Return to VK3YE historical interest articles
|
Packet radio
Put simply, packet radio is a means of
exchanging computer data by radio. With the extension of Novice (= current Standard) license privileges and
the proliferation of home computers, packet radio usage increased in the 1990s. Usage declined in the 2000s, and this article is mainly presented for curiousity.
History
Packet transmission is not new; the basic
technique dates back to 1964 (reference 2). American research led to the
creation of the packet-based ARPANET in 1969, while radio transmission was
first used by the Hawaiian ALOHANET the following year. Amateur packet
experimentation began in Montreal, Canada in 1978. Later that year the first
packet radio repeater (digipeater) was installed. Experimentation with packet
radio continued in North America through the 1980s. In 1982 the AX.25
transmission standard (or protocol) was developed. AX.25 (which is still in use
today) is a modification of the standard X.25 protocol. In the ensuing years,
the system was refined and has since spread worldwide.
How Packet Works
It is not until you become involved with
digital communications that you realise how complicated it is to establish and
maintain communication over a radio link. What is implicit and taken for
granted in voice modes must be made explicit to computers in a network. This is
why computers have to formally 'connect' to each other to exchange data.
Likewise, to end a contact, the operator must consciously disconnect his/her
terminal from the other station. These functions are performed by special
software or a device called a TNC. More on this later.
When a user types a message at a packet
terminal, it is broken up into small units of text, to be sent one at a time.
Each unit of text (or frame) may be up to 256 bytes long. Frames also include
additional information, such as the originator of the message, the callsign of
the station to whom the packet is addressed, and control data.
AX.25's error protection feature ensures
that packet transmissions are received as sent. When there is interference, a
packet link will continually re-send packets until the receiving station has
received acknowledgment from the sender that the message has been received
correctly. If the interference persists, and the message cannot be passed, the
packet system will give up after a set number of retries. All this is done
automatically; no human intervention is required.
What Packet can do
Packet radio is a great system for leaving
people messages. The mode is also handy for disseminating written material to
large numbers of people; readers can save messages to disk to read later. If
you need information on a piece of equipment, or want to find out where to
obtain some esoteric component, a message posted on your local bulletin board
system (BBS) will often bring results. Because bulletin boards are linked to
each other via HF, VHF, UHF, or satellite links, you do not need HF to send
messages interstate or overseas.
Information on a variety of amateur radio
topics is available through BBS systems. Most WIA divisions post their weekly
broadcasts on packet. You will also see amateur satellite keplarian elements,
DX information, contest rules and technical 'digests', to name a few of the messages
passed each day through the packet network. Figure One shows a typical BBS's
listing of messages. Packet users can also converse via the keyboard, though
such contacts are slow and impersonal compared to voice operation.
Packet Equipment
There are two main ways of transmitting
packet; one hardware-based and the other software-based. If you opt for the
hardware method, you need a device called a Terminal Node Controller (TNC)
which fits between your radio and computer. TNCs contain circuitry that assembles
computer data into individual packets for transmission. During reception, the
reverse process occurs, with received packets being converted to text that is
displayed on the screen. TNCs also contain a modem, which converts computer
data (after it has been assembled into packets) into audio tones which are then
transmitted. Once again, this is done in reverse for reception. TNCs suitable
for VHF/UHF packet operation cost around 250 dollars.
With the software approach, you can operate
packet without a TNC by using a simple modem and special software that emulate
a TNC's functions. If you already own a suitable computer, this method is the
cheapest way of transmitting packet. With guidance from a more experienced
amateur, it is not difficult to convert a 1200 baud telephone modem for packet
operation.
My own ex-telephone modem required a couple
of circuit board tracks to be cut, a few extra connections and a simple
transistor switch circuit to operate the radio's PTT. Assuming you have an
IBM-compatible computer, all that remains is to obtain a copy of Baycom
shareware, and connect the modem to the computer's serial port and the radio.
