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TUCoPS :: Radio :: pac_intr.txt

Introduction to Packet Radio






INTRODUCTION to PACKET RADIO
------------ -- ------ -----

   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
with efficient use of the spectrum, and is resistant to inter-
ference due to other stations and to signal degradation due to
adverse band conditions. Not only can packet radio be used for
informal amateur QSO's and traffic handling, but it has additional
possibilities for exchange of data between hams with computers,
"bulletin boards" and message systems, and remote computer
programming.

WHAT IS PACKET RADIO?
   Packet radio is a communication system in which information is
digitally encoded. In this respect it is similar to RTTY or ASCII,
but with important differences. These differences are the key to
insuring error-free reception and at the same time allowing max-
imum use of the spectrum through shared frequency use.
   Data integrity is provided by packet radio through a "hand-
shaking" technique and error detection. Along with each trans-
mission, a computed value called a "frame check sequence" (FCS)
is sent, which allows the receiving station to check for errors.
The receiving station acknowledges an error-free packet with a
special acknowledgement (ACK) signal. If the sending station does
not receive such a signal within a certain period of time, it
automatically retransmits the packet.
   A packet also contains identification of the destination
station, permitting several QSO's to take place on the same
frequency. A packet radio station can automatically ignore any
packets which are not addressed to it. Due to the fact that the
duration of most packet transmissions is very short, a user does
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
will notice an increased delay time before getting replies to
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
ham who already has a computer in his shack. However, it does
not require that the operator be a programmer, or even that the
station have a personal computer. All that is really nec-
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
display.
   The way in which most terminals encode ASCII characters is
in "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
radio system. It has one port that is connected to the term-
inal 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 TNC.
   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
passing a successfully received packet to the terminal at
the terminal baud rate.
   The part of the TNC that does the translation between the
sequence of tone levels and the characters is called a
"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
wpm, although the FCC now authorizes much higher baud rates
on some amateur bands. The audio tones used are 1200 hz and
2200 hz. This choice of frequencies is that of the Bell 202
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
has been in the 2-meter band. The only important requirement
of the radio is that its audio frequency response at 2200
hz be adequate. In other words, the 2-meter FM rig you
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
with your terminal, assemble a packet, operate your trans-
mitter and receiver to send and receive a packet, and decode
a packet. The special functions of the TNC which would be
difficult to implement with an ordinary personal computer are
the use of protocol to communicate with other TNC's and
real-time control.
   The encoding and decoding of packets involves a carefully
standardized set of procedures called "protocol". The proto-
col basically determines the exact form of the header and tail
parts of the packet. The header allows receiving TNCs to auto-
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
determine whether the packet was received correctly, and if so,
to automatically acknowledge it. Since the protocol is pro-
grammed into the TNC, the operator does not need to know exactly
what his packet looks like. In particular, he does not need to
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
is 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
speed requirement, an assembly-language program or equivalent
is required. While BASIC looks pretty much the same on any
computer, assembly language is different for every machine.
If the TNC were replaced by personal computers, program dev-
elopment would have to be redone for each variety of com-
puter. In addition to maintaining the right pace, the TNC also
must be constantly "listening" at both ports simultaneously
while putting packets together or taking them apart. The
hardware of personal computers may not even be capable of this
sort of multi-task application.
   Programming of individual TNC's must be as easy as possible,
since there will inevitably be unforseen problems in the
initial software. In addition, hardware changes may necessitate
software changes. For this reason, TNCs are designed around
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-
inarily consists of a text message typed in by the operator,
sandwiched between the header and tail information required by
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
sending and receiving TNC's don't get swapmed with information.

   A packet need not consist of ASCII or Baudot character strings,
however. It could contain information in other coding systems, such
as BCD or EBCDIC, or even binary data such as a compiled computer
program. The TNC, which uses a "bit oriented protocol" based on a
standard called High Level Data Link Control (HDLC), can encode
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-
ware and software. A second advantage of HDLC protocol is that the
beginning and end of the entire message are flagged, making the
"start" and "stop" bits for each character unnecessary when the
packet is transmitted in "synchronous" format.

   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
in 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
protocol being used.

   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
request for information from net control. It may also contain a
sequence number for a multi-packet message which must be received
in 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
has been received correctly. If the FCS calculated by the
receiving TNC matches the FCS of the packet, an acknowledgement
is 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
station may be a single-frequency simplex repeater which re-
transmits any correctly received packets, or it can be "normal"
split frequency repeater.

   As operators sign on to the net, they are recognized by the
net control and given net address codes. An operator desiring to
start a QSO with another net station will subsequently have his
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
station is functioning as net control participates in exactly the
same way as other operators. The net control functions are taked
care of automatically by his TNC.

   As more packet radio LAN's become active, there will be the
possibility of link stations with access to two distinct LAN's.
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"
station, which will be a specialized station having access to some
long-distance mode of communications. The gateway station will
reformat packets with another layer of protocol containing inter-
network linking information and transmit it to another gateway
station in a distant LAN. Three possibilities are being explored
for long-distance links.

   TERRACON will be a high-speed ground-based linking system
utilizing UHF and/or microwave relays. It could potentially
handle most long-distance packet radio communications in the
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
special-services channels on the AMSAT Phase III-B satellite.
AMICON will allow intercontinental linking and contact with
isolated areas not accessible to TERRACON. High data rate exper-
iments are being planned for the 23cm uplink/70cm downlink (mode
L) translator. There are also plans for a packet radio digital
repeater aboard the AMSAT Phase III-C satellite.

   SKIPCON is AMRAD's projected HF network of LAN gateway
stations. The nature of HF propagation will require slower data
rates (75 to 600 baud) and error correction as well as error
detection protocol. SKIPCON experiments have been conducted
since the end of 1981.

HOW TO GET IN ON PACKET RADIO

   There are currently two TNC designs available. The first
packet radio TNC was designed by the Vancouver Amateur Digital
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
power supply. A modem kit is also available from VADCG. The TNC
design is based on the Intel 8085 CPU and 8273 HDLC controller
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
ports) for interface to the terminal, as well as an interface
to the radio.

   The Tucson Amateur Packet Ra…io group (TAPR) is currently
testing a second TNC design. This TNC has the modem, radio
interface, serial and parallet terminal interfaces, and power
supply circuitry (exclusive of the transformer) on a single
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
VADCG-compatible protocol.
   Additional information on TAPR activites is available from
Tucson Amateur Packet Radio, PO Box 22888, Tucson, AZ 85734.
 


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