Sound Card Packet  with AGWPE

Translations of this site
Most recent AGWPE version is:  2013.415  15 Apr 2013

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Packet Engine Pro

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About Packet
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Further Reading

Packet Overview: What is Packet?

Packet is one of several data communications modes used by amateur radio operators (hams) to send text and data files. This web page attempts to explain the basic concepts and mechanics of packet without too much radio jargon. Its primary objective is to build a foundation for a later explanation of the role of the AGWPE program in packet exchanges.

Disclaimer: For explanation purposes, this page contains oversimplifications and generalities that a more advanced operator might consider inaccurate. Keeping in mind the author's objective of promoting a basic understanding of packet without getting bogged down in details, he welcomes corrections and suggestions for improvement.

Traditional Packet Station
AX.25 Protocol
Role of the TNC
Role of the Computer
Terminal Program
Inside the TNC
On To the Radio
Receiving Packet

The name "packet" comes from the technique of sending small packages of information -- packets -- in bursts. It takes one or more packets to convey a single typed sentence and many, many packets to send a paragraph, page, or large file. One of the advantages of packet is that it includes a provision for the receiving station to request a re-sending of any packets it thinks it has not received accurately. This "automatic repeat request" feature, abbreviated "ARQ" by hams, makes packet an excellent data mode when 100% accuracy is desired.

Note: You will also find the term "packet" being used to describe elements of computer networks and digital telecommunications systems, since they too use the transmission of short sentences of data.

Traditional Packet Station

Let's talk about sending packet on the VHF frequencies. Later we'll talk about some differences for "slow"  packet on the HF frequencies and "fast" packet on the UHF frequencies.

Historically, a packet station has included:

  • a computer with monitor and keyboard for entering and receiving messages
  • an intermediate smart device called an Terminal Node Controller or TNC
  • a radio for sending and receiving packet signals

Let's follow the process for sending a single packet. Let's assume you want to send a simple sentence, "Hi Don!" to your friend, Don. You have your computer, TNC and radio running, and he has his computer, TNC and radio running. What next?

AX.25 Protocol

First, we need a guide to help us through the process: a packet rule book. In the 1980s, this set of rules, or protocol, was first set down in a document called the AX.25 protocol. Last revised in 1998, it is the definitive reference on the packet "language" and transmission procedures. At this point, you don't need to know any of the rules, just that there are some and they help packet stations exchange packets effectively. We will explain some of those rules as we go along.

The Role of the TNC

Now, let's look at the role the TNC (Terminal Node Controller) is going to play in the process. The TNC is going to take the sentence you type on the computer and convert it into a series of two tones that the radio will transmit. At our friend Don's station, his TNC is going to convert those tones from your radio signal back into the characters Don will see on his computer monitor. The TNC also has a computer chip that has been embedded with some of the AX.25 protocol rules. As we go along, we will try to explain a bit more about what the TNC is doing. On a later page, we'll try to explain how AGWPE (in its sound card mode) compares to the TNC.

The Role of the Computer

TNCs could have been built with a keyboard and monitor and been an all-in-one device, but that would have been expensive and bulky. Why not just use a computer's keyboard and monitor? That's exactly what the developers decided to do. But for this scheme to work, we need two things:

  1. A physical connection between the computer and the TNC. The developers decided to use RS-232-compatible connections, specifically serial cables. The cables connect a serial/COM port on the computer to another serial/COM port on the TNC.
  2. A software program to facilitate the transfer of data back and forth between the computer and the TNC -- a terminal program.  In simple terms, the terminal program converts text from the computer keyboard and sends it to the TNC for transmission as tones by the radio. The program does the reverse for any received signals -- it converts data received from the radio and TNC and sends it to the monitor for display.

The Terminal Program

Let's look at the terminal program and  what it does to send and receive text from the TNC.

First, a terminal program must be configured to communicate with the TNC. It must set to use serial port settings that include:

  •  the data transfer rate (baud rate) between the two devices,
  •  parity rate (none, odd or even)
  •  number of data bits (7 or 8)
  • number of stop bits (1 or 2)

It's not important to know more about those settings for this discussion, just that the terminal program must match the settings in the TNC. (Note: TNC's come with default serial settings. After the terminal program and TNC connect successfully, it is possible to change the TNCs default serial settings using the terminal program.)

Now, with the program "talking" to the TNC, the terminal program must collect any text that is typed on the keyboard, and seen on the monitor, and send it to the TNC in a form the TNC can use.

According to the AX.25 protocol (above), the TNC needs to receive any letters, numbers or punctuation marks translated into into series of eight zeros (0) and ones (1) that represent an ASCII character. For example, an uppercase "A" is ASCII character  number 65 and 65 is represented by these 8 digits in binary code:  01000001. (The series of eight zeros and ones is called a byte.)

