Packet loss by digi collision

It has been apparent to me that the two local digipeaters often collide.

One uses UIDIGI and therefore is conformant with the p-persistence channel access algorithm.

The other is a Argent Data T3-135 which does its own thing.

To discover how many packets are detected with bad CRC, I installed 6PACK on a TNC. 6PACK reports receive CRC failures which are logged by the Linux kernel. Data was collected for seven hours through the middle of the day to ensure that there was traffic, but that channel utilisation was low. Continue reading Packet loss by digi collision

Another Argent Data T3 incompatibility

I have written on incompatibility of Argent Data TNCs with other devices.

In pursuing apparent packet loss, I have run up a Paccomm Tiny-2 MK-2 TNC with 6PACK firmware, and my aprx server using Linux AX25 kernel support and 6PACK driver.

It has become apparent that although the system reliably decodes packets from a multi-packet burst from VK2AMW-1, it only ever decodes the first packet of a multi-packet burst from VK2RHR-1. Frame check errors are logged. Continue reading Another Argent Data T3 incompatibility

The KISS TNC – too simple, too stupid?

An important element of early AX.25 networks was the Terminal Node Controller (TNC). Essentially, a TNC was a packet assembler / disassembler (PAD) pretty much equivalent to the PAD of X.25 networks but adapted to AX.25, and commonly, an embedded modem.

The TNC-2 was a hardware configuration which became a de-facto standard, and various firmware packages became available each with their own advantages and disadvantages. and a range of protocols for the host connection.

One of the inventions was the KISS protocol (KISS for keep it simple stupid) from Mike Chepponis (K3MC) & Phil Karn (KA9Q), and an implementation for TNC-2. Continue reading The KISS TNC – too simple, too stupid?

RPi headless aprx server using soundmodem

I have posted several articles on headless APRS servers based on RPi. This article describes one based on RPi, AX25 soundmodem using a $2 USB sound card, and aprx v2.09.


Above is the server hardware. It uses the Sailer sound modem and kernel AX.25 support. This is currently configured as a RX only iGate for 30m, hence no tx audio path and no PTT (though on HF, PTT can be done simply using transceiver VOX). The small black USB module is a Belkin WLAN adapter. Continue reading RPi headless aprx server using soundmodem

APRS: does pre-emphasis make much difference

This morning I have been observing a weak APRS signal from a digipeater located on a prominent hill about 200km away, VK1RGI-1.

VK1RGI-1 is characterised by having a pre-emphasised transmission, but about 1.5KHz deviation, about 6dB low.

Nevertheless, VK1RGI-1 packets can be fairly reliably decoded at a strength that does not show any segments on the IC2200H S meter, less than -115dBm (by measurement). Continue reading APRS: does pre-emphasis make much difference

APRS: how popular are Kenwood trackers

APRS has evolved to be all things to all men.

It inventor, Bob Bruniga, generalised the original meaning Amateur Position Reporting System to Amateur Packet Reporting System as he extended APRS to include all manner of generalised broadcast and point to point messages.

Perhaps in doing that,it was recognition of the inability for many reasons of APRS to provide reliably good position reporting performance in practice.

Any thinking person understands that there is conflict between use for timely reporting of position and loading the radio channel up with generalised message traffic, or even extending the RF coverage of a position report by repeating it many times into adjacent precincts. But simple minds muttering “use it or lose it” ruin APRS for position reporting by sending repeating traffic without a demonstrated demand.

Attempts to emphasise position reporting performance tend to be opposed in my experience mostly by owners of Kenwood trackers (eg DM-710), and the weather propagators. Many Kenwood owners like to see position reports from a very wide area repeated into the local area so that they can be displayed on their radio. There are other features that generate traffic that appeal to Kenwood owners.

This article looks at how significant the Kenwood users are in terms of unique packets reaching APRS-IS, the basis of mapping for most users these days. Continue reading APRS: how popular are Kenwood trackers

Adjusting KISS TNC AFSK tx level using an isochronous test packet

This post explains a technique to drive a KISS TNC with a specially constructed packet that contains an ISOCHRONOUS test packet, a packet that will produce equal high and low tone alternation in the transmitted AFSK signal.


Above is the waveform recovered from a receiver without de-emphasis (a Motorola R2009D communications analyser in this case).
Continue reading Adjusting KISS TNC AFSK tx level using an isochronous test packet