What is Packet Radio? 

Packet radio is computer-to-computer communication using radio links 
rather than telephone lines. Instead of sending a continuous stream of 
information, data is assembled into neat little bundles called packets. 
These packets are sent one at a time. When a packet arrives error-free, 
the receiving station sends an "okay" message (called an ACK) and the 
next packet is transmitted. If the receiving station detects an error, 
it discards the faulty packet and does nothing. After a certain amount 
of time has passed without an ACK from the receiving station, the 
transmitting station sends the packet again. These exchanges take place 
at high speeds, resulting in efficient digital communications without 
errors--a remarkable feat for a radio-based computer network! 

Amateur packet exists as a loose global network. Through packet radio, 
you can tap into this continuously flowing stream of data and exchange 
information with stations throughout the nation and the world. From the 
fascinating to the absurd, it's all on packet! 

Most VHF packet activity takes place on 2-meter FM. You'll also find 
packet on 222 MHz and 420 MHz. Packet is alive on 6 meters as well--with 
many opportunities for packet DX. 

What Can I Do With Packet? 

The answer depends on your interests! Every amateur has different uses 
for packet. Some view packet as a tool to enhance their overall 
enjoyment of Amateur Radio. Others take it further, using packet as 
their primary means of communication. 

To make things simple, let's list the most popular applications of VHF 
packet as it exists today: 

[ ] Enjoying live keyboard-to-keyboard conversations. 

[ ] Accessing packet bulletin boards to exchange messages with other 
amateurs and read general interest bulletins. Some bulletin boards offer 
additional services such as electronic call sign directories and 
magazine bibliographies. 

[ ] Participating in the ARRL National Traffic System. (Packet bulletin 
boards can be used to receive and originate NTS traffic.) 

[ ] Using DX PacketCluster systems to hunt HF or VHF DX. Through 
PacketClusters you can determine which DX stations are on the air at the 
moment--and where they are! DX PacketClusters support other useful 
features as well. 

[ ] Monitoring and communicating with amateur satellites. Many 
satellites function as orbiting bulletin boards, relaying packet 
messages around the world. Others transmit images that you can display 
on your computer screen. 

A basic packet station consists of a 2-meter FM transceiver, a personal 
computer and a terminal node controller (TNC) or multimode 
communications processor (MCP). TNCs are priced around $130. MCPs, which 
offer other digital modes in addition to packet, typically sell for 
about $300. Almost any computer can be used with the TNC or MCP of your 
choice. All that's required is a compatible terminal program. Some 
dealers listed in the pages of QST offer terminal programs for as little 
as $10. 

For 2-meter packet operating, fancy antennas are not required. A simple 
ground plane is fine for most areas. If packet activity is hot and heavy 
where you live, just the rubber duck antenna on a hand-held transceiver 
may be all you'll need! If you'd like more information, pick up a copy 
of Your Packet Companion at your favorite amateur dealer. You can also 
order this book directly from the ARRL.--WB8IMY 
 

Subject: HF packet

Many people ask about using PSK at HF, as they have heard claims
that its performance surpasses what they are now using, which 
typically is AX.25 packet using 300- or 1200-baud FSK.  (1200-baud
signals are allowable at 10 meters and above).  First, let's not
confuse the terminology.  AX.25 is a protocol that defines the bits
that are sent.  It doesn't define the modem technology, that is,
frequency-shift keying (FSK) versus phase-shift keying (PSK) or
what-have-you.

There are two fundamental issues with regard to the efficiency of
an HF digital communications link: the modem and the protocol.
With regard to the modem, the question of whether FSK or PSK is
superior is best answered by, "it depends."  It depends on the
particular characteristics of the FSK and PSK systems being used.
As far as I know, the only direct comparisons ever reported in the
amateur press between PSK and FSK are from some tests done using
1200-baud FSK (Bell 202) and 1200-baud PSK (using the TAPR PSK modem
originally designed for satellite work).  In these tests, PSK proved
superior.  That shouldn't be taken as a blanket statement that PSK
is better than FSK, however; there are many other FSK and PSK
systems that can be used, and a given FSK system could be better than
a given PSK system.

Ionospheric effects complicate the picture.  A modem that works well
on a solid, fadeless channel may not work well at all once fading
begins.  In the presence of fading, it may turn out that the modem
that works best is one that didn't stack up so well against other
modems when tested on a fadeless channel.  All of which means that 
there is no one answer to which kind of modem is best.  Indeed, the
push these days is for adaptive modems, modems that change their
characteristics to adjust to changing propagation effects.

With regard to protocol, which is a separate issue, as noted, I don't
think any knowledgeable person would argue that AX.25 is even close
to being an efficient protocol for HF work.  The problem is that even
using the best possible modem, you occasionally will get bit errors
(a transmitted '1' bit received as a '0' or vice versa).  In AX.25,
one bad bit makes an entire packet unusable.  This is the main reason
why AX.25 isn't a good protocol to use on error-prone channels--like
those encountered on HF.  At present, we have no better standard
HF protocol, however.  Candidates for that honor include the PACTOR
system and the soon-to-be-available Clover system from HAL
Communications.  But expect to see more developments over the next
few years.

The ARRL Computer Networking Conference Proceedings and QEX magazine
have carried quite a bit of information on this subject over the past
few years.  Among the most useful are the articles listed in the
bibliography below.  (If you don't have these publications, copies of
articles can be obtained for $3.00 each from the ARRL Technical
Department.  Send a message to bdicosim@arrl.org with a list of the
articles needed and your postal mail address.  You'll receive the copies
and a bill for the copying
charges.)

J. Bloom, KE3Z
Senior Engineer, ARRL
November 17, 1992

BIBLIOGRAPHY

H. Helfert and U. Strate, "PACTOR--RadioTeletype with Memory ARQ and
Data Compression," QEX, Oct 1991, pp 3-6.

H. Price, "Digital Communications: BBS Forwarding on HF," QEX, Dec
1992, pp 20-22.

J. Reed, "PSK Anyone?" QEX, Jul 1991, pp 3-
K. Wickwire, "The Status and Future of High Frequency Digital
Communication, Part 1: Overview," QEX, Jun 1992, pp 3-14.

B. McLarnon, "More Ideas for a Robust HF Packet Signal Design," QEX,
Mar 1990, pp 9-10.

R. Petit, "Clover II: A Technical Overview," ARRL 10th Computer
Networking Conference Proceedings, ARRL, 1991, pp 125-129.

K. Wickwire, "The Status and Future of High Frequency Digital
Communication, Part 2: HF Modems and Their Performance," QEX, Jul 1992,
pp 3-15.

K. Wickwire, "The Status and Future of High Frequency Digital
Communication, Part 3: Simulating the Performance of HF Digital
Networks," QEX, Aug 1992, pp 12-17.

K. Wickwire, "The Status and Future of High Frequency Digital
Communication, Part 4: Where is HF Digital Networking and Where is It
Going," QEX, Oct 1992, pp 10-18.

