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What’s All This RacketAbout Packet?
Packet radio is growing fasf. What’s it like? Read on.By Harold
Price,* NK6K
This is an article about packet radio. If you don’t know speeds;
120 characters per second is standard on the 2-meter band,what
packet radio is, that pronouncement won’t and a recent development
permits ‘960 characters per second onmean much to you. “High Speed
CD/CSMA Digital the 220-MHz band. Below 30 MHz, 30 characters per
second is
Communications in the Amateur Radio Service, Theory and
Ap,-plications” isn’t a real grabber either, so “this is an article
about
used. Assuming that you already have a radio and a computeror
terminal, it will cost you between $180 and $500 (higher cost
packet radio” will have to do. This article will have a slant
for more “bells and whistles”) to get on packet radio. This isd i f
ferent f rom mostprevious articles aboutpacket radio. It will
notdiscuss what you will beable to do with packet inthe future, but
what youcan do with packet now:how to use the existing net-works,
which of severalfrequencies in your area arebeing used for
packet,what packet controllers areavailable and what you canexpect
when you get onpacket radio.
the cost of a terminal nodecontroller (TNC), a boxthat connects
the data-generating device to theradio. In other words,packet
allows hams to ex-change information muchfaster than before with
noerrors at a reasonable cost.
What Is Packet?P a c k e t i s usually
character communication.Letters and numbersentered on a keyboard
orfrom a computer are sentfrom one amateur stationto another. On
the surface,this sounds no differentfrom RTTY, which hasbeen around
in AmateurRadio for many years.Packet radio has threemajor
characteristics andseveral beneficial side ef-fects that make it
stand outfrom other amateur digitalcommunications modes.
After we discuss whatpacket does, there will besome theory-an
explana-tion of how packet doeswhat it does (there’s nosuch thing
as a free lunch).Those of you who want todig deep into
technicaltopics should consult thebibliography at the end ofthis
article.
A final word of warning:This is a sales pitch! Thegoal is to get
you interestedenough in packet radio thatyou will get
involved.Whether you visit afriend’s packet-equippedshack, see
packet in action.at a radio store or Field
’ >
lllllllllltllillllllllilllllllllilllllIlll~~~~i~l~~
Day site, go to localpacket-radio meetings, orjump in and buy or
build a packet controller, you’ll learn farmore by doing than by
reading.
Packet radio guaranteesperfect reception.
Information sent viapacket is checked to seethat it was received
exactlyas it was transmitted. Datais automatically retransmit-ted
until it is accurately re-
ceived. There is only a very small chance (one in millions)
thatbad data will sneak through.
Executive SummaryPacket radio, or simply “packet,” is the common
name for
a digital communications mode in Amateur Radio that
provideserror-free communications. It is designed to allow
automatic link-ing of systems for cross-country networks. Packet
uses high
l 1211 Ford Ave., Redondo Beach, CA 90278
Packet permits a single frequency to be shared by
severalsimultaneous conversations.
This also makes possible a bit of magic called a
“simplexrepeater” -a repeater with its input on the same frequency
asits output.
Packet allows for “routing” information among stations, so
thatany packet station can be part of a linked set of “repeaters.
”
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For example, there is a set of packet sta-tions in California
that allows informationto be sent from San Diego to Sacramento,a
distance of 480 miles, on 145.01 MHz.Before users of mega-linked
VHF/UHFFM repeater systems start chuckling at this“paltry”
distance, consider that each sta-tion on the packet link uses a
single fre-quency, a single antenna and a singletransceiver.
Stations do not require addi-tional telemetry, diplexers,
duplexers, cir-culators, link radios or large amounts ofmoney. The
packet-radio link can also sup-port multiple simultaneous
conversationsbetween the ends of the path, as well asbetween points
in the middle. Similarpacket-radio “networks” exist in Florida,the
Northeast and Mid Atlantic states, andin several Midwestern
states.
Like the man says on TV, “And whatwould you expect to pay for
all this? Don’tanswer yet, there’s more . . .”
Packet allows computers to speak directlyto each other in their
“native tongue.”
Most personal computers use the ASCIIcode, which has 256
separate characters.Baudot, the code most often associatedwith the
term “RTTY,” has 32 charactersand a trick called “shifting,” which
allowsan additional 26 characters to be recog-nized. Transmitting
ASCII computercharacters over Baudot RTTY causesserious problems
for computer users.Packet radio can transmit ASCII characterswith
no restrictions or shifting.
