127 lines
5.9 KiB
HTML
127 lines
5.9 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
|
|
<HTML>
|
|
<HEAD>
|
|
<META NAME="GENERATOR" CONTENT="SGML-Tools 1.0.9">
|
|
<TITLE>Sat (Satellite Technology) HOWTO: Technical Information</TITLE>
|
|
<LINK HREF="Sat-HOWTO-4.html" REL=next>
|
|
<LINK HREF="Sat-HOWTO-2.html" REL=previous>
|
|
<LINK HREF="Sat-HOWTO.html#toc3" REL=contents>
|
|
</HEAD>
|
|
<BODY>
|
|
<A HREF="Sat-HOWTO-4.html">Next</A>
|
|
<A HREF="Sat-HOWTO-2.html">Previous</A>
|
|
<A HREF="Sat-HOWTO.html#toc3">Contents</A>
|
|
<HR>
|
|
<H2><A NAME="s3">3. Technical Information</A></H2>
|
|
|
|
<H2><A NAME="ss3.1">3.1 General working</A>
|
|
</H2>
|
|
|
|
<P>We now try to understand how satellite connection works and at what conditions.
|
|
<P>We can imagine a satellite link as a classical Wireless link, I mean a
|
|
link between 2 systems which don't use a real cable to talk each other.
|
|
<P>Wireless link is very different from Wired link cause we have some additional
|
|
problems to solve, such as reachability, privacy problems and so on. Also there
|
|
could be weather problems, particularly in snow or rain conditions.
|
|
<P>Anyway, we have to consider the first principle behind Wireless communication:
|
|
line of sight free, which is a MUST unless we are unable to talk. For more
|
|
you can see the
|
|
<A HREF="http://www.fatamorgana.com/bertolinux/">Wireless-HOWTO</A>.
|
|
<P>In sat connections we use a special kind of antenna, a parabolic one, that
|
|
gives us a very high gain in RX, needed to receive satellite signal: in fact
|
|
satellite has a geostational orbital at 36.000 km and the only kind of antenna
|
|
we can use for receiving is just a parabolic one.
|
|
<H2><A NAME="ss3.2">3.2 Frequencies</A>
|
|
</H2>
|
|
|
|
<P>Frequency we receive is from 11GHz up to 12.7 GHz (from the satellite transponder,
|
|
the transmitter sending us datas), a very high freq., but the feed (converter
|
|
in the center of the parabola) converts it to, in output, 1-2 GHz so that we'll
|
|
able to send signal to the receiver through the cable (up to 40m depending
|
|
to cable loss).
|
|
<H2><A NAME="ss3.3">3.3 DVB Receiver Card</A>
|
|
</H2>
|
|
|
|
<P>
|
|
<PRE>
|
|
|
|
1 GHz Signal --> |RX|--> |ADC| --> |Low Level Network| --> |O.S. TCP/IP Stack|--> Data
|
|
|____________________________________|
|
|
DVB Card
|
|
</PRE>
|
|
<P>Now we can imagine a classical RX at 1 GHz receiving analog signals from
|
|
the Sat, converting it to digital signals and giving all to the low level network
|
|
layer (ISO OSI 1,2): here, card firmware builds a 2 level packet (pretty like
|
|
ethernet) to be sent to our PC with Linux, Windows, or other system, and in
|
|
the end, we will only have to transform it to a TCP/IP packet.
|
|
<H2><A NAME="ss3.4">3.4 Data Link Level DVB setting</A>
|
|
</H2>
|
|
|
|
<P>Here we have to config some settings, directly to the DVB card:
|
|
<P>
|
|
<OL>
|
|
<LI>Frequency working, we have to set the satellite frequency (as from magazines):
|
|
it is between 11.8 GHz and 12.8 GHz for example 12640000 KHz</LI>
|
|
<LI>Symbol rate, measured in symbols per seconds, typically 22 MS/s (Mega Symbols
|
|
per seconds)</LI>
|
|
<LI>Polarization, which is an antenna setting configurable in software: possible
|
|
values are H (Horizontal) or V (Vertical).</LI>
|
|
<LI>PIDs. The PID is used for select a trasmission between many signal from
|
|
same frequency. </LI>
|
|
<LI>Also we have to specify the bitfilter mask, which tells us how large is
|
|
the destination MAC addresses group (bitfilter mask is similar to TCP/IP netmask,
|
|
the only difference is that the bitfilter mask is 48 bit size while TCP/IP
|
|
netmask is 32 bit size). Always Internet data is sent with Unicast PID, while
|
|
Video always is sent using Multicast PID .</LI>
|
|
<LI>Calculate MAC address: here we specify what will be our MAC address (for
|
|
our DVB card). You need to do this cause some ISP uses an algorithm (see Appendix
|
|
A for more) which calculates your MAC address from your IP address, some other
|
|
ISP uses your own MAC address.</LI>
|
|
</OL>
|
|
<H2><A NAME="ss3.5">3.5 TCP/IP working</A>
|
|
</H2>
|
|
|
|
<P>As we said in 2.2 section, first we have to make a request using the modem
|
|
interface (i.e. ppp0 or whatever we use to reach Internet), then the answer
|
|
will return to our DVB interface (dvb0).
|
|
<P>Modern O.S. allow us to receive packets from an input interface, different
|
|
from the output interface from where we made the request: to do it we have
|
|
to "disable" some packets flow control, such as type an
|
|
<P>echo "0" > /proc/sys/net/ipv4/conf/dvb0/rp_filter (for Linux).
|
|
<H2><A NAME="ss3.6">3.6 Authentication</A>
|
|
</H2>
|
|
|
|
<P>It remains only one thing to complete our description: authentication method.
|
|
<P>Some Sat ISPs use the so called "Proxy Authentication": when you used their
|
|
proxy, you also need to give login and password to continue the request (you
|
|
should have been subscribed some kind of account to use their sat service):
|
|
once done, the ISP use your IP address to calculate your MAC address (see Appendix
|
|
A for more), to which send the answer.
|
|
<P>Some other ISPs require you make a VPN connection (using your login and
|
|
password) first, then they will control your registration account (where they
|
|
retrieve your MAC address) and will send data directly to (and only to) your
|
|
card (your MAC address).
|
|
<P>Anyway noticed that you can modify your dvb sat filter value to be able
|
|
to receive packets destined to EVERY mac address (related to a single frequency).
|
|
<H2><A NAME="ss3.7">3.7 Services I can use with a sat connection</A>
|
|
</H2>
|
|
|
|
<P>Typically services you can have from sat connections depend on what authentication
|
|
system is used by ISP:
|
|
<P>
|
|
<UL>
|
|
<LI>with classical "Proxy Authentication" you can only use HTTP and FTP service,
|
|
while</LI>
|
|
<LI>with VPN connection you have (ideally) no limit to services you can use,
|
|
the only limit is decided by RTT (time access) to satellite, about 500-1000
|
|
ms average: so you can forget to use "Voice" or other "Real Time" services!! (see
|
|
<A HREF="http://www.fatamorgana.com/bertolinux">VoIP-HOWTO</A> for more). Anyway you still can use: mail, chat, telnet, ping, dns and so
|
|
on.</LI>
|
|
</UL>
|
|
<HR>
|
|
<A HREF="Sat-HOWTO-4.html">Next</A>
|
|
<A HREF="Sat-HOWTO-2.html">Previous</A>
|
|
<A HREF="Sat-HOWTO.html#toc3">Contents</A>
|
|
</BODY>
|
|
</HTML>
|