Most packet activity occurs on two metres,
though there is some on UHF. Any synthesised hand held or mobile FM transceiver
should work on packet, provided there is access to the microphone, speaker and
PTT connections. Alternatively, you may convert an ex-commercial crystal-locked
transceiver for packet. A Philips FM 828 is a good choice. The main
disadvantage of this approach is the high cost of crystals.
A vertically-polarised antenna similar to
that used on FM will normally suffice. It should be mounted as high as
possible. Two metre packet frequencies vary; some areas use frequencies around
145 MHz, while others operate near 147.6 MHz.
Figure Two shows a typical VHF/UHF packet
station (refer to original article - not included on Novice Notes Online).
Operating Packet
Packet can be confusing to the newcomer, but
several weeks operating should allow you to become quite proficient. Before you
can get on the air, your callsign and other settings must be programmed into
your TNC or emulation software. Either the instruction manual for your TNC or
another packet user can assist.
Once you have found out the frequencies used
in your area, it might be a good idea to try receiving local packet activity.
As you hear the raucous packet tones through the speaker (and displayed on your
modem's LEDs), you should be rewarded by lines of text appearing on the screen.
A lot of these will be just callsigns, but you should be able to see people
having packet conversations with others, or reading messages from bulletin
boards.
Once packet reception has been mastered, try
listening for the DOVE amateur satellite. If you leave your packet system on
overnight, tuned to 145.825 MHz, in the morning you should see a brief message,
followed by lines of telemetry code from the satellite. Because of its low
orbit, DOVE's signals are very strong.
Initially, try to arrange keyboard
conversations with nearby amateurs to prove that your packet station is
working. Later, you could try connecting to BBSs and other packet users, some
of which leave their equipment operating 24 hours a day. Keep a list of
stations to which you are able to connect; using these stations as digipeaters
is handy if unable to access stations directly. As packet requires almost a
noise-free signal, the your station's range on packet will be somewhat less
than what you'd expect from FM voice operation. This is especially so where
many are simultaneously transmitting on the one frequency.
A Session on the BBS
After you have experimented with receiving
packet, and perhaps had a few keyboard contacts, it's now time to log on to a BBS.
You must decide on a home BBS, to which messages sent to you by other amateurs
will be sent. It is probably wise to make the closest BBS your home BBS. As the
BBS will not recognise your callsign the first time you connect, it will ask
you to register. This is a simple process, and normally entails answering a few
questions. If you can reliably use the BBS without need for digipeaters, so
much the better, as operation will be faster.
While connected to a BBS, you may send or
receive mail, as well as read general bulletins. As a new user, there will be
no mail addressed to you (unless prior arrangements have been made), so you
should start by reading bulletins. Refer to the list of commands below. For
instance, if you would like to list the last 20 bulletins, type in 'll 20'.
Within a couple of minutes, you should see a list of these messages rolling
down the screen (see Fig 1). Each message is numbered. To read the contest
calendar, for example, you would type 'r 30953'.
Every packet user has a packet address to
which mail for them is sent. An address consists of your callsign, the callsign
of your home BBS followed by various geographic identifiers. For instance, as
VK1BBS is my home BBS, my address is VK1PK@VK1BBS.ACT.AUS.OC.
After some experimentation, you should be
able to do the following:-
* list bulletins sent to a single subject
designator (eg WIA, DX, QRP, etc)
* issue bulletin messages within one's own
state, nationally, or worldwide
* read messages and save them to disk
* send mail to other packet users
* post text stored on disk on to the packet
network
This list is not exhaustive, but should give
you an idea of packet radio's capabilities. Patience is a great virtue when
operating packet; when many are sharing the one frequency, there is a risk of
packet 'collisions'. These slow the rate of data transfer. The problem is
particularly acute in a hilly city (such as Canberra) where many packet
stations are shielded from each other. The necessity for people to use
digipeaters in such a situation only adds to the delays.
Conclusion
This article has provided a basic overview
of VHF/UHF amateur packet operation. While the information contained herein is
insufficient on its own for a complete newcomer to assemble a packet station,
with guidance from a more experienced amateur and/or the references below, it
should provide a sound starting point for experimentation.
