Calculates the frequency slot for a given Meshtastic channel name.
In the US region, Meshtastic uses the LONG_FAST modem preset by default. This works well in many areas, but I live in the San Francisco Bay Area which has many nodes and thus the local group BayMe.sh has recommended that users use the MEDIUM_SLOW preset instead to minimize network congestion.
BayMe.sh provides instructions to set up one's node to use that configuration on the primary channel. If one keeps the frequency slot set to the default value of 0, Meshtastic will use a hash-based algorithm for determining the frequency slot corresponding to that channel name. However, it can be desirable to use a private primary channel and the default as a secondary channel.
Unfortunately, there's a complication: Meshtastic uses the hash-based algorithm based only on the name of the primary channel and the number of frequency slots in that region. Put simply, both the primary and any secondary channels share the same frequency slot as the primary channel.
When a node is configured to use the default LONG_FAST modem preset, the default primary channel name is LongFast1 In the US, the LongFast channel uses frequency slot 20 (906.875 MHz). If one uses a private primary channel with a different name and moves the default LongFast channel to a secondary channel, they need to explicitly set the frequency slot for the primary channel to 20 (for LongFast) in order to see the default traffic on the secondary channel.
I was interested in setting up a private primary channel and moving the defaults to a secondary channel, but I did not know the frequency slot for the MediumSlow channel (the name of the default channel for the MEDIUM_SLOW modem preset). Using 20 wouldn't work, since that's the slot for LongFast, not MediumSlow. In order to get the local MediumSlow traffic and participate in the mesh, I needed to know the frequency slot for the MediumSlow channel.
Fortunately, Meshtastic is open source and I was able to read the source and implement the frequency slot calculation algorithm in python so I could calculate the slot for the MediumSlow channel. It turns out the slot for MediumSlow is 52.
Since the frequency slot value depends only on the channel name and the number of frequency slots in the region (104, in the US), it's possible calculate the frequency slot for any arbitrary channel name, even ones not associated with the built-in modem presets. However, the modem presets, channel names, and frequency slots all must exactly match those of other people one wishes to communicate with.
For example, it's possible to configure your Meshtastic node to use the LONG_FAST modem preset2 with slot 52 (which correponds to the MediumSlow channel name), but that won't allow one to communicate with people using the MEDIUM_SLOW modem preset even if they are also using the MediumSlow channel name. I can't think of any reason why someone would want to do that, but I wanted to mention that it's possible in case someone accidentally does it and wonders why they can't communicate with anyone.
python3 frequency_slot.py
Channel Name: LongFast
Number of Frequency Slots: 104
Frequency Slot: 20
Selected Frequency: 906.875 MHz
python3 frequency_slot.py -n MediumSlow
Channel Name: MediumSlow
Number of Frequency Slots: 104
Frequency Slot: 52
Selected Frequency: 914.875 MHz
python3 frequency_slot.py -n xyzzy
Channel Name: xyzzy
Number of Frequency Slots: 104
Frequency Slot: 36
Selected Frequency: 910.875 MHz
- Currently only produces output for the US region. I hope to add additional regions in the future.
Footnotes
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Although similarly named, the modem preset and channel name are different things entirely: the modem preset defines the bandwidth, spreading factor, and other parameters for the LoRa mdoem itself, while the channel name being essentially a chat room name. ↩
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Or any valid, manually-configured modem settings. ↩
