Amateur Radio Guides – Packet Radio

TopicPacket Radio
SubtopicDigital Modes
Equipment RequiredComputer + Radio + Sound interface
Document last reviewed and updated (reviewed each year)5th February 2023


Note: Please note that this article is generalistic in nature and it is intended to be. It is impossible to cover a subject that has had many changes over 40+ years. The purpose of ths article is to explain a few basics that will allow the reader to perform further investigation that particularly relates to their field of interest. It was necessary to provide this article as many recent hams that “play” with Winlink and VarAC do not fully understand where this originated from, how it is changed and ultimate how to locate and resolve faults.It should be also noted that their are Hybrid designs e.g. Some Modem TNC‘s are using USB connections instead of older Serial Connections.

Packet Radio has been around for quite a while, in many cases longer than you might have expected.

AX.25, also known as Amateur X.25, was originally derived by the Layer 2 of the X.25 Protocol Suite and was designed for use by Amateur Radio operators. Layer 2 refers to the OSI Layer model which is outside the scope of this article, but well worth understanding (even at a high level) as it is the conceptual model used to describe the functions or a networking system. The OSI model was publish n 1984 by the International Organisation for Standardisation (ISO).

OSI Layer Model

As mentioned AX.25 is a Data link Layer protocol. AX.25 is commonly used as the data link layer for the network layer such as IP4 and then TCP is placed on top of that.

In Australia, whilst an exact date cannot be put on it, Packet Radio gained a lot of popularity around the Mid 80’s, with regular articles in related magazines by 1986. I know I remember seeing my first Packet Radio TNC in roughly 1987 at a local Amateur operators shack, with my interest piqued, as I was working with a recently introduced 1200/1200 direct connect kit modem (about 30 discreet IC’s) at the same time.

Packet Radio is still around and is utilised for APRS (as you know it stands for Automatic Packet Reporting system), for Email such as Winlink, Chat applications like VarAC which is a Freeform Chat FT8 alternative.

Back in those early days, your only choice to utilise Packet Radio was through the purchase of a reasonably expensive Modem TNC which prevented a large number of people getting interested in packet radio.

Nowadays, it is lot less to get involved and your choices are

  • Modem TNC (Serial Interface) – Modem and Control done by the TNC Modem
  • Modem TNC (USB Interface) – Modem and Control done by the TNC modem with the serial interface being a USB connection (as most computers are no longer coming with serial interface)
  • Sound Card interface (like DigiRig / SignaLink) – The main difference is that the PC is doing the part of TNC and Digital Sound Processing, with Digirig or SignaLink being the sound interface
  • Sound Card interface of their Transceiver now that many modern rigs have this built in, making Packet Radio even simpler and more reliable.

Needless to say, a lot of Amateurs are opting for the Sound Card interface usually already in their Transceiver (costing them very little to get on Packet Radio) or using a Sound Card Interface like DigiRig/SignaLink which can be bought in most cases for less than $AU150 (I assume less than $US100)….Even with the cost of the VARA licence at $100AU, it is still cheaper than $500 min for a brand name Modem TNC at lower rates.

Design differences between Hardware TNC and Software TNC

Traditional Modem TNC

This first diagram shows the traditional configuration of the Packet Modem TNC

Typically the PC could be either a dumb terminal or a PC running a terminal emulator.

The PC would connect to the Packet Modem TNC with a typical serial cable with TX/RX.

From the Packet Modem TNC to the Transceiver, you would have a cable and you would, in most cases be a 5 to 6 Pin cable, but this could differ based on your Transceiver

Typically the pins would be

Pin 1 – Microphone audio
Pin 2 – Common Ground
Pin 3 – PTT – allows the TNC to key your transmitter.
Pin 4 – Receive audio
Pin 5 – Squelch input (this may or may not be used

The one critical difference is that the Modem TNC has the “software” to perform the control of the Transceiver, so it key the transmitter when it is ready to send.

You will find many of the past and modern transceivers have some sort of interface that has these signals exposed.

Sound Card Interface and PC Software based TNC

This second diagram shows a USB connection to the Digirig or SignaLink

In this case, the PC / MAC / SBC is required as it will perform the TNC function and the Digital Sound Processing (these are normally the functions of the TNC Modem).

Digirig’s website could not say it better ” Digirig itself doesn’t implement any digital modes or TNC functionality, it acts a conduit between your digital computer and your analog transceiver”.

In this case, we will be depending on VARA or Dire Wolf (or other) software to provide the processing functions that we would normally expect from the TNC modem.

Finally from the Sound Card Interface, you have a combination of Microphone and Speaker Audio AND a Serial connection.

In the case of a Transceiver with a CAT control the commands (e.g. Push to transmit, frequency changes are sent via the CAT interface) along with the audio interfaces.

In the case of a transceiver with no CAT interface (e.g. HT or older Rig), then the control lines of the Serial Interface is used to perform the PTT toggle (such as RTS). If you look at software such as FLRig and other software that requires the ability to transmit, they normally list the PTT methods as VOX, CAT, DTR or RTS. Again the software would control this.

Now you will note that in the last sentence I mentioned VOX. This is another method of controlling transmit, however how successful or reliable it will be can depend on a number of factors, with the main one being the amount of delay before VOX kicks in and this may differ between Transceiver models. If the delay is too long, transmission of the packet will fail. For this reason, if you have a way of implementing CAT control or a hardware method of PTT (RTS), this should be your first choice.

Further Reading – AX.25 Link Access Protocol for Amateur Packet Radio -Digirig – SignaLink

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