Packet Commands
The following list gives some commonly used
packet commands.
b bye - used after you have finished using a
BBS
c connect - use when you want to get on to a
BBS
km kill messages addressed to me (do this
after you have read messages to you)
l list messages unlisted since previous time
on BBS
lm list messages to my station
l n list message number n
l< vk* list bulletin messages from VK
amateurs only
rm read messages to me
r n read message number n
sb send bulletin message (to everyone)
sp send personal message (to a specific
person)
Glossary
1200 baud:- The data transmission rate of most VHF amateur
packet. Higher speeds (such as 4800 and 9600 baud) are increasing in
popularity.
AX25:- the technical standard (protocol) to which amateur packet conforms.
BBS:- Bulletin Board System - a computer system connected to a TNC and
transceiver where messages can be left for other users. Where BBSs are linked
to other BBSs, messages can be sent worldwide.
Bit:- The smallest unit of digital information (either 0 or 1).
Bulletin:- A message addressed to everyone. You can see them
listed on your local BBS. If you want to read a particular bulletin, type in
the bulletin's message number, and it will be displayed on your screen.
Byte:- A group of bits (normally eight).
Digipeater:- A packet radio repeater. Unlike a conventional FM
voice repeater, digipeaters receive, temporarily store and re-transmits
incoming packets. Used if you are unable to connect to a BBS or other packet
station direct. Any packet station left running will function as a digipeater.
Mailbox:- Similar to a BBS except this is used privately by one person. You can
send messages to people with mailboxes without going through a BBS.
Sysop:- person who runs a bulletin board system (short for SYStem OPerator).
TNC:- Terminal Node Controller - the black box between your radio and the
computer. Not required if you are using TNC emulation software (such as Baycom
and Digicom).
TPK:- A means of automatically receiving messages for you from a BBS
without you needing to manually connect to it. Well beyond the scope of this
article.
Wormhole:- a link between two packet bulletin boards via a
telephone line or through Internet.
References and Further Reading
1. Day, J Packet Racket, ARA/RAC June -
September 1995
2. Horzepa, S Your Gateway to Packet Radio,
ARRL, 1989
Figure 1: A typical listing of messages
on a bulletin board system
Msg# TSLD Dim To @ BBS From Date/Time
Title (LC-choice: *)
30954 B$L 17546 WIA @VKNET VK1PK 0822/2106
VK1WI NEWS 23/8
30953 B$ 4313 WIAQ @VKNET VK4BB 0822/2014
contest calender
30952 B$ 9398 NEWS @AMSAT K5ARH 0822/2012 *
SpaceNews 21-Aug-95 *
30949 B$ 7203 KEPS @AMSAT K5ARH 0822/1942
2Line Orb. Elements 230.AMS
30948 B$ 2761 ALL @AMSAT SP2UKA 0822/1851 Cd
listow !! Hi Hi....
30947 B$ 2940 KEPS @AMSAT K5ARH 0822/1850
Orbital Elements 230.MISC
30946 B$ 1126 AMSAT @AMSAT YV5AMW 0822/1845
INFO REQUEST
30945 BFL 9429 LOGS @ZAOIP ZAOBBS 0822/1731
Logfile VK1ZAO 1995 wk33
30944 BFL 482 LOGS @ZAOIP ZAOBBS 0822/1731
Epurmess.Log
30934 B$ 1499 MIR2 @VKNET VK1DSN 0822/1453
Mir-2 Docking Module
30933 B$ 3362 SPACE @VK1 VK1DSN 0822/1451
Refurbished Wind Tunnel
This article appeared in Amateur Radio December 1995.
Items were chosen for likely usefulness and a satisfaction rating of 4/5 or better.
|
Books by VK3YE
Ham Radio Get Started (USA)Australian Ham Radio Handbook (Aust) More Hand-carried QRP Antennas 99 things you can do with Amateur Radio Getting back into Amateur Radio Illustrated International Ham Radio Dictionary Make your Passion Pay (ebook writing)
All material on this site |