Stuart,
It’s good to hear someone else is looking at applying 6LoWPAN to amateur radio.
I’ve studied 802.15.4 a fair bit. I find the standard is growing beyond an
individual’s ability to manage. So I distilled the 802.15.4 frame to the
essential pieces. My result <https://github.com/dwhall/HeyMac> is not 802.15.4
compliant, but it’s much more manageable for an individual to understand and
write code. My frame format will support RFC6282 (header compress and UDP
compression).
My target radio is the Semtech SX127X running in LoRa
<https://en.wikipedia.org/wiki/LoRa> mode. That’s a Chirp Spread Spectrum
modulation with selectable bandwidth from 7.8 to 500 KHz. It has built-in FEC
and supports payloads up to 255 Bytes. Not enormous paylods, but twice that of
802.15.4. My first LoRa configuration is (Bandwidth=250KHz, FEC code rate:
4/6, spread factor: 128). This results is a data rate around 9000 bps.
My current status is that I have a physical layer driver working and I’m
building the MAC layer. It’s time-slotted, but not channel hopping. Time
slots are 250ms to allow a full frame and an ack. The radio boards
<https://www.tindie.com/products/edwin/loragps-hat/> have a GPS chip and
synchronize to its Pulse-Per-Second (PPS).
In the MAC layer, I have an extended Beacon working. I’m experimenting with
what to put in the beacon at the moment. My next endeavor is to create a MAC
command for simple text messages. This is all still in prototype stage. I’m
coding in Python3 on a Raspberry Pi 3. Yes, I’m getting millisecond-level
accuracy with Python. I start range-testing the 1/10th Watt radio modules this
weekend. There are 1W modules if this protocol proves promising.
regards,
!!Dean
KC4KSU
> On Mar 24, 2018, at 4:57 PM, Stuart Longland <stua...@longlandclan.id.au>
> wrote:
>
> On 25/03/18 06:25, Bruce Perens wrote:
>> FreeDV voice packets are around 7 bytes per 40 milliseconds. Even a
>> single IPV6 /address /is twice the size, not to mention the rest of the
>> header. So, realistic expectations are called for. Header compression is
>> helpful. Retransmission of a TCP packet could easily be a 1500 or more
>> byte repeat, taking most of a second. The best mix of FEC to reduce
>> repeats while minimizing overhead is still open for your research.
>
> Actually, I have been looking at how 6LoWPAN does things. In
> particular, I've been working with OpenThread at work, and considered
> rejigging that to work within AX.25 UI frames which could be transmitted
> with existing AX.25 hardware for FM, or utilise the FreeDV modem for SSB.
>
> 6LoWPAN ordinarily runs over IEEE 802.15.4. There, they have a 128-byte
> limit on link-layer frames, and a radio network of nodes where not all
> nodes can directly communicate.
>
> For nodes within the mesh, they actually use a shortened address to
> abbreviate the IP address to help compress the header down, and the
> protocols typically used over 6LoWPAN are typically geared to keep the
> packet size down (e.g. CoAP instead of HTTP). If you do send a big
> packet, it gets fragmented.
>
> I don't think there's repetition of a single frame within that segment,
> so it'd be up to upper layers to re-send the lot. Forward erasure
> coding would help a lot there as the probability of us losing a packet
> is so much higher.
>
> Thread is officially supposed to work over 2.4GHz, but there's nothing
> to suggest it couldn't work on HF. We just have to do our own modem to
> replace 802.15.4, and rejig the crypto so that shared keys are used to
> authenticate (a message integrity code) instead of encrypt.
>
> In short though, I think it could work.
> --
> Stuart Longland (aka Redhatter, VK4MSL)
>
> I haven't lost my mind...
> ...it's backed up on a tape somewhere.
>
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