3. Technical Information

3.1 General working

We now try to understand how satellite connection works and at what conditions.

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.

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.

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 Wireless-HOWTO.

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.

3.2 Frequencies

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).

3.3 DVB Receiver Card

 
1 GHz Signal --> |RX|--> |ADC| --> |Low Level Network| --> |O.S. TCP/IP Stack|--> Data
                |____________________________________|
                                DVB Card

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.

3.4 Data Link Level DVB setting

Here we have to config some settings, directly to the DVB card:

1. 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
2. Symbol rate, measured in symbols per seconds, typically 22 MS/s (Mega Symbols per seconds)
3. Polarization, which is an antenna setting configurable in software: possible values are H (Horizontal) or V (Vertical).
4. PIDs. The PID is used for select a trasmission between many signal from same frequency.
5. 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 .
6. 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.

3.5 TCP/IP working

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).

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

echo "0" > /proc/sys/net/ipv4/conf/dvb0/rp_filter (for Linux).

3.6 Authentication

It remains only one thing to complete our description: authentication method.

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.

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).

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).

3.7 Services I can use with a sat connection

Typically services you can have from sat connections depend on what authentication system is used by ISP:

• with classical "Proxy Authentication" you can only use HTTP and FTP service, while
• 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 VoIP-HOWTO for more). Anyway you still can use: mail, chat, telnet, ping, dns and so on.

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Generated: 2007-01-26 17:57:56