111 lines
4.1 KiB
HTML
111 lines
4.1 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
|
|
<HTML>
|
|
<HEAD>
|
|
<META NAME="GENERATOR" CONTENT="SGML-Tools 1.0.9">
|
|
<TITLE>Wireless Howto: Technical info about Wireless </TITLE>
|
|
<LINK HREF="Wireless-HOWTO-4.html" REL=next>
|
|
<LINK HREF="Wireless-HOWTO-2.html" REL=previous>
|
|
<LINK HREF="Wireless-HOWTO.html#toc3" REL=contents>
|
|
</HEAD>
|
|
<BODY>
|
|
<A HREF="Wireless-HOWTO-4.html">Next</A>
|
|
<A HREF="Wireless-HOWTO-2.html">Previous</A>
|
|
<A HREF="Wireless-HOWTO.html#toc3">Contents</A>
|
|
<HR>
|
|
<H2><A NAME="s3">3. Technical info about Wireless </A></H2>
|
|
|
|
<P>Here I report some technical info to understand basic Wireless environment.
|
|
<P>
|
|
<H2><A NAME="ss3.1">3.1 Physical Layer </A>
|
|
</H2>
|
|
|
|
<P>At first layer ISO/OSI we can have 3 kind of spec:
|
|
<P>
|
|
<OL>
|
|
<LI>FHSS, Frequency Hopping Spread Spectrum </LI>
|
|
<LI>DSSS, Direct Sequence Spread Spectrum </LI>
|
|
<LI>Infrared connections, not covered by this howto</LI>
|
|
</OL>
|
|
<H2><A NAME="ss3.2">3.2 Configurations </A>
|
|
</H2>
|
|
|
|
<P>2 types of configurations:
|
|
<P>
|
|
<OL>
|
|
<LI>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. </LI>
|
|
<LI>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. </LI>
|
|
</OL>
|
|
<P>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).
|
|
<P>
|
|
<PRE>
|
|
Adhoc mode
|
|
|
|
A - - - - - C
|
|
\ /
|
|
| \ / |
|
|
/\
|
|
| / \ |
|
|
/ \
|
|
B - - - - - D
|
|
</PRE>
|
|
<P>In a Infrastructure environment you use the Access Point to which ALL other
|
|
hosts must connect to share the network.
|
|
<P>
|
|
<PRE>
|
|
Infrastructure mode
|
|
ESS
|
|
|
|
A - - - | - Access Point - - Access Point - | - - - D
|
|
|
|
B - - - | BSS1 BSS2 | - - - E
|
|
|
|
C - - - | | - - - F
|
|
</PRE>
|
|
<P>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.
|
|
<P>In addition there are terms like indoor and outdoor to distinguish short
|
|
area coverage from long area coverage.
|
|
<H2><A NAME="ss3.3">3.3 Compatibility </A>
|
|
</H2>
|
|
|
|
<P>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:
|
|
<P>
|
|
<OL>
|
|
<LI>same configuration mode: all Adhoc or all Infrastructure </LI>
|
|
<LI>same physical layer: all DSSS or all FHSS </LI>
|
|
<LI>same protocol (for example Proxim has its particular proprietary protocol
|
|
OpenAir that cannot talk with other FHSS cards). </LI>
|
|
</OL>
|
|
<H2><A NAME="ss3.4">3.4 Should I use Adhoc or Infrastructure? </A>
|
|
</H2>
|
|
|
|
<P>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?
|
|
<P>
|
|
<P>Anyway be aware that if you spend much money probably all works well while
|
|
spending less you could have some trouble.
|
|
<H2><A NAME="ss3.5">3.5 A Linux Box cannot act as an AccessPoint? </A>
|
|
</H2>
|
|
|
|
<P>Good asking!
|
|
<P>Recently there is a kind of hardware that allows this feature: Prism2
|
|
<P>You can find something at
|
|
<A HREF="http://people.ssh.com/jkm/Prism2/">http://people.ssh.com/jkm/Prism2/</A><HR>
|
|
<A HREF="Wireless-HOWTO-4.html">Next</A>
|
|
<A HREF="Wireless-HOWTO-2.html">Previous</A>
|
|
<A HREF="Wireless-HOWTO.html#toc3">Contents</A>
|
|
</BODY>
|
|
</HTML>
|