chang206TiSCH

Goal

The goal of this paper is to show an entire synchronized protocol stack running on SCµM under certain circumstance. Specifically, the authors show that, through 3 levels of calibration and compensation, they are able to have SCµM and OpenMote drift by as little as 10 ppm, and stay synchronized with a maximum synchronization error of 300 µs.

Paper organization

1. 6TiSCH protocol stack and OpenWSN reference implementation of that stack

2. Main features of SCµM, including its clock system

3. How the clocks are calibrated to allow SCµM to communicate with OpenMote

4. Compensation algorithm needed for porting OpenWSN onto SCµM, and some experimental synchronization results

Introduction

• Time Synchronized Channel Hopping (TSCH) is at the core of all main industrial standards. Today, these standards can run on virtually any IEEE802.15.4-compliant chip. All of the commercial chips use stable oscillators as a time reference.

• A crystal oscillator have the property of oscillating at a frequency that is precise and characterized over temperature, supply voltage, and aging. The problem of needing a crystal is cost, space and energy.

• The goal of the Single-Chip micro Mote project (SCµM) is to remove all external components, including crystals, capacitors and other passives, and indeed ultimately even the battery and antenna, integrating everything into the wafer fabrication process. The current version of the chip still requires external power and antenna.

Challenges and advantages of SCµM

• The main challenge with SCµM is that its internal oscillators are far less accurate than crystal/MEMS-based external circuits. Another big challenge is the fact the environment changes, such as temperature, voltage or humidity, heavily influences the RC/LC oscillator frequency error, keeping sustainable frequency error while environment changes.

• The main advantage of SCµM is that it is based on TSCH technology proven common in Industrial IoT applications and which provides ultra-low level of power consumption.

Background

• Texas Instruments combined an ARM Cortex-M4 and an IEEE802.15.4 radio on a single IC, and a separate MEMS BAW (Bulk Acoustic Wave) oscillator.

• Similarly, Wiser et al. build a prototype Bluetooth Low Energy (BLE) radio which uses a thin-Film Bulk Acoustic wave Resonator (FBAR) as a replacement for a crystal oscillator.

• The Single Chip micro-Mote, or SCµM, is a true crystal-free chip taped out in 2019 by the authors of this paper. It is a 2×3 mm² single-chip crystal-free mote-on-chip which contains an ARM Cortex-M0 micro-controller, a 2.4 GHz IEEE802.15.4 radio, and an optical receiver for optical programming.

• The Michigan Micro Mote (3M) partly realizes that vision known as “world’s smallest computer”. However, none of the 3M versions are standards-compliant, i.e., they cannot communicate with off-the-shelf radios.

• The authors showed a calibration algorithm to tune the oscillators on SCµM so it can send and receive
  frames to the OpenMote, a popular off-the-shelf IEEE802.15.4 mote built around the CC2538 chip.

Highlights

• Open

  

  The 6TiSCH stack.

• Using the 6TiSCH protocol, all communication is secured, and the Constrained Join Protocol (CoJP) is used by a node to securely join a network, through mutual authentication between the network and the joining node.

• OpenWSN is the reference open-source implementation of 6TiSCH.

• Open

  

  SCµM contains 4 main oscillators. The crossbar switch is configured by a series of registers called the Analog Scan Chain (ASC).

• The RFTimer orchestrates the transition between the different states of the TSCH state machine.

• A 20 MHz crystal oscillator of the FPGA is used.

• One of the goals of porting OpenWSN onto SCµM is to show that this platform is perfectly capable of running an off-the-shelf completely standards-based full stack. Little changes to the OpenWSN implementation was made:

  • The port of OpenWSN on SCµM does not come with link-layer security, nor secure joining. This is because this version of SCµM does not come with an AES-128 cipher.

  • The OpenWSN TSCH state machine is simplified thanks to the RFTimer.

  • The slot length is increased to 82 ms, this is because an FPGA is used for the digital side of the chip.

• SCµM first listens to a blinking LED to provide coarse calibration of its oscillators. Using an OpenMote, which can measure and report the frequency offset, provides a second level of more precise tuning. Finally, as SCµM and OpenMote are communicating, the OpenWSN port on SCµM uses a digital trimming compensation algorithm based on tick skipping (periodically adds or substracts a tick from the lengths of the slot) to turn a 567 ppm apparent drift due to a rounding error into a 10 ppm apparent drift. This allows a synchronized fully functional 6TiSCH network to form between SCµM and OpenMote.

• The resulting offset when applying digital trimming shows two effects working together. First, the digital trimming causes the rapid saw-tooth like compensation, resulting in a much more manageable 10 ppm apparent drift. Second, SCµM regularly synchronizes to OpenMote, causing larger jumps.

New terms

• TSCH (Time Synchronized Channel Hopping) is a channel access method for shared-medium networks. It is used by Low-Power devices to communicate using a wireless link. It is designed for low-power and lossy networks (LLNs) and aims at providing a reliable Media access control layer. TSCH can be seen as a combination of Time division multiple access and Frequency-division multiple access mechanisms as it uses diversity in time and frequency to provide reliability to the upper network layers.

• IEEE 802.15.4 is a technical standard based on TSCH which defines the operation of a low-rate wireless personal area network (LR-WPAN). It specifies the physical layer and media access control for LR-WPANs, and is maintained by the IEEE 802.15 working group.

• 6TiSCH is the latest standardization based on TSCH, and is developed by the IETF influenced by the 6TiSCH WG activities.

• OpenMote is a popular off-the-shelf IEEE802.15.4 mote built around the CC2538 chip.

• CoAP (Constrained Application Protocol) is a specialized Internet application protocol for constrained devices, as defined in RFC 7252. It enables those constrained devices called "nodes" to communicate with the wider Internet using similar protocols.

• RPL (Routing Protocol for Low-Power and Lossy Networks) is a routing protocol for wireless networks with low power consumption and generally susceptible to packet loss. It is a proactive protocol based on distance vectors and operates on IEEE 802.15.4, optimized for multi-hop and many-to-one communication, but also supports one-to-one messages.

• ASC (Automatic Squelch Control) is a function used to reduce background noise when there is no communication. It is an adjustment which must be made precisely and repeated frequently in order to avoid reducing the reception capacities of the radio.

• CoJP (Constrained Join Protocol)

Review