High Expectations for WSNs

Wireless sensor networks have experienced a decade or so of growing pains. For years, the accuracy and precision required by Industry 4.0 was head and shoulders above the capabilities of the latest technology. Networks which worked in theory failed to work in practice and many businesses wisely waited for a technology suited for cutting-edge applications. Future Industry 4.0 networks will permit connections between a wide range of devices and services. Complex structures are needed to form hierarchies of devices which adhere to four main requirements.

 

Must-Haves for Cutting Edge Applications

Total coverage: The entire space within a facility needs to be covered by the network with no dead zones or blind spots. Coverage loss due to concrete pillars, metal doors, and WiFi interference is unacceptable.
Zero Fails: Networks must provide high performance. Every packet must arrive at its final destination with a success rate touching 100% and a Packet Loss Rate (PLR) touching 0.
Sustainability: Both software and hardware must be low-power to ensure energy efficiency and cost effectiveness.
Scalability: Solutions need to be flexible, fluid, and mutable. Deployments need to adapt to changing workplace environments and must not require renovations to existing spaces.

This is a lot to ask. Until Bluetooth Mesh, no standard seemed up to the task. Now,  Bluetooth is changing our outlook on the future of IoT as it moves in to become the lingua franca of the smart object world, coming built into mobile phones, vehicles, luggage, pet tags, medical wristbands, and other wearables. Bluetooth Mesh is a new type of topology which allows smart objects to communicate in a casual network, passing on packet data to each other, and ensuring that all packet data makes its way to a gateway which pushes the data to a cloud server. Prior to this set up, Bluetooth objects could organize in two main topologies.

 

Old Bluetooth Topologies

  1. Connection Topology. Bluetooth objects are able to exist in groupings called piconets in which one Master Device is able to communicate with up to eight Slave Devices. In this grouping, Slave Devices are not able to communicate one to another.

  2. Broadcast Topology. In this topology, devices are set into one of two modes: advertising mode and scanning mode. In advertising mode, a Bluetooth device can transmit data to any listening device in scanning mode within range.

Unfortunately, Broadcast and Connection Topologies don’t approach the high expectations of Industry 4.0. In favourable broadcast deployment conditions, less than 70% of sensors were able to receive the highest RSSI from the nearest broadcaster (Hortelano et al., 2017). When proximity and location sensing is the entire basis on the IoT deployment, such a margin of error is unacceptable. Simple Broadcast Topologies have a high PLR (Packet Loss Rate) and are plagued with blind spots. Connection topologies are equally limited by the piconet structure. 8-1 connections repeated in however many star topologies may work for small deployments, but there are serious issues with using piconets. The failure of one master device affects up to eight other devices. Furthermore, until Bluetooth 4.1, two Masters could not form a relationship as a Master – Master/Slave. This limitation from a few years back inspired researchers to experiment with new topologies.

 

CSR Mesh Topology: An Early Mesh Configuration

The BLE Mesh Network is built on work by the CSR and Bluetooth Special Interest Group. In traditional broadcast topologies, Bluetooth smart objects were able to alternate between two roles.
In CSR Mesh networks, devices are able to use three different BLE roles simultaneously:

  1. Broadcaster: Mesh devices typically broadcast data packets three times. This results in higher network usage but ensures that the Packet Loss Rate (PLR) is next to 0.
  2. Observer: While broadcasting packets, Mesh devices are also able to observe for incoming packets. If the packets received are in the proper format, the device will relay the received packet to nearby devices.
  3. Advertiser: Mesh devices may still form Master-Slave connections which introduce new devices to the Mesh, ensuring that the new device’s data is sent and relayed in the proper format.

BLE Mesh: An Open, Better Protocol

The ability for devices to listen and broadcast simultaneously is a huge improvement from the older connection and broadcast topologies. However, the introduction of new devices necessitating Master-Slave connections is clunky. To solve this problem, newer, open Mesh protocols have been developed which do away with bridge devices and master-slave connections. This new BLE Mesh uses a new packet format which includes an 8-bit preamble, a 32-bit advertising address, and 4-3-bit advertising data which includes the sensor data. The format allows for two possible configurations: individual and collaborative. Individual Mesh saves on network usage because devices only transmit their own data packets and do not retransmit the packets of other devices. Collaborative mesh devices do retransmit neighboring device’s packets.

Imagine the possibilities of this collaborative BLE Mesh and its scalability! By retransmitting data packets along a chain of BLE devices, you do away with the need for multiple gateways. Each BLE device increased the overall range of the deployment by 40-100 feet depending on the hardware used. This is a critical advantage since network coverage range in factories and office buildings varies in dynamic ways due to construction materials, WiFi interference, and the movement of objects and humans which block Line of Sight (LOS) signal. Both invidual (low network load) and collaborative (high network load) mesh experienced a nearly perfect Packet Loss Rate of 0.21% (Hortelano et al., 2017). In collaborative mesh under a low network load, meaning that packets are transmitted once instead of thrice, the number of packets which reached the server revealed a Packet Loss Rate of just 2%. A high network load resulted in a Packet Loss Rate of 0.11%

Both the invidual and collaborative configurations of BLE Mesh offer a reduced Packet Loss Rate and improved network performance. They increase the coverage area of a network and allow all BLE devices to retransmit packet data – ensuring total network coverage. These offerings improve scalability dramatically by doing away with restrictive piconet structures. They improve user experience by opening networks up to meshing with personal BLE wearables and smartphones without needing manual pairing. Finally, unlike CSR, BLE Mesh is an open packet format. Anyone can use it as a jumping off point towards future innovation. We hope you are excited as we are for the future of Mesh.

 

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