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PACKET
RADIO
Packet Radio is yet another data mode used for sending text messages in
digital form from one station to another. There are two things that separate
Packet from other data modes. One is the fact that full error correction
is used, and the second is that more than one contact can take place on
one frequency at one time. These two advantages have led to a network
over the world of real-time and non-real-time contacts. Packet can be
used in the same way that RTTY or PSK31 is used on HF, but is slower by
comparison.
The REAL advantage
is the non-real-time use for the mode. If you look at Packet as a container
for a message with an address along with it, then the data is contained
within the Packet and if the data, or message, is long, then it is broken
down into smaller parts called frames. These are sent sequentially, and
each frame consists of a start "flag", an address, the text
and an ending "flag". Each frame is identified by the receiving
station by a unique number and if this co-incides, then the receiving
station sends an acknowledgement and the next frame is then sent. If the
two numbers are different, the receiving station asks for a repeat. You
can therefore see how more than two stations can exist on one frequency
at a time, with this type of corrective sequence. It is not necessary
to go into lengthy technical discussions of the operations of the TNC,
so I will merely explain the setting up of a typical station.
Installation
A typical installation
for a user would be a computer, a TNC (Terminal Node Controller or Modem)
and a transceiver. Using the equipment rather depends on what the user
wishes to do. However, since a simple installation can cover most uses,
I will describe these as we go along. The "typical" installation
I just covered is not strictly true in the year 2003. When packet radio
was first introduced, a TNC was essential, but now there are software
solutions that have taken over as a cheaper way to obtain the same, or
better, results. So, the PC with a sound card is the basis of a packet
( or any data mode ) station. The software to use is written by SV2AGW
and is available to download on the Internet. Once installed, it will
allow you to run 300 baud packet for HF operation, 1200 baud for local
use and also 9k6baud for local use, if there are any other similar users.
Information
One of the first
things to do is to read as much as you can. There are several books available
to introduce you to all the advantages of Packet Radio, and the following
would be a good start:
Packet Radio Primer
by Dave Coomber G8UYZ and Martin Croft, G8NZU
Your Gateway to Packet radio by Stan Horzepa WA1LOU
Digital Modes for All Occasions by ZL1BPU
These are readily
available and will be a worthwhile investment.
Do a search on the Internet, and probably one of the best places to start
would be the BARTG
Web Site. In any case, if you are serious about Data communications,
you should join BARTG. I think there are some tutorials on there but a
check would be a good idea.
Setting Up
Setting up the station
has to be done properly in order to gain full advantage of the system.
One of the most important things to check is your deviation. Assuming
you are using an FM transceiver, set up your deviation so that it is not
more than 2.5kHz for a 12.5kHz channel. Most channels on two meters should
be 12.5kHz now. Then check your tone frequencies and make sure they are
accurate, and lastly the various parameters on the TNC or in your software
program. The most important of these are the timing parameters. If these
are not set correctly you will not connect to the other station.
TXD 65
MAXFRAM 1
RESP 15
DWAIT 4
Ppersist 60
Frack 7
Slottime 120
Retries 10
Paclen 200
These are the most
important to get right in the first place. Once you have these set correctly
then try a connect with another station. If all works, then you are set
to proceed further.
Uses of Packet
One of the most common
uses for the mode is the accessing of the local Bulletin Board System
or BBS. This is run by a volunteer, who leaves his equipment running 24/7
with a PC and a Mailbox program. This is where the non-real-time use comes
into its own. It's a bit like posting a letter. You compose it, either
off-line or on-line and then send it, addressed correctly to the BBS.
The BBS then routes it to the address you have given it and -hopefully
- delivers it to your correspondent. If the route is within the UK, then
it follows a terrestrial network around the houses until it ends up at
the nearest BBS to your correspondent. He then reads it, and replies and
the reply comes back via the same route.
If the mail is for
abroad, say Australia, then your mail is routed via the UK Network to
GB7LDI, the Satellite Gateway. It is
then automatically up-loaded to an amateur satellite, UO-22 carrying store
and forward software, and is downloaded when the satellite is above Australia,
by a VK Satellite Gateway. It then follows the VK terrestrial route to
its destination. This routing has been spoiled lately by BBS changing
over to Telnet, an Internet based telephone forwarding system, non-amateur.
The Internet has been responsible for a decline in the interest in Packet
Radio, but having described the journey of mail destined for VK, using
an amateur RADIO based network, surely this should be supported instead.
There is more satisfaction from talking to a VK on the air than phoning
him!
You can find our more about the Satellite Gateway system from another
article I have written, also on here.
The BBS has all sorts
of servers, all to make your use of it much more interesting. You can
ask to have your mail sent and collected by the BBS, and by leaving your
gear on as well, you can have bulletins of just your interests, sent to
you too. Conferencing is possible using the BBS, with several stations
on at once this can prove quite interesting, just like chatting.
There are various means of asking for all sorts of information, from a
histogram of the use of the BBS to orbital data for about 40 different
satellites.
The teletext server has lots of interesting articles and up to date information
on all sorts of subjects. The Library is another place where you can spend
a few hours just reading. There is a full call-book server on CD ROM,
and also an up-to-date QSL manager server.
If you get fed up with the BBS, then take a look at your local DX Cluster.
If you are interested in HF DX-ing this is the place for you. You don't
have to listen on the bands for hours on end any more. Just monitor the
DXX Cluster and watch the spots come up. Then just tune your transceiver
to the frequency given and - hopefully - work him! Propagation can be
checked and the latest solar data found here. You can even use your packet
station to connect to the International Space Station!
It is far too complex
a subject to cover it in full here so take my advice and invest in a book
or two. Software is all important and if you are an end user of a BBS,
take a look at Winpack
or TPK, or as I suggested above, the SV2AGW
program. Above all, enjoy it, and if you don't understand something,
never be afraid to ask. We all had to start and we all had to ask.
If the person you ask takes offence or laughs at a simple request, then
he is not worth talking to. Just ask somebody else. A genuine Radio Amateur
will always help.
Above all, enjoy Packet
Radio - whatever you decide to do with it.
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