So "Hi Don!" is converted into to binary code as follows::



Binary code






















The complete sentence to Don would look like this in binary code:


Next, the terminal program must be configured with addressing information for the packets you want to send:

  • your callsign (often entered and stored when the terminal program is first configured)
  • the callsign of the target station for this packet, in this case Don's callsign
  • the callsigns of any digipeater stations that you want to relay the packet to Don, if any.

These callsigns will also be converted to binary codes representing ASCII character numbers.

With the addressing information and data (Hi Don!) converted, the terminal program forwards the binary code to the TNC.

Inside the TNC

The TNC has three primary parts which are used to send and receive packets.

1. The Packet Assembler and Disassembler or PAD

In simple terms, the PAD takes the addressing information and the data from the terminal program and combines them into a packet, also called a frame. It also adds in some processing characters (from the AX.25 protocol), such as flags to mark the beginning and end of a frame.

A complete frame might contain a string of up to 2048 digits (zeros and ones), which equals 256 characters/bytes. Usually, the frame is limited to 128 characters or less, since shorter frames have a better chance of making it through the radio transmission process accurately.

2. The Modem

To send a packet, the TNC uses a device called a modem to convert the zeros and ones into tones that are fed to the radio's audio input circuit and thus change (modulate) the radio's signal. Typically with 1200 baud (baud=bits/tones per second) packet, the tones are 1200 Hz and 2200 Hz (Bell 202 Standard).

The modem uses changes between the tones to indicate a zero and no change to indicate a one. So a low tone followed by a low tone would be no a zero, but so too would a high tone followed by a high tone.  Source:  (The TNC also has a clock to determine when one tone has ended and another has begun.)

At the receiving station, another modem must detect the radio signal changes/tones (demodulate) and convert them back to zeros and ones. The name "modem" is short for modulate-demodulate, since the same device can be used to send (modulate) or receive (demodulate) tones.

3. Logic Chip

Also in the TNC is an EPROM logic chip. The chip has been programmed with the AX.25 protocol transmission rules. These rules provide for efficient exchange of packets by:

  • establishing an exchange process with a target station
  •  making sure packet frames are received in the correct order and without errors
  •  asking for a retransmission of a packet frame that has been lost or damaged

In addition the chip logic provides:

  •  a way for the user to 'talk' to the TNC and set various timing settings to improve the efficient flow of packets, for example, how long to wait before sending a reply packet
  •  a way to beacon a general announcement at specific intervals
  •  digipeater services, to automatically relay packets for others
  •  optional services such as a personal bulletin board system to receive and store messages while the operator is away from the keyboard

On To the Radio

From the TNC, the packet frame is then sent to your radio for transmission. A cable between the TNC and radio carries:

  •  audio output, i.e. the stream of tones representing the packet frame
  •  a signal to trigger the PTT (push to talk)/transmit circuitry of the radio
  •  audio input, i.e. the stream of tones being sent to you

On some radios, the cable from TNC is connected at the the microphone, the microphone being removed temporarily (and on some radios, the audio input may have to come from the radio's speaker jack instead of the microphone jack). On newer radios, the TNC connection can be made at a special data port/jack on the front or back of the radio.

Because the TNC can initiate the radio's transmit circuitry, there is nothing the operator needs to do to facilitate the packet transmission, other than tune the radio to the appropriate frequency.

The audio tones from the TNC will modulate (change) the radio signal by shifting the sending frequency ever so slightly. Since the TNC tones  that create this modulation are input to the radio's audio circuit, this type of modulation is known as Audio Frequency Shift Keying or AFSK.

Listen to packets sent at 1200 baud

Note: AFSK is the modulation method used for common 1200 baud packet on the VHF frequencies. Different baud rates and modulation methods are used for "fast" UHF packet. You can read more about this on the Operating Modes page on this website. 


On the Receiving End

On the receiving end, the process is reversed. Your friend's radio will convert your received signal back into tones that are fed to his TNC. The TNC's modem will convert the tones to a binary bit stream that represents the packet frame. The TNC's PAD will then extract the data from the packet frame, in the form of zeros and ones, and send it to the computer's terminal program. There they will be converted back to text, numbers and punctuation symbols and then displayed on your friend's monitor:

Hi Don!


This page has offered a simplified overview of the packet process -- from typed text to ASCII character numbers to binary code to audio tones to radio signal modulation -- and back again. It has glossed over the some of the functions of the TNC and the AX.25 rules, but they will be covered a bit more on the follow-up page, Packet Modes. 

Last Updated:

by Ralph Milnes NM5RM


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