Note: This post borrows entirely from the VAPN feasibility study website, but the information is perfectly relevant here.


AX.25 packet under attack

New techniques are emerging in the digital landscape including the 802.11 MeshNet folks and impressive voice channel digital modes by FLDigi and the NBEMS efforts. Competition is a good thing, but it’s important to see where packet fits in this new digital landscape.

Where AX.25 fits into the ARS digital landscape

Figure 1 – Where AX.25 fits into the ARS digital landscape

There is no doubt other digital modes belong on the graph in figure 1 (i.e. Pactor, Clover, etc.), but the VAPN is focusing on minimizing footprints in many ways including spectrum use in the MF/HF segments. Hence the focus for the MF/HF bands is on HF Packet and Robust Packet.

The digital landscape

In figure 1, HF, 1200 and 9600 AX.25 packet radio modes are shown as circular icons in comparison with a variety of other digital modes including:

  • PSK31 [+ icon] – shown simply as a 31.25 bps reference at 160 – 6m.
  • 8-PSK500F and 8-PSK2000F [Triangle icons w/o bars] – New modes from the continuing and impressive work from the FLDigi developers. Like HF and 1200 packet, these modes fit into the bandwidth of a voice grade radio link. Combined with NBEMS these phenomenal new PSK modes challenge the AX.25 packet based message concepts.
  • Freewave [diamond icon with bar] – This is an example of a 915 MHz FHSS serial modem made by Freewave Technologies. Personal experience confirms exceptional performance. While not specifically an amateur radio product, the 902-928 MHz ISM band may be used under Part 97 making industrial radios a valid option in the digital radio landscape.
  • 802.11 Networks [triangle icons with bars] – The HSMM-MESH and AREDN efforts represent a real leap forward in data movement. Many non-amateur efforts exist as well in the Part 15 regime. A significant advantage of these 802.11 efforts is use of the Internet Protocol thereby leveraging all known Internet applications. Greatly increased data rates are alluring as well. Time will tell if Part 97 or part 15 reigns supreme in the ongoing development. Security and legality concerns aside, the VAPN community can benefit by provisioning a telnet port to the BBS/Chat node on the nearest Mesh network.

Merits of the packet approach

An AX.25 BBS/Chat capable node is one of the few applications that can service nearly any band from DC to daylight with a consistent, centralized and well known interface.

The need for 9600 bps

One thing is abundantly clear about figure 1, in the middle ground VHF and UHF bands, 9600 packet is no longer a luxury, but a necessity. Therefore, if AX.25 is to keep pace with competing developments, new packet projects must consider 9600 bps VHF/UHF ports along with 200-600 bps HF packet frequencies.  Add a telnet port on a Mesh network and the user has a broad palette of choices for message movement. Frequency diversity is a key feature of the VAPN.

 

About John Huggins

John is an electrical engineer working in astronomy and aerospace including antenna/RF design spanning 36 years with experience in antenna design including improvements (US20170201002A1 & USD798847S1) to the J antenna.

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