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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.
No updates have been made and it appears solely for historical interest.

 

 

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