Full Duplex Packet

Hidden Transmitter Syndrome

One of the easiest ways for AX.25 packet radio to cut its own throat is the built-in collision detection of all stations that cannot hear all the stations on channel. Even with VAPN high in the sky, the problem plagues any packet frequency with multiple users.

Full Duplex

What if we configure VAPN to operate not simplex, but duplex with a, for example, 600 kHz split on the 2m port. Properly configured, this should result in all users hearing all users and make the CSMA-CA/CD (carrier sensed multiple access – collision avoidance/collision detection) actually work for us.

Not insane and nothing new

TAPR has an article from long ago detailing exactly the above concept.


One of the concluding paragraphs…

This discussion would not be complete without bringing up the enhanced operation the users enjoy. Beyond the higher efficiency local area network operation we can enjoy the novelty of true roundtable operation. By utilizing the essentially true to form CSMA-CA/CD nature of the layer two protocol that duplex affords, we connect up in pairs and hold discussions while in full channel monitoring mode with large numbers of ‘conversational-mode’ packet users. This is an excellent way to operationally verify the correct repeater transmission to reception balance. (As well as any one user’s similar balance.) When a single user can request the retry of a ‘lost’ packet to the sender for the entire local area network, then excellent free-flow of discussion can be realized with a minimum of lost time in distributing ‘information’ within the network.


If we are to re-invent packet radio in the 21st century we may as well kick over the furniture and be accepting of more radical ideas such as the TAPR full duplex approach. Solving the hidden transmitter, or “hidden terminal” as TAPR calls it, will be a major improvement towards users’ experience. It requires a fairly costly solution for the hilltop sight. Fortunately the users all have FM radios that can do full duplex making them a mere reprogramming step away from partaking of the benefits.


Let's banish the hidden transmitter syndrome for good.
Let’s banish the hidden transmitter syndrome for good.

Graphic is the public domain from https://commons.wikimedia.org/wiki/File:Repeater-schema.svg