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Radio amateurs in Canada, Sweden, and the United States have
been experimenting with packet radio, a system of computer-based
communications. This new mode can provide high-speed communication
ference due to other stations and to signal degradation due to
adverse band conditions. Not only can packet radio be used for
nformal amateur QSO's and traffic handling, but it has additional
"bulletin boards" and message systems, and remote computer
WHAT IS PACKET RADIO?
Packet radio is a communication system in which information is
but with important differences. These differences are the key to
nsuring error-free reception and at the same time allowing max-
mum use of the spectrum through shared frequency use.
Data integrity is provided by packet radio through a "hand-
mission, a computed value called a "frame check sequence" (FCS)
s sent, which allows the receiving station to check for errors.
The receiving station acknowledges an error-free packet with a
not receive such a signal within a certain period of time, it
automatically retransmits the packet.
A packet also contains identification of the destination
frequency. A packet radio station can automatically ignore any
not need the channel most of the time. The time between trans-
missions is available to other users on frequency. This system is
called time-domain multiplexing. On a very busy channel, the user
transmissions, but the packet radio equipment will take care of
automatic retransmissions and sorting out the replies meant for
the station. The user never "hears" the QRM.
WHAT IS A PACKET RADIO STATION?
Packet radio requires the use of a microprocessor-based con-
troller at each station, and it will obviously appeal to the
not require that the operator be a programmer, or even that the
essary is a terminal, a terminal node controller (TNC), and an
amateur radio transceiver.
The terminal can be a simple display (CRT) or typewriter
terminal that produces ASCII characters, a personal computer,
or even a commercial mainframe computer. What you need is a
terminal with a keyboad to allow you to talk and a screen or
a printer to allow you to read incoming information. You can
even get an inexpensive terminal that uses a TV set for the
The way in which most terminals encode ASCII characters is
n "asynchronous" format. SInce characters are encoded as they
are typed, there is a flag consisting of one or more "mark"
(binary 1) values to mark the beginning and end of each char-
acter. The device decoding the characters expects a specific
"baud rate", or number of transitions from "mark" to "space"
(binary 0) per second during the character, but no particular
time interval between characters themselves.
The terminal node controller is the heart of the packet
nal or computer, and communicates through it by asynchronous
ASCII format at the baud rate required by the terminal. The
TNC converts the data stream from the terminal to a packet
by attaching a "header" showing the destination of the packet
and control information for the network, a "tail" containing
the result of the FCS calculation for error detection, and
flags to mark the beginning and end of the packet.
The second port of the TNC connects it to the trans-
ceiver microphone and speaker audio lines, and the PTT line.
Ordinarily, the TNC will produce AFSK modulation by putting
one of two tones into the microphone input, corresponding
to a "mark" or "space". In this fashion, the packet is sent
out on the air at the packet channel baud rate, which is
unrelated to the terminal baud rate at the other port of
The receiving TNC reverses this procedure, decoding the
audio tones from the speaker audio line of the radio, re-
moving and reading the header and tail information, and
the terminal baud rate.
The part of the TNC that does the translation between the
"modem", short for MOdulator-DEModulator. This device may or
may not be built into the TNC board. Most packet radio
modems operate at 1200 baud, which corresponds to about 1200
on some amateur bands. The audio tones used are 1200 hz and
modem, which is available as surplus.
The final component of a packet radio station is an
amateur radio transceiver. Most packet radio activity so far
of the radio is that its audio frequency response at 2200
already have is probably just fine.
WHAT THE TNC DOES
The TNC consists of a special purpose microcomputer, con-
taining all the necessary software and hardware to communicate
mitter and receiver to send and receive a packet, and decode
a packet. The special functions of the TNC which would be
the use of protocol to communicate with other TNC's and
The encoding and decoding of packets involves a carefully
col basically determines the exact form of the header and tail
matically determine the purpose of the packet, e.g., net
check-in, part of a QSO, or ACK to a previous transmission. The
tail contains the FCS which allows the TNC to automatically
to automatically acknowledge it. Since the protocol is pro-
know how the destination of his packet is indicated. The oper-
ator communicates with other amateurs by call sign, and the
TNC translates the call sign into the identification required
by the protocol.
The TNC is required to perform a number of tasks simultan-
eously, including responding to events such as the receipt of
a packet or instructions from the operator in "real time", in
other words, as they happen. This makes programming in BASIC,
the common language of personal computers, undesirable. This
s because BASIC useĀ an "interpreter" which reads each line
of the program and translates it into machine-type instructions
every time the line is executed. The time required for the
translation would prevent a program from responding rapidly
enough in a packet radio environment. In order to meet the
s required. While BASIC looks pretty much the same on any
computer, assembly language is different for every machine.
elopment would have to be redone for each variety of com-
must be constantly "listening" at both ports simultaneously
Programming of individual TNC's must be as easy as possible,
nitial software. In addition, hardware changes may necessitate
erasable programmable read-only memories (EPROM's), which
normally function like the ROM of a personal computer, where
the vital software is storaed in an indestructible form.
