Packet radio is a method of communicating digitally over radio frequencies. It is much like using a phone modem to
access the Internet, only without the phone line! Operators on packet can do keyboard chatting, file transfers and email.
While the protocols used on packet radio are not exactly like the Internet, there are many similarities, so anyone
familiar with using the Internet will find themselves at home on packet.
How can this work? To allow digital communications to be passed over radio links, the signals in the computer (1's and 0's) must first
be converted to a format suitable for transmission over a radio. This conversion process utilizes a device called a
TNC (Terminal Node Controller). This device (usually external to the computer) consists of a controller, called a
PAD (Packet Assembler Disassembler) to process the serial data from your computer or terminal into packets. These
packets are then turned into audio tones by a MODEM (MOdulator/DEModulator) that can be fed into your radio's
Fig 1. Packet Communications
Fig 2. Packet TNC Function
TNC's come in all shapes and sizes, with a lot of different features. In general though, there are
two basic types of TNC, Packet-Only and Multimode. Multimode TNC's include the ability to convert digital modes
other than packet, as well as handling normal packet operations. Some current multimode TNC's can operate Packet,
ASCII, RTTY, SSTV, WEFAX, PSK, AMTOR, and other digital modes.
Due to advances in computer technology and digital signal processing, it is now possible to dispense with the
external TNC altogether. Using a simple interface from radio to PC soundcard, software can emulate the functions of
a TNC. For the average user, this can alleviate the strain on the wallet that even a modest station setup can
Fig 3. Various TNC's
Several commercial vendors make soundcard interfaces:
Or you can build your own:
KB4OID's Homebrew Sound Card I/F
Fig 4. Sound card interface
A good primer on packet radio can be found at PacketRadio.com.
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