ARRL's VHF Digital Handbook

173 Pages·2008·13.98 MB·English

by  Steve Ford

Copyright©2008 by
TheAmericanRadio RelayLeague
CopyrightsecuredunderthePan-AmericanConvention
II1~ternationalCopyrightsecured
This workispublicationNo.332ofthe RadioAmateur's
Library,publishedbythe League.Allrights reserved.No
./ partofthiswork maybereproduced inanyform except
withwritten permissionofthepublisher.Allrights of
translationarereserved.
PrintedinUSA
-Quedanreservadostodoslosderechos
ISBN: 0-87259-122-0
FirstEdition
SecondPrinting
Foreword
Acknowledgements
Chapter 1 • Packet Radio Fundamentals
Chapter 2 • The Automatic Position Reporting System
Chapter 3 • Packet and Public Service
Chapter 4 • D-STAR
Chapter 5 • Digital Meteor Scatter and Moonbounce with WSJT
. .Chapter 6 • APCO-25
Chapter 7 • High Speed Multimedia (HSMM) Radio
Appendix A • AX.25 Link Access Protocol for Amateur Packet Radio
Appendix B • D-STAR System (Technical Description)
Appendix C • APCO-25: Anatomy Of The Common Air Interface
Index
When the Internetbecamepart ofeveryday life, the amateurpacket radio networks that
had flourished in the 1980s and early '90s declined sharply. Tosome,the collapse ofpacket
spelled the end of digital AmateurRadio above 50 MHz.How wrong they were!
Although packetnetworks see less activity than they did decades ago, packet radio itself
is far from dead. Packetradio has been "repurposed" to createthe popularAutomatic Position
Reporting System, and traditional packet networks still exist to supportpublic service activi
ties. New software applications have greatly enhancedtheir function.
Thanks to pioneering work by Joe Taylor,KUT, hams cannow enjoy digital meteorscatter
contacts and even moonbounce on VHF and UHFfrequencies with modeststations. His WSJT
software is available free of charge and requires little more than an ordinary computer sound
device.
TheJapan AmateurRadio League developed the D-STARdigital voice and data standard,
and ithas seen significantgrowth in the United States as hams establish D-STAR repeater
networks on VHF, UHFand microwave bands.
Amateurs are even experimenting with the APCO-25 standardused bypublic service
agencies. They'rereprogramming commercial APCO-25 transceivers for use on 2 meters and
70cm.
All ofthese topics, and more, are discussed in this edition ofthe ARRL VHFDigital
Handbook. My hope is that you'll use this booknot only as a helpful reference, but also as an
inspiration to try your own VHF+ digital experimentation.
David Sumner, KIZZ
Executive Vice President
Newington, Connecticut
January 2008
Theauthor wishes to thankthe following individuals and organizations, whose
contributions helpedmake this book a reference that many readers will enjoy.
• Allan Crosswell, N2YGK and Bill Covey,WIGTT, for theircontributions to
Chapter2.
• Rick Muething, KN6KB,AlanIsaachsen, KB2WF and Jim Oberhofer, KN6PE,
for materials used in Chapter3.
• Ray Novak, N9JA,ofICOM Americaand Ward Silver,N0AX,fortheir
contributions to Chapter4.
• Joe Taylor,KIJT, for his WSJT UsersGuide andReference Manual, portions of
which appearin Chapter5.
• Pete Lunness,AScT,Training and SpecialProjects,Daniels Electronics Ltd, for
the use of Chapter4:Anatomy OfThe Common AirInterface from the Daniels
Electronics APCO-25 training manual,which has beenreprinted in Appendix C.
• T.J.Molenkamp, KC8LTS,for his contributions to Chapter6.
• John Champa, K80CL,for authoring Chapter7.

l'
-,
acket radio is not a new phenomenon. Nor is it octets (2 and 10call signs) and consists of a destination,
- confined to Amateur Radio, or to VHF, for that source and up to 8 intermediate repeating stations. The
,
matter. address is 7 octets consisting of the call sign followedby
Inthebeginning,there wasX.25,aprotocolforwide the 4-bit (SSID) and 4 flag bits. Flag bits ofnote include
area digital networks that typically communicated over the repeatedand end oflist (last repeating station) mark
telephonelines.Withoutgoingintogorydetail,X.25works ers.
by chopping data into strictly definedpackets, orframes Control-ThisisusedmostlyforAX,25 connection
of information. This is accomplished by a device known oriented protocol.
as a PacketAssembler/Dissembleror PAD. Each packet PID - The protocol ID identifies what higher-level
issenttothedestinationdevicewhereanotherPADchecks protocol the frame carriesdata for.Examplesinclude:
it for errors. Iferrors are discovered, the packet must be
• AX.25 layer3(virtualcircuits - connections)
sent again.This ensures that the datathe user receives is
.InternetProtocol(IP frames inside UI frames)
100%errorfree.