However, if the need arises, they can be reprogrammed by
"burning in" the new program using special equipment.
WHAT IS A PACKET?
A packet is the basic message unit in packet radio. It ord-
narily consists of a text message typed in by the operator,
the protocol. In a typical QSO, a packet would be encoded and sent
out by the TNC when the operator ends a line of typing by hitting
the RETURN or ENTER key. In any event, the length of a packet is
limited, usually to 128 characters. This helps to prevent a single
user from "hogging" the channel, as well as making sure that the
A packet need not consist of ASCII or Baudot character strings,
as BCD or EBCDIC, or even binary data such as a compiled computer
any of these equally easily. An advantage to this choice of proto-
col is that the functions it requires are available on a single
large-scale integration (LSI) chip, which simplifies the TNC hard-
beginning and end of the entire message are flagged, making the
"start" and "stop" bits for each character unnecessary when the
The "frame" of an HDLC packet is represented below. Each field
of the packet is encoded as a sequence of 1's and 0's (bits) to
be transmitted as "mark" and "space" tones. With the exception of
the DATA field, all these fields are generated by the TNC as it
assembles the packet for transmission. The operator is concerned
only with the contents of the DATA field.
| Flag | Address | Control | Data | FCS | Flag |
The FLAG is a unique bit sequence which identifies the begin-
ning of a packet to the HDLC controller. This pattern corresponds
to no sequence which would be encountered in any of the other
fields, except possibly in the transmission of binary data. Even
n this case, there are provisions for distinguishing data from
the flag sequence.
The ADDRESS field contains routing information for the packet.
This information may include the destination station, the origin-
ating station, and possibly intermediate routing information if
the packet will be relayed to the destination. The destination
and originating stations mights be identified by a network address
number of by amateur call sign, depending on the exact form of the
The CONTROL field describes the purpose of the packet to the
network. It identifies packets with such functions as network
check-in or check-out request, packet acknowledgements, or
n the correct order.
The DATA field contains the message being sent, which will
ordinarily be the text typed in by the user, converted into an
ASCII data string. In the case of a packet identified in the
control field as performing a control function, the DATA field
may be absent.
The FCS allows the receiving station to verify that the packet
s sent; otherwise the packet is ignored.
WHAT IS A PACKET NETWORK?
A local area packet radio network (LAN) consists of a net
control station and a number of individual operators. The net
control station is sometimes referred to as the "station node"
and the individual stations as "terminal nodes". The net may also
contain a digital repeater or "digipeater", which may be the net
control station or a separate repeater station. The repeater
transmits any correctly received packets, or it can be "normal"
As operators sign on to the net, they are recognized by the
net control and given net address codes. An operator desiring to
transmissions addressed to that station. Any operator may choose
to have his TNC receive all transmissions, rather than just those
addressed to his station. Of course, the TNC will only acknowledge
those transmissions intended for that station. The operator whose
care of automatically by his TNC.
As more packet radio LAN's become active, there will be the
These stations can be members of both nets and serve as communic-
ations links thorugh which packets originating in one net can be
funneled to an addressee in the other net.
A more sophisticated possibility is that of a "gateway"
long-distance mode of communications. The gateway station will
network linking information and transmit it to another gateway
for long-distance links.
TERRACON will be a high-speed ground-based linking system
utilizing UHF and/or microwave relays. It could potentially
United States and Canada. It will probably be a few years before
TERRACON is implemented as a useful system, and somewhat longer
before the continent is linked.
AMICON will be a satellite-based network utilizing one of the
AMICON will allow intercontinental linking and contact with
solated areas not accessible to TERRACON. High data rate exper-
ments are being planned for the 23cm uplink/70cm downlink (mode
L) translator. There are also plans for a packet radio digital
SKIPCON is AMRAD's projected HF network of LAN gateway
HOW TO GET IN ON PACKET RADIO
There are currently two TNC designs available. The first
Communications Group (VADCG). The Vancouver TNC is available
as a bare board, and requires a power supply, and external
modem, and parts. It comes with instructions and notes on the
and includes 4K bytes of 2114 RAM and 4 K bytes of 2708 EPROM.
The TNC requires an 8250 (serial ports) or an 8255 (parallel
to the radio.
The Tucson Amateur Packet Ra
io group (TAPR) is currently
testing a second TNC design. This TNC has the modem, radio
nterface, serial and parallet terminal interfaces, and power
board. It is based on the 6502 microprocessor (Editor's note:
the board will now use the Motorola 6809E microprocessor),
and can hold a total of 48K bytes of RAM and ROM on the board.
the 1933 HDLC chip it uses is compatible with the 8273 chip
used on the VADCG board, and the TAPR TNC will be capable of
Additional information on TAPR activites is available from
Tucson Amateur Packet Radio, PO Box 22888, Tucson, AZ 85734.