Getting Started in Packet Radio

Equipment Configurations...

The Classical Terminal Model

Let's begin by considering the simplest, classical assemblage of packet equipment. In the "old days" which was not so long ago, computers were incredibly expensive. Only large universities and corporations could manage to purchase and run them. So how did you communicate with the central unit? You used a terminal. A terminal is like a very simple monitor except it runs over an RS-232 line, not a parallel tri-color cable. If you look carefully the next time you are in the check-out line, you might see a terminal displaying the money amounts for your transaction. So they haven't gone away, just been re-allocated as eventually happens to all "new" technologies...

OK, what's next? You need a TNC, a Terminal Node Controller. And what does this do? It links the terminal to the radio. But, remember, the terminal is not too smart, so it needs an operating system and that is what is built-in to the TNC, hence the words node and controller. The TNC has a UART chip which talks to the terminal, a CPU which manages all these events, memory, a modem, and sometimes a pretty high price because it has alot of software on board as well as its local operating system. It is really a dedicated micro-processor. (However in fairness, the costs have decreased steadily over the years.)

Now you need a radio, usually a VHF radio, but it could be an HF rig as well. The audio side of the TNC, or modem part, is connected to your radio. There are usually three lines, plus the ground, that control the radio side: the PTT or Push-to-Talk, the RX or receive line and the TX or the transmit line. And, the radio is connected to an antenna , usually a groundplane vertical up about 30 feet or so.

So, there you have it, the classical packet station that has been in effect for probably over 30 years, and still finds relevance even today. Often emergency portable stations still use this Terminal-TNC-Radio classical model because it is light-weight, flexible, and dependable. Even if a laptop computer were used, it would probably be running a terminal emulation program like ProComm, or one from a wide variety of others currently available.

"Vintage" Model 100

Once, I even used an RS Model 100 as the viewing device (terminal), and despite its eight lines of display, I was able to copy any and all traffic over not-too-busy frequencies! Using a TNC like a KPC-3, a handie-talkie for the radio, and a portable J-Pole or groundplane antenna, would make a very dependable, low-power emergency packet station.

In summary, here is what you need to set up this system:

• A Terminal, or Computer with its Monitor
• A TNC: A Terminal Node Controller
• A Radio System for either VHF or HF, i.e., my local setup

And, you will also need the appropriate software if using the computer, i.e., a terminal emulation program, such as ProComm, etc. For extra hardware, you will need an RS-232 nine (9) or 25 pin connector between the computer and the TNC, and audio cables between the TNC and the radio.

The Newer Computer Based Model

The next level up the hierarchy of performance "moves" the reliance on the operating system out of the TNC and into a standalone computer. The OS would be something like DOS/Windows, UNIX, or Linux . The software running on these platforms would either be a terminal emulator or a "Network Operating System," a NOS, such as JNOS , TNOS or the AX25 Utilities . (Such network systems function as servers enabling multiple, simultaneous connects from many users, even on DOS platforms!)

If you only use the terminal emulator capability, you really haven't gained that much by using a computer; but, if you use one of the NOS's, you gain all the power that your computer has to offer in terms of speed and memory, plus you have at your disposal all the extra functions and features inherent in the NOS software. It is obvious that a TNC can only hold so much ROM code, whereas your computer can run larger programs that can do so much more than the TNC. (It is also easier to upgrade software when installed on your PC than to burn code into the TNC's ROM.)

So let's assume for the sake of argument that we are running a Linux box, and the network software is JNOS. What does this look like? Well, we still need a TNC but we are basically only going to use the "packetizer" and the modem part of it. Almost all of the software internal to the TNC will be skipped over. The JNOS program will assume all of the functionality that the TNC had before, plus more as noted above. And we will employ a special protocol, or data format for serial communications, to send our packet data to the TNC. That protocol is called "kiss." (We still use the RS-232 line and its associated parameters, but the data will be encapsulated in this "kiss" protocol.)

Why is it called "kiss?" Because this is supposed to be the simplest possible protocol known. There is only one special character at the start of the data string and one special character at the end of the string. And to "keep it simple" here, that is all we need to know for right now. When this data packet arrives at the TNC, it is "de-encapsulated" from these special "marks" and then run through the High Level Data Link Controller, the "packetizer," and sent out over the air. When data comes in, it is also "kissed" and sent back to the computer program where it is also de-encaped from the "kiss" protocol. (So when using the NOS programs, the TNC must be set into "kiss" mode or it won't work at all!) A "Kiss" Example ...

So if you are running a laptop with Linux on board, you still have the portability and low-power capability that you have with the classical setup above, except you have all the power of Linux and JNOS combined, a very significant advantage! (However, the dollar cost of this system is also significantly greater than the classical model! Tradeoffs, tradeoffs...)

In summary, here is what you need to set up this system:

• The Computer with its Monitor, Desktop or Laptop
• An OS capable of running the Application Software
• The Application Software: JNOS , TNOS, or AX25 Utilities
• A TNC: A Terminal Node Controller, running in "kiss" Mode
• A Radio System for either VHF or HF, or both if dual TNC

And, for extra hardware, you will need an RS-232 nine (9) or 25 pin connector between the computer and the TNC, and audio cables between the TNC and the radio.

Extentions to The Computer Model

The extentions comprise a group of software components that continue to replace hardware functions with software. As you can see, this is the current trend and will probably continue for some time...

A typical replacement example is that the "packetizer" is now running inside the computer, that is, the High Level Data Link Controller which is what breaks the data stream up into packets is now actually a module on Linux systems called hdlcdrv.o. Another replacement has even gone as far as replicating the modem itself using the soundcard! And who knows, one day we may even see the radio with some of its hardware functions managed by an enclosing computer!

The network software for this "advanced model" inherits all the previous capabilities from the earlier computer model and can therefore run either JNOS, the AX25 Utilities, or any NOS-based system. So the applications have not changed when attaching to these extentions, just the methods of handling the packets themselves at the lower levels in the AX25 communications stack (HDLC), or at the interface to the radio (Soundcard).

In summary, here is what you need to set up this system:

• A Computer with its Monitor
• If Linux OS, kernel 2.2.0 or greater
• The Application Software: JNOS , TNOS, or AX25 Utilities
• A TNC, or an FSK Thru-modem , or a Soundcard
• The Drivers for any or all of the above hardware, some examples...
• A Radio System for either VHF or HF, or both if dual TNC

And, for extra hardware, you may need an RS-232 nine (9) or 25 pin connector between the computer and the TNC, and audio cables between the TNC and the radio, or the soundcard and the radio.