Buck Rogers K4ABT Handbook page 5

AN INTRODUCTION TO
PACKET RADIO IS:
c
ommunications between computers using radio (RF) as the medium rather than wires or solid mediums. Your
personal computer in your ham shack coupled to your VHF or UHF transceiver via a terminal node controller (TNC) is
the makeup of the Packet Radio station. Computer terminal programs enable computers to send and receive Ax.25
Packet along with other well know digital modes such as CW and RTTY. The mode we are most interested in it called
"Packet Radio."
Packet Radio is faster and is completely error-free as compared to CW and RTTY.
allows us to deliver ASCII text, binary data, and even digital video via this high speed and error-free communications
medium.
One of the many benefits of using Packet Radio is that it preserves spectrum by allowing several stations can share one
frequency at the same time. By using the X-1J4 network node base described later in this book, you will discover other
ways that Packet radio can link your State, Country, or even the World into one massive network. One of the elements or
off-springs of Packet communications is the internet. Much like Packet radio, the internet uses every conceivable type or
kind of medium to transverse from point A to destination B. The internet uses longer packets with a protocol based on
Transmission Control Protocol (TCP) with Internet Protocol (IP) addressing.
THE TERMINAL NODE CONTROLLER (TNC):
By now you have heard the phrase, "terminal node controller" or "TNC" several times. The terminal node controller
(TNC) performs as an interface between the computer and the transceiver. The TNC combines a modem and a packet
assembler/disassembler that accepts information from your computer and sends received data to your computer. The TNC
(prior to 1983 it was called a "PAD" or packet assembler/disassembler) breaks data into "packets." These packets are
normally 128 letters or characters in length. In our Packet TNC we have the option to increase this packet length up to 255
characters in length. The TNC command that allows us to make this change is called "PacLen."
Once the TNC has received the data from your computer, it then breaks it into packets about 100 characters long and
combines additional (bytes) information to the outgoing data. The added bytes include the destination addressing, error-
checking, and frame control information. Address and destination information that is added in by the TNC includes the
callsign of the target station and the callsign of the station sending the packet (data). This same frame or data packet will
also contain the path or nodes that is used between the two connected stations. The forward error-checking intelligence
within the AX.25 Packet frame enables the target station to ascertain that the was received without any errors. If errors
were received, the receiving station sends a non-acknowledge (NACK) Packet indicating to the sending station that it must
repeat the Packet again or until the Packet is received "error free." Once the Packet is received error-free, the receiving
station will send an "ACKnowledge" packet to indicate that the data was received as sent.
DIGIPEATERS & NODES:
Any packet radio station can act as a digipeater. A digipeater is a "store-and-forward" Packet station. Most TNC's are
setup to digipeat automatically without any effort on the part of the station being used as a digipeater. To reach a distant
station, we connect to any remote node that is in range of our station. From this point on, we use the remote node as if we
have a long cable between our computer and the remote node. Once connected to the remote node we can instruct the node
to connect our station to a distant Packet station. The node will "ACKnowledge" packets between our station and the target
or destination station. The node is also a "store-and-forward" device that may be located atop a mountain or tall structure
that enables it to hear Packet stations that are beyond our stations reach. This handbook has a complete section dedicated to
the construction, configuration, operation, and use of nodes. Read on as you are about to discover how much fun and
enjoyment Packet Radio
; Packet Radio "The Basics"
Section 1
PACKET RADIO:
by
Buck Rogers K4ABT
A Packet Primer for the new Packeteer
This mode of data communications
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