Disable preamble? shortest possible packet? APRS?

[craigerl]

I'm still new here, but getting on well with modem73 experimentation.

https://www.youtube.com/watch?v=ZoTFmKCBr7o

Let's say I want to implement APRS, on a busy channel, so I need very short packets with minimal error correction on FM/VHF with rigs that have eq emphasis. I also need CSMA, possibly COS for busy-channel collision detection.

QAM16, 1/2, Short has been working well so far.

At the beginning of each packet there seems to be a RSID preamble? Can we turn that off, and possible have the listener be in a fixed mode that doesn't need RSID (again, thinking APRS).

Has anyone else gone down this path? best practices?

curiously,
-craig
KM6LYW

[craigerl]

following up with myself,

--no-lead-tone gets rid of what I thought was an RSID preamble, but I suspect it's there to trigger CSMA/keyup detection.

Which brings me to tx-delay that only goes down to 250ms? Can we make that even smaller? Again, this is an APRS network that has lots of short packets in a single collision domain.

[RFnexus]

The 2-FSK ID you hear is a random hash that's turned on by default in SYNC and RANKED CSMA is called the leading tone.

The purpose of the lead tone is to track station population in a shared channel, and have a faster turnaround time for carrier detection when CSMA is enabled. The station population tracking will adapt the CSMA wait times in SYNC mode using the known population.

TXDelay can't go any lower than 250 ms on the majority of HTs, especially cheap ones like the Baofeng UV-5R or UV-K5 due to the minimum turn around time required for keying up the radio. I could add an option to set it all the way to zero though, but wanted a reasonable ceiling so people don't do that and wonder why it isn't working. The less foot guns the better.

If you want I can add an "ADVANCED" checkbox setting that will allow you to set it all the way to 0, but the wait you are probably experiencing is most likely related to CSMA. What CSMA mode are you using?

[craigerl]

GREAT information - thank you! I guess what I'm getting at is that I'm looking for FAST turnaround AX25, ultimately for IP over AX25. I agree 250 is a good default to get all radios online without hassle, but, yah, and advanced option would make this really convenient.

So, I'm curious, how would you setup ip-over-ax25 with modem73? assuming VHF and radios with emphasis.

thanks again,
-craig

│  CSMA                               #                                        │
│  Enabled        ON                  #  NETWORK                               │
│  Mode           THRESHOLD           #  Port: 8003  0c                        │
│      [?] Which mode                 │                                        │
│  Threshold      -24 dB -56          │                                        │
│  Level        [====-----|----]      │                                        │
│  Band           HF                  │                                        │
│  Preset         CUSTOM              │                                        │
│  > [-] Advanced                     │                                        │
│    Quiet        AUTO                │                                        │
│    Window       8 x 500ms           │                                        │
│    Lead Tone    ON                  │                                        │
│    Reply Offse  250 ms              │                                        │
│    Burst        2 pkts              │                                        │
│    [?] Info     

[RFnexus]

Your CSMA band is set to HF. Is that intentional? VHF/UHF will have sharper preset timings for slot and wait times, and it's intended to be used with faster bitrate modes

[craigerl]

not intentional - I'll flip it to vhf - thank you!

[RFnexus]

@craigerl Let me know how that went. For the Pi Zero 2 the SYNC CSMA detection can be a bit delayed. You may want to use THRESHOLD which will be snappier. On faster devices SYNC will work just fine though .

Also make sure you have disabled the decoders you aren't using, if you are using a device like the Pi Zero 2.