Packet is fast.Packet is faster than Baudot RTTY
because of technical standards, equipmentavailability,
convention and regulatoryissues. For whatever reason, you won’t
seemuch Baudot above 100 words per minute(WPM). On the other hand,
you won’t seemuch packet below 360 WPM on HF orbelow 1440 WPM on
VHF. We are startingto see packet at 11,500 WPM on 220 MHzand
above, and packet has been sent ex-perimentally at 300,000 WPM on
the 7O-cmband. I’ve taken a small liberty in ex-pressing packet
radio speeds in WPM; theactual speeds in bits per second (bit/s)
are300, 1200,960O and 250,000, respectively.You say that you can’t
type 300,000 wordsper minute, or even 360? Keep reading-packet
isn’t just for typists.
Packet provides “non-realtime”communications.
What this means is that you and theperson you are talking to
don’t have to behome at the same time. Much of the presentuse of
packet radio is leaving messages forothers on a centrally located
bulletin board.A bulletin board is a message-storagedevice, usually
maintained at the home ofa local ham. If the person you want to
talkto isn’t on the air when you are, you canleave a message for
him or her on thebulletin board. The message can be about
plansanything: breakfasttomorrow, the fact that you worked
theClipper-ton DXpedition on 160 meters, yournew antenna, etc.
Although similar systemshave been available on traditional
RTTYsystems, packet lends itself nicely tobulletin-board operation.
Because packetis fast and many users can share the samechannel, a
properly designed mailbox canshare the frequency with several
non-mailbox conversations, or severalmailboxes can be on the
frequency at thesame time without mutual interference.
Packet is information transfer.Because of these characteristics,
packet
Iends itself well to connecting a central storeof information,
usually called a hostsystem, to a local network of users. For
ex-ample, in Southern California we have ahost run by WB6YMH that
is kept stockedwith the latest Amateur Radio informationavailable.
Electronic versions of the ARRL
Letter, Gateway (the ARRL packet-radionewsletter), the W5 YI
Report, AMS’A TSateUite Report and newsletters fromseveral other
organizations are madeavailable via packet radio and the hostsystem
to any suitably equipped amateurin the area.* A typical issue of
the ARRLLetter, around 20,000 characters, wouldtake about an hour
to send at the standardRTTY speed of 60 WPM. At the standardVHF
packet rate of 1200 bit/s, it takesabout 3 minutes.
Now, what would you pay for all of this?Wait, there’s more . .
.
Packet i& 97.1(b).Part 97 of the FCC rules under which
we
live states the purpose of the AmateurRadio Service. One of the
subparagraphscontains these words: “Continuation andextension of
the amateur’s proven abilityto contribute to the advancement of
theradio art.” One of the better recent ex-amples of amateurs
advancing the radio artis the current activity in packet
radio.Packet didn’t originate in the AmateurRadio Service, but we
have taken the basicidea and have shaped it into things thatdidn’t
exist before, or which have a slantdifferent from what has been
tried before.We have also added the traditional
amateurtouch-extremely low cost. Amateur-designed and -built
packet-radio controllersflew in official weather planes through
theeye of a hurricane. Army and Navy MARSstations are integrating
amateur packet-radio technology into their activities.Several
commercial manufacturers havetaken the amateur-designed controllers
andhave begun to sell them both in and out ofthe amateur
market.
Packet is satellites.AMSAT-OSCAR 10 is an excellent
medium to use for packet radio. La,rgcamounts of data have been
sent ,cross thecontinent via OSCAR 10. Using a specialpacket device
called a teleport, a packet sta-tion in northern Canada was
connectedthrough the satellite to a station in LosAngeles; data was
then relayed throughanother packet controller to a station inSan
Diego. The UoSAT-OSCAR 11satellite carries a packet-radio
controllerthat can store 120,000 characters andretransmit them
later to any other point onthe globe. PACSAT, an amateur
satellitecurrently under development, will usepacket radio to store
up to 4 millioncharacters for relay between stations.
Packet is international.The packet-radio protocol, AX.25, is
now accepted as an international standard.
‘Gateway, the ARRL packet-radio newsletter,is available from the
ARRL. U.S. subscriptionsare $6 for ARRL members and $9 for
non-members.
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!: ‘8e&u& ik breaks’ up ai data stream into short
secfions called frames, packet:T looks, to the oDerator, like
full-dunlex communications+ FuH duDIex means both
2 puti& can speak at :..;‘;;rin& .yTk’i;, .:,(I’<
y&L ,_ ?:, ;::“~$~ ’ ‘. , ;,; ., I
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channel interference and other problemscommon in amateur
communications.
