Technology

Introduction

This write-up introduces the working principle and improvements made by our Arach mesh stack over the competition. Detailed technical white papers and manuals can be obtained by contacting us directly.

Problems with current solutions

Current market solutions for deploying wireless mesh networks suffer from one major drawback, the routing nodes consume a significant amount of power. The design of these protocols requires them to be powered on at all times to receive data. This means that in most cases only the edge nodes can be battery powered for a long time, restricting the area where the network can be successfully deployed.

Newer solutions like BLE Mesh, IEEE 802.15.4e and WirelessHART have been proposed to counter this, but they have their own drawbacks which are discussed later.

Working principle

The end goal for the protocol is simple: reduce energy consumption. This in turn translates to reducing the duty cycle of the radio. Upon analysis of the aforementioned mesh technologies, it is evident that routing nodes keep their radios on longer than they need to, either due to suboptimal routing techniques(BLE mesh) or due to protocol overhead(IEEE 802.15.4e, WirelessHART). Our solution was to create a time scheduling algorithm to synchronize the radio activity in the network to obtain the lowest duty cycle. This, along with improvements in other aspects of networking allow us to achieve some of the lowest power consumption numbers for routing nodes.

Using extremely precise timing and adjustments, Arach manages to minimize the idle radio time. Combined with extremely low protocol overhead using tight data packing and compression, we can achieve extremely low device duty cycle.

Improvements

Arach comes with significant improvements over the competition in the areas of:

1. Network time independent scheduling: better slot alignment with nodes at different network depths
2. Adaptive drift correction: allows operation without external crystal clocks in some cases
3. Efficient link layer: works with an overhead as low as 16 bytes per data packet in the worst case
4. Timeout negotiation: dynamic slot timeout negotiation based on the number of pending packets
5. Efficient downstream communication: low overhead for routing from root to network nodes
6. Passive redundant paths: redundant paths that do not consume power when inactive
7. Routing: routing techniques that do not require tables or graphs to be maintained

These, along with other major innovations throughout the network stack allow extreme energy efficiency with reliability. Due to using easy-to-compute algorithms and state machines, Arach can run on relatively low-end processor cores. With its optimized code and overlapping data paths, Arach stack can fit in less than 20kB of flash and use under 2kB of SRAM, with the rest left for the user application or reserved for larger network buffers. This allows the usage of much cheaper hardware for Arach deployments, giving it a serious edge over the competition.

Comparison with competing standards

The main competing technologies in low power mesh are BLE Mesh, IEEE 802.15.4e and WirelessHART. Here is how Arach improves over each of them.

BLE Mesh uses a managed flooding algorithm for data routing. In this, the message is transmitted to each neighbor which has not received the message till the time-to-live(TTL) expires. This causes a lot of unnecessary transmissions with no guarantee of the packet reaching the intended node, sacrificing both power efficiency and reliability.

IEEE 802.15.4e defines a TSCH link layer mechanism for scheduling transmissions. It is usually coupled with the RPL routing protocol for ultra low power networks.
For constrained devices it uses a source-routing protocol which lists the entire path to be followed resulting in large overhead in downlink communication. Arach with its path addressing algorithm achieves the same performance level at a fraction of the overhead.
TSCH defined in the standard does not provide efficient connection management methods for nodes connected to multiple coordinators for redundancy. They must treat them as a full-fledged connections even if they are not being actively used. Arach allows finer control over these connections, which saves a significant amount of power.

WirelessHART is a mesh protocol intended for industrial sensors supporting the HART protocol. It relies on the gateway node to create and maintain a network graph, calculate paths based on network metrics and distribute the path information to the nodes routing tables.
Being a centralized architecture, it is heavily reliant on the working of the central gateway node, a failure in which means a failure in maintaining the graph structure and routing tables. A centralized architecture also adds computational overhead for the gateway to run the graph algorithms. Arach eliminates these concerns by using a completely distributed architecture with multiple gateway support.

Parameter	Arach	WirelessHART	IEEE 802.15.4e	BLE Mesh
Frequency Band	Any	2.4GHz	2.4Ghz, sub-GHz	2.4GHz
Channel Access	Channel Hopping	TSMP	TSCH	FHSS
Slot Scheduling	Distributed	Centralized	Distributed	Distributed
Routing	RPL+Optimized source	Centralized Graph	RPL+ Source	Network Flooding
Physical layer spec	Minimal	802.15.4	802.15.4	BLE