Shareware Grab Bag/Shareware Grab Bag.iso/090/pkttut.arc/PACKET2.INT

Text File | 1986-04-02 | 7.9 KB | 157 lines

TYP PACKET.IN2
(Continued from PACKET.INT in the XA9 HamNet Database)

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 is0implemented 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 Raàio 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.