Wireless HOWTO for Linux Systems

3. Technical info about Wireless

Here I report some technical info to understand basic Wireless environment.

3.1 Physical Layer

At first layer ISO/OSI we can have 3 kind of spec:

1. FHSS, Frequency Hopping Spread Spectrum
2. DSSS, Direct Sequence Spread Spectrum
3. Infrared connections, not covered by this howto

3.2 Configurations

2 types of configurations:

1. 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.
2. Infrastructure mode, where a number of networks (with a BSS each one) can communicate each other by means of an Access Point (one for each BSS) to create a ESS (Extended Service Set). 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.

In addition there are terms like indoor and outdoor to distinguish short area coverage from long area coverage.

3.3 Compatibility

Keep on mind that there are a number of Wireless cards in the world, but not every card can communicate with every other one. For talking together the cards have to use to:

1. same configuration mode: all Adhoc or all Infrastructure
2. same physical layer: all DSSS or all FHSS
3. same protocol (for example Proxim has its particular proprietary protocol OpenAir that cannot talk with other FHSS cards).

3.4 Should I use Adhoc or Infrastructure?

Access Point are very useful and killing problem but they are expansive. Ideally, for a more concentrated network you could use Infrastructure mode, while for few hosts you can choose Adhoc: why to spend much money for few hosts?

Anyway be aware that if you spend much money probably all works well while spending less you could have some trouble.

3.5 A Linux Box cannot act as an AccessPoint?

Good asking!

Recently there is a kind of hardware that allows this feature: Prism2

You can find something at http://people.ssh.com/jkm/Prism2/