Dragan Dan-Stefan - Master AAC

The Constrained Application Protocol (CoAP) is a web transfer protocol that is intended for use with reduced-capability, low-power devices. These nodes often have 8-bit microcontrollers with small amounts of ROM and RAM. CoAP provides a request/response interaction model between application endpoints, supports built-in discovery of services and resources, and includes key concepts of the Web such as URIs and Internet media types. This project uses an existing CoAP implementation over an IPv6 over Low power Wireless Personal Area Networks (6LoWPAN) stack and its main goal was porting the existent environment on Sparrow v4.

6LoWPAN is a networking technology or adaptation layer that allows IPv6 packets to be carried efficiently within small link layer frames. It has defined encapsulation and header compression mechanisms that allow IPv6 packets to be sent and received over IEEE 802.15.4 based networks.

In the figure below, one may see an IPv6 network, including a 6LoWPAN mesh network. The 6LoWPAN network is connected to the IPv6 network using an edge router. This edge router or coordinator as we call it in our project performs the following actions: * the data exchange between 6LoWPAN devices other IPv6 network * local data exchange between devices inside the 6LoWPAN * generation and maintenance of the radio subnet

The starting point CoAP implementation was supported by 6LoWPAN mesh networks that used as Coordinator a development board with an Atmel ARM-based microcontroller and the Routers and Devices were development boards with Atmel ATMega128rfa1 microcontrollers. In this model the Coordinator contained a Real Time Operating System (FreeRTOS) with a scheduler designed to provide a predictable execution pattern. The scheduler was used to plan for execution both initialization tasks for network, low-power IP and CoAP tasks and events like responding to device join/leave requests, responding to CoAP requests.

The new approach required using Sparrow v4 for all three types of nodes. Porting the Device and Router applications was not very difficult because the previous model used the same microcontroller, so the only changes were for pin enablement (sensors and LEDs) and build configuration files. On the Coordinator side the effort was more consistent. The figure below shows the new model with a redesigned network. First of all, the build configuration files needed to be rewritten, because of the switch from ARM to AVR, not only for the CoAP application, but also for other dependencies, like ROM, 6LoWPAN subroutines and CoAP subsystem. Since the flash size was now, reduced to half, the Real Time OS required to be removed. The execution pattern was predictible, so the RTOS scheduler was easily replaced with bare metal support for the initialization tasks and callbacks.

6LoWPAN defines two categories of routing: mesh-under and route-over. Our project is based on the mesh-under routing method, which uses the layer-two data link layer addresses (IEEE 802.15.4 MAC and short address) to forward data packets. In a mesh-under system, routing of data happens transparently, hence mesh-under networks are considered to be one IP subnet. The only IP router in such a system is the edge router. A broadcast domain is established to ensure compatibility with higher layer IPv6 protocols such as duplicate address detection. These messages have to be sent to all devices in the network, resulting in high network load.

In order to set up a new Sparrow wireless sensor network using COAP the following instructions should be used:

* Request access to Dresden Elektronik 6LoWPAN stack repository [1]. * Install the next packages on your Linux machine: avr-libc, avrdude, binutils-avr, gcc-avr (example for Debian based systems)

sudo apt-get install avr-libc avrdude binutils-avr gcc-avr

* Clone the git repository

git clone https://gitlab.cs.pub.ro/dan.dragomir/dde-stack

* Switch to sparrow_support git branch

cd dde-stack/
git checkout sparrow_support

* Build ROM static library for Coordinator with MAC address support

cd ROM/lib/Sparrow_v41/
make all

* Build SLOW static library for Coordinator with MAC address support

cd SLOW/lib/Sparrow_v41/
make all

* Build the COAPEcho Application for Coordinator and upload it on a Sparrow node

cd Applications/SLOW_Examples/COAPEcho/Coordinator/Sparrow_V41/GCC/
make all
cd bin
sudo ~/arduino-1.6.12/hardware/tools/avr/bin/avrdude -C~/arduino-1.6.12/hardware/tools/avr/etc/avrdude.conf -v -patmega128rfa1 -carduino -P/dev/ttyUSB0 -b57600 -D -Uflash:w:COAPEcho-Coordinator.hex:i

* Build the COAPEcho Application for Router and upload it on a Sparrow node

cd Applications/SLOW_Examples/COAPEcho/Router/Sparrow_V41/GCC/
make all
cd bin
sudo ~/arduino-1.6.12/hardware/tools/avr/bin/avrdude -C~/arduino-1.6.12/hardware/tools/avr/etc/avrdude.conf -v -patmega128rfa1 -carduino -P/dev/ttyUSB1 -b57600 -D -Uflash:w:COAPEcho-Router.hex:i

* Build the COAPEcho Application for Device and upload it on a Sparrow node

cd Applications/SLOW_Examples/COAPEcho/Device/Sparrow_V41/GCC/
make all
cd bin
sudo ~/arduino-1.6.12/hardware/tools/avr/bin/avrdude -C~/arduino-1.6.12/hardware/tools/avr/etc/avrdude.conf -v -patmega128rfa1 -carduino -P/dev/ttyUSB2 -b57600 -D -Uflash:w:COAPEcho-Device.hex:i

The image below represents the output of a serial monitoring of the Coordinator when three other Sparrow nodes joined the network. The Coordinator assigns MAC addresses and short addresses to the routers and devices and registers them in a children table.

The below topology presents a coordinator, a router in the middle and two devices. The router may be used to route messages from one node destined to another. The hosts can also be sleepy devices, waking up periodically to check its parent for data, enabling very low power consumption.

* [1] https://gitlab.cs.pub.ro/dan.dragomir/dde-stack * [2] Jonas Olsson, 6LoWPAN demystified, Texas Instruments Incorporated * [3] https://tools.ietf.org/html/rfc7252