scenario: In the future, it is possible to update your node on your 200-foot tower by simply sending the firmware file directly to the node with a zero hop direct connection
I know short fast can get about 10kbps (1KB/s) across the link, but thats just too slow to send a 1.5MB firmware update. I know that short turbo is illegal in some places, and I believe it’s because it’s using a bandwidth of 500KHz instead of the 250KHz of short fast.
So if you turn the error correction way down to almost off what is about the maximum bandwidth you could hope to possibly get out of the LoRa connection at 250KHz?
Even if you could pull off 125kbps (12.5KB/s) that would only require two minutes to send the firmware file from your node to the receiving node up on the tower.
According to the SX1261/2 datasheet, the maximum RAW bit rate of this LoRa chip is 62.5 kb/s for 500 KHz bandwidth and spreading factor of 5.
The bit rate is constrained by the spreading factor and the bandwidth. Bandwidth is constrained by law, spreading factor is constrained by design. You can find the details in LoRa™ Modulation Basics, AN1200.22, section 4.
For 250 KHz Lora, you can use the equation to get the theoratical max:
If we set SF = 5, BW = 250,000, bit rate is 39.0625 kb/s for a raw bit stream.
So, I think that the limit of 62.5 kb/s is a safe speed ceiling to consider. I am not sure how you would get 125 kbps using LoRa and current constraints. You could switch the chip to FSK mode and pull at 300 kb/s IF the FSK link is strong enough.
Sooo, 1998 “USR X2” dial-up speeds.
1.5mb is okay. 32mb we used ‘at’ to schedule (just so we’d get the mail then it was done) and go to lunch.
Keep in mind, when USENet was in full swing, we had entire institutions daisy-chained off a single 56k incoming line like a trunk line, from America to Vancouver through Edmonton and then Calgary. The best and brightest surfed binaries and alts like mad off that single chain.
It’s really usable if we tune our workflow.
I agree. I think that a firmware update taking several minutes to complete would be alright. If loss of service is a concern, they can keep more than one device at the tower and one acts as a backup while the other updates.
One problem is that the data rate I quoted is the maximum capacity for a continuous stream. In practice, that is often illegal due to duty cycle rules. So, you might get 40 kbps while transmitting, but local laws may let you transmit only 1% of the time. If you choose to be obedient it will take a lot longer to get the firmware across.
Even then, let’s say it takes a few hours to get the full firmware, I think this can be alright for sporadic firmware updates.
Us bandwidth restrained by law cos frequency ranges are limited?
The frequency ranges, power of the transmitter, signal bandwidth, the fraction of time that the transmitter is transmitting (duty cycle), and some other parameters are regulated. LoRa usually transmits in an unlicensed ISM region of the radio spectrum. So, you don’t need a license to transmit, but devices still need to comply with the regulations to remain lawful.
I’m not sure about the legal bandwidth limits in different locations. The commonly used LoRa chip (SX1261) can support up to 500 KHz bandwidth, and the OP mentioned 250 KHz as their limit.
Interesting. So what’s the limiting factor for Meshtastic data throughput? Is it legal or technological?
Bandwidth is a resource that limits the amount of information that you can transfer per unit time. You can get a higher throughput if you can increase the information density or by using more bandwidth. Physically, both are possible. Increasing bandwidth of a 868 MHz radio signal is primarily limited by law, not technology. Increasing the information density is limited by the technology of the receiver, transmitter, and modulation.
As for Meshtastic, they often rely on the SX1261/2 chips that have a bandwidth limit by design. The people who manufacture that chip could design it to support a wider bandwidth, but it makes more sense for them to optimize the specs in a way that falls within the legal boundaries of the target applications. This comes from the chip’s datasheet:
So… It is both, legal and technological. The technology is designed considering the law. But it is not limited by the laws of physics.
And there is an additional layer of constraints that are imposed by the Meshtastic firmware itself.
Thanks for the info.