antenna; 1 km with a directive one; 2-3 km with a omnidirectional amplified (200mW); some km with parabolic antenna. 50-60 km with parabolic or directive antenna amplified (some Watts).

Be aware that it is not always legal to amplifier Wireless cards, cause you could violate FCC/CEPT (and also your country relative) specifics.

2.3. What's the difference between wired and wireless networks?

I assume that you are allready familiar with the bassics of wired networks, at least you should know what is a IP used for and what routing does in a network. When i speak about networks i mean a group of computers which share the same Ip address space. This could be a LAN, WAN or even a MANET ( Mobile ad-hoc network).

Wired networks are very simple to setup (at least at low level). Wireless networks are very difficult to setup, to manage, to debug... Typical problem with wired networks like hardware install, software install, debug and so on become very critical with Wireless:

In wired networks we got IP addresses which help us to distinguish between different computers or networks. Thats not so different from wireless networks, here are also IP adresses used , but there is a tiny difference.

While in wired networks we got wires to separat oure networks physically we are not physically separated in wireless networks. This means more networks share one physicall area. To be able to separate between different networks in a wireless network we got some parameters to help us knowing which computewrs or nodes are in oure network group.

• ESSID : The ESSID is basically a name used to define a wireless network group, if you have ever used Windows filesharing or Samba you are familiar with Workroups, an ESSID is very similar to that, it helps us to know which nodes are in oure network.

• CHANNEL : The bandwith used by a wireless devices is splitted into channels, this basically reduce radio noise produced from other wireless network groups in oure area. Think on a walkie talkie, you got different channels, and when one channel is in use by someone you have to use an other one, in a wireless network different network groups can share one channel, but when they get more it will get more noisy.

  In the US are 11 channels. In Europe are 13 channels.

There is one more parameter which is not so important but we will mention it here. Since there are wireless devices which can handle different data transmission rates, they need a way to deal a data transmission rate of which both devices are capably, when they negociat. Therefore is the RATE parameter. The default setting of this parameter is "auto" and should not be changed until you realy know what you are doing.

2.4. Wireless network access modes.

There are two types of wireless networls, the differece is in how the nodes access each other.

• AdHoc mode (also called Independent mode), where you have independent networks with a BSS (Basic Service Set) each one. Each station has the same BSS. In this mode all nodes are independant from each other, every node can talk to all nodes close enough to send and receive radio signals. It's a decentral network structure

• Infrastructure mode, where a number of networks (with a BSS each one) can communicate with each other by using an Access Point, it's a more centralized network, easyer to manage . But if the Access Point fail's for some reason and there is no other to take over the whole network managed by this Access Point is down. Also there is a roaming function letting a station "attach" to the nearer Access Point.

Adhoc is the simpler method (and the also the less scalable) and let many hosts communicate each other directly. The restrictive requirement is that all one are to be visible directly to reach a complete coverage of the network. (at least Ideally, because this problem could be solved at IP level! For more see Par 5.4).

	
                                 Adhoc mode 

                               A - - - - - C          
                                 \       /            
                               |   \   /   |             
                                     /\                              
                               |   /    \  |   
                                 /       \     
                               B - - - - - D
		

In a Infrastructure environment you use the Access Point to which ALL other hosts must connect to share the network.

Infrastructure mode

 
                                    ESS
 
          A - - - | - Access Point - -  Access Point - | - - - D
 
          B - - - |   BSS1                   BSS2      | - - - E
 
          C - - - |                                    | - - - F
		

B and C could not see D,E and F, but they can communicate as well cause all one are using the same ESS. Important: A,B and C could also not see each other.

2.5. Security in wireless networks.

There have been different security methodes developed, since wireless networks are very open to attacks. But while this HOWTO is written for open community networks, i will not go to discuss this here.