64
Modes of Operation
This chapter covers the major ways in which you may use your Kantronics TNC. For
details on particular commands, see the ―Command Reference‖ chapter.
Packet Mode of Operation
This section adds to the information presented earlier (in the Getting Started and other
sections) on how packet radio uses digital information for communication. By
familiarizing yourself with the concepts presented here, you‘ll be aware of and, if you
wish, able to change the default value(s) of the command parameters in your TNC to
optimize your packet activities.
Introduction
Information is Organized into “Packets”
A packet is a group of characters with a flag and header at the beginning and a
checksum and flag at the end. A flag is a specific character used to signify the
beginning and ending of a packet. The header is information indicating who the packet
is from, who it is to, any relay stations needed to get to the destination and some control
information. A checksum is a complicated mathematical formula that produces a
number based on the combination of characters that are in the packet. This number is
recalculated by every station that receives the packet, and if it does not match the
number that is in the packet, the packet is thrown away, thus near error-free
communications. A packet is also called a frame.
Your Packet Unit is a Terminal Node Controller (TNC)
Packet radio modems, or packet modems, are generally referred to as TNCs. This
―label‖ or ―moniker‖ was adopted when the Tucson Area Packet Radio Group (TAPR)
developed their first ―TNC-1" packet radio modem kit in the early 1980s. The TNC is the
workhorse of packet radio. As a listening device it hears an audio signal from the radio,
changes the data to digital form, determines if the data is a good packet and sends it to
whatever device is attached, usually a computer. As a relay device it also checks the
packets it receives and determines if the packets need to be resent, then does so if
appropriate. As a sending device it receives digital data from the computer, packetizes it
and changes it into audio tones, which are sent out to the radio. The rule the TNC uses
to do all of this is called a protocol.
Protocol for Amateur Packet Radio: AX.25
The most commonly used protocol in amateur packet radio is AX.25. The details of the
inner workings can be found in a book titled ―AX.25 Amateur Packet-Radio Link-Layer
Protocol‖, which is available from the ARRL. Most TNC users will not need to or care to
go into the details. The TNC takes care of the nitty gritty work for you, although there
are parameters you can set that determine how efficiently some of that work is done.
To Next Page
Loading...
