Coming changes & improvements in 2.3.0

[RFnexus]

In 2.3.0, modem73 will introduce new CSMA (Carrier sense multi access) features to mitigate overhead and allow for even higher throughput on channels with multiple concurrent users.

The new round robin CSMA can be enabled by turning "Sync Only" ON, and "Ranked" ON. When you are sharing a frequency with multiple stations, this is recommended. All stations will need to update to the latest modem73 version for these features.

The process works like this:

1. Every transmission starts with a tone. Before the data, each station sends a short 2-FSK burst that carries its own 16-bit station ID. The station ID is based on a PRNG id from 0 to 65,535 determined on startup
2. Everyone listening keeps a list of those IDs
3. The list is then sorted by ID. All stations sort those IDs in the same order so that each one knows which turn to take
4. Your turn is a ranked time slot, x out of the total population.
5. The last station to transmit goes to the back of the line. Whoever's station tone was heard most recently anchors the rotation, so the next station up in sorted order takes the first slot and the one that just finished takes the last. This is what keeps any single station from hogging the channel.

For channels that are idle, there is no more waiting. The new CSMA removes the contention window entirely.

If a collision occurs, it is self-sorting as both stations will rank themselves as the winner which puts them in a different order next round, preventing future collisions.

These changes increase throughput on population saturated channels by 3-7x, and is the first of a kind implementation for amateur radio software modems.

When 2.3.0 is released, it is recommended to upgrade modem73. 2.3.0 also includes some bug fixes and other improvements which will be documented in the release notes

[RFnexus]

Presence Tone for establishing turn order:

When Ranked CSMA is enabled, modem73 will transmit a short, half-second presence tone containing the presence ID every 31 to 58 seconds when the two conditions are met:

• No frame was transmitted in the last presence tone window(a presence tone is already included leading each frame) AND
• The station has transmitted within the last 20 minutes

Stations that have dropped off for 20 minutes with no activity, "lurkers", will switch back to random contention windows on their first transmission that re-joins the channel after the 20 minute inactivity period if their population table is empty.

A station also counts as participating for the first 20 minutes after it launches.

This allows stations listening on the frequency to build the same turn order table and keep a consistent track of population.

The presence tone can be decoded to around -1 dB. A station can have one missed presence tone in the 120 second population tracking window without any massive errors in the ranked turn order.

[RFnexus]

The presence interval is now configurable from 45 to 90 s, and the expiry is now set at a 300 second floor