
An intoduction to Packet radio
------------------------------
By Ron Upton WB0WSI

Packet radio is a communication system in which information is diitally 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 maximum use of the spectrum through

shared frequency use. Data integrity is provided by packet radio through a
"handshaking" technique and error detection.  Along with each transmission, 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 acknowledgment (ACK) signal.  If the sending
station does notreceive 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 QSOs 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 transmissions 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 controller at each
station, and it will obviously appeal to the ham  who already has a computer in
his shck.  However, it does not require that the operator be a programmer, or
even that the  station have a personal computer.  All that is realy necessary
is a terminal, a terminal node controller (TNC), and an amateur radio
transceiver. The terminal can be a simple display or typewriter terminal that
produces ASCII characters, a personal computer or even a commercial mainfram
computer.  What you need is a terminal with a keyborad to allow you to talk and
a screen or 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 an "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
character.  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 terminal 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 inormation 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 transceiver  microphone and speaker audio
lines, and the PTT line. Ordinaryily, 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 unrealted to the terminal baud rate at the other
port of the TNC. The receiving TNC reverses this procedure, decodin the audio
tones from the Speaker audio line of the radio, removing 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 MOdualtor-DEModulator.  This device may or may not be built
into the TNC borad.  Most packet radio modesm operate at 1200 baud, which
corresponds to about 1200 wpm, althought 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 Bell 202 modem. 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 IS
A PACKET?
=================
A packet is the basic message unit in packet radio. It ordinarily consists of a
text message typed in by the operator, 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 TNCs don't get swamped with
information.

A packet need not consist of ASCII or Baudot character strings, however. It
could contain information in other coding system, 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?
the TNC hardware 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 is represented below. Each field of
the packet is encoded as a sequence of 1s and 0s (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 beginning of a packet to
the HCLD controller. This pattern corresponds to no sequence which would be
encountered in any of thefields, except possible 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 originating
station, and possibly intermediate routing information if the packet will be
relayed to the destination. The destination and originating stations might be
identified bya network address number or by amateur call sign, depending on the
exact form of protocol being used. The CONTROL field describes the purpose of
the packet to the receipient. It identifies packets with such functons as
initialization or termination of communications, packet ackowledgment, or
request for retransmission. It may also contain a sequence number for a
multi-packet message which must be received in the correct order. The DATA
filed contains the message being sent, which will ordinarily be 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 acknowledgment is sent; otherwise the packet is ignored. The
final FLAG marks the end of the packet.

WHAT IS A PACKET NETWORK?
=========================

A local are packet radio network (LAN) consists of a number of individual
packet stations, which are ordinarily within simplex range. The net may also
contain a digital repeater or digipeater", which may also function as an
individual station. The digipeater is a single-frequency relay which
retransmits any correctly received packets. The protocols currently implemented
by anateur packet radio call for stations to communicate pairwise, through
"connections" which are set up through exchange of special packets. An operator
desiring to start a QSO with another net station will subsequently have his
transmissions addressed to that station. In order to simulate a conventional
anateur "net", stations can monitor transmissions of stations other than the
ones to which they are connected. Of course, the TNC will only acknowledge
those transmissions intended for that station. As more packet radio LANs become
active, there will be the possibility of link stations with access to two
distinct areas. These stations can serve as communication links through which
packets originating in one area can be funneled to an addressee in the other. A
more sophisticated possibility is that of a "gateway" station, which will be
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. Threee possibilities are being explored for
long-distance links. Ground Relays 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 th 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. Satellite
Service AMICON wll be a satellite-based network utilizing one of the
special-services channels on the AMSAT Phase III-B satellite, using a reserved
5 kHz segment o the Mode-B transponder (up on 435 MHz, down on 145 MHz). AMICON
will allow intercontinental linking and contact with isloated areas not
accessible to TERRACON. High data rate experimetnts are being planned for the
23 cm uplink/70 cm downling (mode L) translator. PACSAT is a new class of
satellite designed solely for digital communications. current designs call for
multiple packet radio uplinks on 435 MHz into a low earth orbiting (LEO)
UoSAT-calss OSCAR satellite containing a packet radio repeater (digipeater). A
common downlink on 145 MHz would provide either real-time repeating service, or
a delayed messge storage facility, using up to one megabyte of on-board
storage. This "flying packet radio mailbox" could also have more traditional
digital channels, like RTTY and ASCII. There are also possible plans for a
packet radio digital repeater aboard the AMSAT Phase III-C satellite. Short -
Wave Links 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. A form of
error-correcting code for amateur radio known as AMTOR may be the best
candidate for handling the vagaries of HF channels. SKIPCON experiments have
been conducted since the end of 1981. LOCAL INFORMATION FOR PACKET RADIO. There
is a local(front range) group "RMPRA", ROCKY MOUNTAIN PACKET RADIO ADDOCIATION.
This group is now publishing a newsletter, editorial office (1626 27th St.,
Greeley, Co. 80631). The frequencies in use for packet radio are 145.01 -
145.09. A band plan has been accepted for this region as follows. 145.010 is
the standard calling channel for packet radio and is the local access frequency
for network links. 145.030, .050, .070 and .090 are other (locat) packet
frequencies. 220.950 is the interim 9600 baud link frequency. 145.01 MHz is the
National Callin Frequency for packet radio, though a few areas of the country
do not have this frequency available yet. Long term, we expect to use 145.01 as
input to the high speed link system. In the interim, 145.01 has achieved so muc
h use that it is often congested and slow. Therefore the recommend use of
frequencies is as follows:

145.010 for calling frequency and link inputs.
145.030 for local use in Colorado Springs and Fort Collins.
145.050 for local use in Pueblo, Denver and Cheyenne.
145.070 and 145.090 not allocated, available for local QSO and for future
allocation. Bulletin Boards are urged to move to their local frequency during
prime time at least, and to use 145.010 for file forwarding at non-prime hours.
This information may appear like a lot of knowledge on my part but is
information that I have collected or picked up from the real experts in this
area. If you have a question, I will find out the answer for you or put you in
contact with the right individual.
73s. Ron Upton, WB0WSI

For more information about Amateur Radio write:
ARRL 225 Main Street, Newington, CT 06111  
or call the HAM HOTLINE in Boston (voice) 617 437-0111   

  
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