***CONNECTED TO W6IXU
What Does It Look Like?So far we’ve talked about what packet
can do for you and about how it does it.But what does a packet
contact look like?In the following examples, we’ll look at
theprocedures used by the TAPR, AEA,Heath and Kantronics TNCs.
Other TNCsfollow similar, but not identical,procedures.
First, you must tell the TNC your callsign. For example:
MYCALL NK6K
is the command to enter a call sign. MostTNCs allow you to
change your call signat any time and have a way to rememberit while
the power is off.
‘As in all other modes of Amateur Radio,packet allows you to
“read the mail” ormonitor channel activity. This is called
themonitor mode, and looks like this:
WA6JPR> WB6YMH: HELLO SKIP, WHEN ISTHE NEXT OSCAR 10
PASS?WB6YMH >WA6JPR: HANG ON WALLY, I’LLTAKE A LOOK.
The call signs of the stations involved ap-pear as “from> to,
” and the contents ofthe packet appear after the ‘2”. In
thismanner, you can monitor all traffic on thefrequency. You can
also watch for a sta-tion calling CQ, which might look like
this:
WB6HHV >CQ: MIKE IN SAN DIEGOFOR ANYONE IN SIMI VALLEY.
LOOKING
You can send a CQ by entering the con-versation mode of the TNC.
You go to theconversation mode by typing:
CONVERSE
You can then type your CQ:
MIKE IN SAN DIEGOIN SIMI VALLEY.
LOOKING FOR ANYONE
Your TNC adds your call as the FROM ad-dress, and CQ as the TO
address. Thereceiving station’s TNC adds these ad-dresses to the
front of the displayed text.
You answer a CQ or establish a contactby using the CONNECT
command. This“connects” your TNC to another stationand begins the
acknowledgment procedurediscussed earlier. An example of a
connectcommand is
CONNECT W6IXU VIA WA60ZJ, K6TZ, WB6DAO
This asks for a connection between you andW6IXU routing through
(via) three otherstations. When the connection has beenestablished,
the TNC notifies you byprinting
This means that the computer in your TNChas exchanged some
preliminary informa-tion with the other TNC and is ready toproceed.
If the other station had alreadybeen in a connection with a third
TNC, youwould get a busy signal:
***W6IXU BUSY
If W6IXU is not on the air, your TNCwould make several attempts
to establishthe connection and then print a messagetelling you that
it has not succeeded.
Assuming you get connected, everythingyou send to your TNC will
now be sent toW6IXU with all the error checking andretransmission
just described. Each timeyou hit the ENTER or RETURN key, a
packetis formed and sent. Packets received fromthe other station
are displayed between thelines you enter, much as if a
full-duplexRTTY QSO were taking place.
When you are done with the con-versation, you disconnect by
enteringCONTROL-C and typing DISC.
The commands and scenario above areall you need to know to carry
on a packet-radio QSO. There are many other options(around 60) and
several other combinationsof connected and monitor modes, but
theyare like the 40 knobs, switches and meterson most modern HF
rigs; there areoperators who constantly twiddle, andthose who only
use the push-to-talk switchor the key.
So What Are You Waiting For?We’ve only touched briefly on what
can
be done with packet and mentioned evenless the technical details
of how it works.For some, packet is an end to itself-experimenting
with new ways to transferdata. For others it is just a tool-anew
wayto pass traffic, spot tornadoes, run aparade, score points on
Field Day or meetnew people. To find out more, look intoany of the
references listed at the end ofthis article. Or, wait for the
second part ofthis article, which describes a TNC indetail. See you
on packet!
BibliographyAX.25 Amateur Packet-Radio Link-Layer Protocol.
Newington: ARRL, 1984.First ARRL Amateur Radio Computer
Networking
Conference. Newington: ARRL, 1981.Fourth ARRL Amateur Radio
Computer Net-
working Conference. Newington: ARRL, 1985.Johnson, L., “Join the
Packet Radio Revolution.”
73 Ma azine, Sept. and Oct. 1983, andJan. 1984
Morrison, Ml, and D. Morrison, “Amateur PacketRadio” Ham Radio,
July and Aug. 1983.
Rinaldo, P., “ARRL Board Approves, A:$?&kW:;dio Link-Layer
Protocol, I
Second ARRL Amateur Radio Computer Net-working Conference.
Newington: ARRL, 1983.
Tannenbaum, A., Computer Networks. EnglewoodCliffs, NJ:
Prentice-Hail, 1981.
Third ARRL Amateur Radio Computer Net-working Conference.
Newington: ARRL, 1984.
The 1985 A RRL Handbook for the Radio Amateur,pp. 19-21 to
19-31. Newington: ARRL, 1984W