Amateur Radio Packet Basics

If you have an amateur radio license, you must have heard about Amateur Radio Packet. Let's see if we can push aside all the complex stuff and explain the basics as simply as possible.

Amateur Radio Packet is a form of text communication using amateur radio equipment. A packet station needs a TNC to communicate with other packet stations. The TNC, short for Terminal Node Controller, is a small box connected to the radio on one side and to a PC on the other side. The diagram below shows two packet stations, and the lines of communication.

Packet Data Transmission

At the transmitting station, a Terminal program (like Hyperterm, IPSerial, etc.) or a other software application (like Outpost, etc.) running on the PC, is used to send text data to the TNC. The TNC breaks up the text into small chunks (like 32 characters each) and encodes it into AX.25 packets. The Packet Modem in the TNC coverts the digital data in the packets into audio signals, which are then used to modulate the radio signal transmitted by the radio. Typically, Frequency Modulation is used to modulate signals on the VHF and UHF bands.

Packet Data Reception

At the receiving station, the radio captures the FM signal and extracts the audio signals. Then the Packet Modem in the TNC decodes the received audio and converts it into a digital data. The TNC interprets the data per the AX.25 protocol, and the packets are strung together and converted to text. The text is passed on to the program running on the laptop, which either displays or processes the received text.

Interfaces & Connections

Although pretty obvious, a quick note about the interfaces may not hurt. The radio is connected to the TNC by an interface cable carrying four signals - Audio IN, Audio OUT, Push To Talk and Carrier Detect. The TNC connects to the PC via RS232 cable or an RS232-to-RS232 adapter.

A note about IDs

To keep everything legal and easily identifiable, a couple of unique IDs are used. Per FCC regulations, all radio transmissions should be identifiable, hence every data transmission from the TNC will carry with it a Station ID. Typically this is the FCC assigned call-sign of the station owner, and a numeric suffix (for example, -1), to keep it differentiated from other IDs.

Similarly, a Mailbox ID is used to identify the mailbox in the TNC. Usually, this is also the callsign of the station owner with a different suffix (for example, -2).

In some cases, like public events, it may be better to use a tactical identifier, like "MEDICAL" or "LOGISTICS", as the Station ID. In this case the software applications running on the PC will switch the Station ID temporarily to the preassigned Tactical Station ID.

Where's the fun in that !

Come to think of it, it is kind of cool to be able to send text from one station to another several miles away without a cell phone. It's useful too, if you have to send someone's address or a list of materials without having to say and spell it out over voice radio. See Keyboard-to-Keyboard in the diagram.

At 1200 baud, it is pretty slow by today's expectations but with this capability it's also possible to send formatted text like a list of names or things or the weather data, or a filled-in form etc, without errors. The receiving end will understand the format and associate the data in the expected manner, sort of like what a web server would do !!

Not convinced? Then how about this...

Most TNCs also have a small mailbox, yes a mailbox! The mailbox lets people to post messages, addressed to specific persons, to the mailbox and read messages from it. But wait, the mailbox also lets senders to post bulletins, for everyone to read ! Pretty cool eh? (see Mailbox Operations in diagram)

Well, what else ?

Think of this setup as a Lego-box for Wireless Digital Communication. By writing your own software, you can do build any type of communication system you desire. Several amateur radio enthusiasts have done just that. Now, its your turn... so ask yourself what would you like to do with it ?

... but most importantly, have fun doing it and share it with the world !