.AddressResolution Protocol (call sign-to-IP address)
In the early 1980s,amateurs began adaptingX.25 for
.Nolayer3(UI frames)
over-the-airdigitalcommunications. TheresultwasAX.25.
The new AX.25 protocol worked in much the same way, Information- Thisisthe "text" ofthe message.
although itidentifiedeach messagebysenderanddestina FCS - A checksum used to detect garbled
tion station call signs and added a Secondary Station ID packets so they can be ignored.
number (SSID) in a range from 0 through 15.The entire Instead of a PAD to create and decode these AX.25
AX.25 protocol descriptionisincluded as an appendix to packets, hams invented the Terminal Node Controller;or
this book. TNC. Unlike PADs, TNCs do much more than assemble
As withX.25,each AX.25 framehas adefinedstruc and disassemble data. A TNC is programmed to work
tureasshowninFigure1-1.Theframeislogicallybroken withinaradionetworkwheretheremaybeothercompeting
upinto the following fields: signals. For example, to maximize the throughput for
Flag- Theflag is adelimiterbetween frames. The everyone on the same frequency, a TNC is designed to
01111110 pattern is unique due to bit-stuffing (any time detect the presence ofother data signals.Ifithas apacket
five Isare seen,azeroisstuffedandvice-versafordecod to send, but detects a signal on the frequency,it will wait
ing).Extraflags arepermittedbetweenframes. This gives untilthefrequencyisclear.TNCsalsohaveavarietyofuser
receiver time to sync up to the received signal and also adjustments and other features, such as mailbox functions
allows the transmittertorun continuously ifithas to. that allow them to store messages when the operators are
Addresslist- The address listisbetween14and70 away.
Packet Radio Fundamentals • I-I
ARRL0150
r----.--------,---,--------,---.----,--------.------r~:_:_:_:_::T:":~::T~:_::::I
Flag Flag Flag Address Gtrl PID Information FGS Flag Flag Flag
Figure 1-1-TheAX.25packet frame structure(seetext).
The First TNCs and the with the demand. Soon after, several US manufacturers
Packet Revolution began producing their own TNCs based on ,the TAPR
TNC-2standard.Infact,theTNC-2becamethe standard
InMarch1980theFederalCommunicationsCommis
for packet radio world wide.
sionapproved the use oftheAmerican StandardCodefor The packet fever spread quickly. For the first time,
Information Interchange, or ASCII, for Amateur Radio. hams discovered that they could use ordinary VHF FM
Prior to 1980, hams had been restricted to the limited transceivers to create over-the-air data networks. These
Baudotcodefamiliartoradioteletypeenthusiasts.Baudot networks began springing up around the country, most
can communicate the English alphabet, the number 0 centered on collections of stations that functioned as
to 9 and some punctuation. ASCII, in contrast, contains
Packet Bulletin Board Systems, or PBBSs. Hams could
128letters, numerals,symbolsand specialcodes,each of connect to PBBSs directly, or through relaying stations,
which is represented by a unique binary number. Every and read or send Amateur Radio e-mail. Some PBBSs
keyboardcharacterisrepresented inthis set.WithASCII, offered small filedownloads,too.Itwasevenpossibleto
hams finally had access to what was then the standard configure your TNC mailbox function to automatically
languageforcomputer-to-computercommunication. respond to queries from the PBBS and transfer e-mail
The FCC approval came 18months after Canadian without you ever lifting afinger. "-
hams hadbeenauthorizedtotransmitASCIIandtheyhad
Most user activity was conducted at a signaling rate
already been working on a protocol for doing so.Tothat of 1200 baud, although there were PBBSs that accepted
end, Doug Lockhart, VE7APU, of Vancouver, British 9600 baud connections.On the HF bands,hams are lim
Columbia,developedthefirstTNC. It worked withamo itedto300baud,butthatdidn'tstopamateursfrom setting
demtoconvertASCIItomodulatedtonesand convertthe up HF links torelayinformation'betweenscattered pack
demodulated tonesbacktoASCII. Doughad alsoformed
et networks throughout the nation and, eventually, the
the Vancouver Amateur Digital Communications Group
world. (Beware of confusing baud with bits per second.
(VADCG) and namedhisTNCthe "VADCG board".
See the sidebar "BaudvsBPS vsThroughput.")
Hams in the US started experimenting with the
VADCG board, but in December 1980Hank Magnuski,
KA6M,put adigital repeateron2meters using aTNCof
hisowndesign.Agroupofhams interestedinHank'sTNC
started working together on further developments in
packet radio and formed thePacificPacket RadioSociety
(PPRS).At the sametime,AMRAD,the Amateur Radio
Research and Development Corporation,in Washington,
DC became the center forpacketwork on the eastcoast.
In 1981 a group of hams in Tucson, Arizona, founded
the Tucson Amateur Packet Radio Corporation .
