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Weekly Report

06

Apr

2015

Since the last report I've been reading through the 802.15.4 standard to get an idea of what to expect from silicon radios that say they are compliant.

Both Dean and Richard made the point that just because they use the 802.15.4 radios doesn't mean they follow the standard and as such I shouldn't expect to see a whole lot of structure from the standard. For example the standard is very vague when referring to a network not using PAN controller beacons which would allow the network designer to make whatever decisions they like. This is an issue when interoperability is desired as the IEEE802.15.4 standard really doesn't offer anything that would facilitate such interoperability - the application and the networking (in cases where it is not a star network with PAN controller beacons enables) are not defined by the standard.

I also attempted to trace through the source for both RIOT OS and ContikiOS. RIOT OS is a microkernel meaning that messages get sent between different processes by the kernel. As such, I was able to trace the transmission of a TCP packet through the RPL routing, through the 802.15.4 framing to a call where the frame was sent to a transceiver. ContikiOS was far more complicated as they use a lot of preprocessor macros for various things which makes tracing a little more difficult. I wasn't interested in spending a lot of time tracing when my main interest was just figuring out how the 15.4 layer worked (PAN beacons? PAN controller elections? free for all?). I actually didn't figure out how either worked, but I imagine reading through the RPL routing document would give more information.

Finally I tried to evaluate both ContikiOS and RIOT OS. I like the RIOT OS source as it is very clean and readable. For this reason I was really hoping that I could use RIOT OS for this project. However, support for the CC2538 transceiver (distinct from the microcontroller core) is not in the mainline branch. I was able to checkout a branch that claimed support (and is in the process of a pull request) however it was hard to tell if it was actually working. ContikiOS on the other hand was ready to go out of the box. My sniffer seemed to have some kind of latency in showing me received packets (or it was rejecting them because they aren't using standard frame checksums?) which made it difficult to figure out how packets were flowing. In any case I was able to create a gateway running SLIP to a tap interface as well as a web server node that I could access over the air using the gateway.

I plan to continue evaluating both options but it appears that ContikiOS is more polished than RIOT OS in terms of CC2538 support. I find it interesting that RIOT OS claims support for the CC2538DK but doesn't support the radio (yet). RIOT OS is an embedded RTOS but I don't think I would choose it over FreeRTOS unless I wanted the networking and radio stack.

07

Apr

2015

RIOT OS

I've put some more time into getting RIOT OS going and I still don't know why it doesn't work. I enabled debug output as much as I could to try and shed some light and there's just nothing. It's clear that no amount of wishful thinking is going to get around the fact that RIOT OS isn't ready.

I'll put some more time into understanding how ContikiOS works, as I can confirm that I had data moving through the network.