Hart & Wireless Hart in IoT

Introduction

• Wireless HART is the latest release of Highway Addressable Remote Transducer (HART) Protocol.
• HART standard was developed for networked smart field devices.
• The wireless protocol makes the implementation of HART cheaper and easier.
• HART encompasses the most number of field devices incorporated in any field network. 
• Wireless HART enables device placements more accessible and cheaper - such as the top of a reaction tank, inside a pipe, or at widely separated warehouses.
• Main difference between wired and unwired versions is in the physical, data link and network layers.
• Wired HART lacks a network layer.

HART Physical Layer

• Derived from IEEE 802.15.4 protocol.
• It operates only in the 2.4 GHz ISM band.
• Employs and exploits 15 channels of the band to increase reliability.

HART Data Link Layer

• Collision free and deterministic communication achieved by means of super-frames and TDMA.
• Super-frames consist of grouped 10ms wide timeslots.
• Super-frames control the timing of transmission to ensure collision free and reliable communication.
• This layer incorporates channel hopping and channel blacklisting to increase reliability and security.
• Channel blacklisting identifies channels consistently affected by interface and remove them from use.

HART Network & Transport Layers

• Cooperatively handle various types of traffic, routing, session creation, and security.
• WirelessHART relies on Mesh networking for its communication, and each device is primed to forward packets from every other devices.
• Each device is armed with an updated network graph (i.e., updated topology) to handle routing.
• Network layer (HART) = Network + Transport + Session layers (OSI).

HART Application Layer

• Handles communication between gateways and devices via a series of command and response messages.
• Responsible for extracting commands from a message, executing it and generating responses.
• This layer is seamless and does not differentiate between wireless and wired versions of HART.

HART Congestion Control

• Restricted to 2.4Ghz ISM band with channel 26 removed , due to its restricted usage in certain areas.
• Interference-prone channels avoided by using channel switching post every transmission.
• Transmissions synchronized using 10ms slots.
• During each slot, all available channels can be utilized by the various nodes in the network allowing for the propagation of 15 packets through the network at a time, which also minimizes the risk of collisions.

WirelessHART Network Manager

• The network manager supervises each node in the network and guides them on when and where to send packets.
• Allows for collision-free and timely delivery of packets between a source and destination.
• The network manager updates information about neighbors, signal strength, and information needing delivery or receipt.
• Decides who will send, who will listen, and at what frequency is each time-slot.
• Handles code-based network security and prevents unauthorized nodes from joining the network.

WirelessHART vs. ZigBee

A wirelessHART node after every message, changing channels every time it send a packet. ZigBee does not feature hopping at all, and only hops when the entire network hops.

At the MAC layer, wirelessHART utilizes time division multiple access (TDMA), allotting individual time slots for each transmission. ZigBee applies carrier sense multiple access with collision detection (CSMA/CD)

WirelessHART represents a true mesh network, where each node is capable of serving as a router so that, if one node goes down, another can replace it, ensuring packet delivery.ZigBee utilizes a tree topology, which makes nodes along the trunk critical.

WirelessHART devices are all back compatible, allowing for the integration of legacy devices as well as new ones. ZigBee devices share the same basis for their physical layers, but ZigBee, ZigBee Pro, ZigBee RF4CE, and ZigBee IP are otherwise incompatible with each other.