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Here I report some technical info to understand basic Wireless environment.
At first layer ISO/OSI we can have 3 kind of spec:
- FHSS, Frequency Hopping Spread Spectrum
- DSSS, Direct Sequence Spread Spectrum
- Infrared connections, not covered by this howto
2 types of configurations:
- 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.
- 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.
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:
- same configuration mode: all Adhoc or all Infrastructure
- same physical layer: all DSSS or all FHSS
- same protocol (for example Proxim has its particular proprietary protocol
OpenAir that cannot talk with other FHSS cards).
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.
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/
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