(TAPR).With three centers of amateur packet re
search in the US, itwasn't long before one group
would take the lead:TAPR.
TAPR pioneered the TNC-1, first commerJ
cially successfulpacketTNCin the United
States.By 1984theyintroduceditssucces
sor, the TNC-2. The TNC-2 design was
much more compact, easy to use and
highly reliable. The TNC-2 was enthusi
astically received by the mushrooming
If.
amateur packet community, so much so •'7
that TAPR had difficulty keeping pace TheTAPRTNC-2.
1-2 • Chapter I
Then Came the Internet quicklybecamethestandardfortextcommunication with
millions (and eventually billions) of people exchanging
TheInternet hadexistedforyears andwaswellknown
messages every day. What was once esoteric was now
ingovernment, militaryandacademiccircles.Itsexposure
commonplace.
to the general public in the late 1980s coincided with the
TheeffectoftheInternetonpacketradiowasdevastat
increasing popularity of personal computers. Ordinary
ing. Unlike amateur packet radio, the Internet was ex
citizens began tapping the Internet through connections
tremely fast, reliable over long distances and capable of
providedbytheiremployers,orbycollegesanduniversities.
easily handlinglargefiletransfers.Theallureof"instant"
The revolutionary potential of the Internet was obvious,
global e-mail wastoogreatformost packet users toresist.
butunless youknew your wayaroundthecrypticTCP/IP
Theyabandonedtraditionalpacketradioin droves,which
language, using the Internetcould be achallenge.Some
resultedin the shrinkageorcollapseofamateurnetworks
thing more wasneededbefore theInternetcouldspreadto
throughouttheworld.The effect wassimilartotheimpact
an even larger audience.
"Somethingmore" arrivedin 1991.Thatwasthe year cellular telephones had on amateur repeater autopatch
theConseilEuropeenpourlaRechercheNucleaire(CERN) systems. Once everyone had an affordable and private
established their newWorld Wide Web projectwith Web wirelesstelephone,the practice of making a call through
"pages"createdinHypertextMarkupLanguage,orHTML. an autopatch wasrenderedobsolete.
In 1993the National CenterforSupercomputingApplica This is not to by that amateur packet radio is dead.
tionsatthe University ofIllinoisreleasedMosaic,thefirst There are many packetnetworks still in place. What has
Webbrowser.Finally,thepublichadanextremely"friend happenedinsteadisthat packetradio hasbecomespecial
ly"toolfornavigatingincyberspace.TheWeb,asweknow izedthroughapplicationsdesignedtomeet specificneeds.
ittoday, wasborn. We'll discusstheseapplicationslaterinthebook.The most
Therest,asthey say,ishistory.TheWebexplodedin popular application of Ax'25 packet radio today is the
popularity andwithin5yearsbecamemainstreamtechnol AutomaticPosition Reporting System, or APRS,and that
ogy,asfamiliar asahousehold telephone. Internete-mail subject hasachapterall itsown.
v.,
Baudvs BPSvsThroughput
These threetermsareoftenconfusedandmany creates a 1200baud outputsignal.Thankstoclever
hamsusethem interchangeably. By definition,however, coding,themodem iscapableofencodingtwobitsfor
theyarequitedifferent. every signalchange,so it isoperating at2400 bps
The baudrateisameasureof how many times per (1200 baud x 2).Sofar sogood,butlet'ssaythe radio
secqnd a signalchanges states (from "mark"to "space" issendingthe2400 bps data on a path that isproneto
inG.lra.dioteletypet~ansmission, for example)inone [email protected]!ectserrors
' . heterm aud"comesfrom EmileBaudot, G.lr1dframeshG.lY'Eltobe re-V . 'ttedm
or
oft nchronoustelegraph printer. thoughthe sen(jingstation'i .. ingd
BRS-"-Bitspe econd-is afneasure of how many bps,thethroughput,based 0 .he amou ....... <> a
bitsper secondaretransmitted.Withsome digital successfully decoded atthe receiving statton;ismuch
codingschemes,itispossibletoencode multiplebits lower. - --
per baud resultinq inbit ratesthatexceedthe baud rate. Bewarywhenyou readmanufacturerclaims about
Throughputisa measureofthe amountofdata equipmentthat cantransferdata atspecific ratesover
transferredinaspecificamountoftime,usually radiochannels.Dothey meanthe encodedbits per
expressed inbitsper second (bps).Thisis acritical secondatthetransmitter,orthe effective throughput?
distinctionbecause throughputcan be independent of Inmost instances,theymean.thedata ratea.tJhe
baudrateorencoded bitspersecond. transmitter.When youtaketheirhardwareintc,thereal.
AILthreetermsqan cometogetherinsome World,your tivethrou ' b . .
, intElr~$ting ways. Imagineyouhavea.radlo modemthat >dIfferent.
-
Packet Radio Fundamentals • 1-)