Explaining WirelessHART communication

HART is a communication protocol commonly used in industrial communication networks. HART communicate data over the 4-20mA control loop in the form of AC signal in audio-frequency tone. Then HART protocol is introduced into wireless communication mode. The transmitters in the field are connected to wireless network, which communicates with a wireless gate device connected to the controller system.

Wireless HART network protocol:

Physical layer:

  • Uses 2.4GHz to 2.5GHz frequency transmission and low power level 10 dBm nominal because meeting these criteria allows WirelessHART devices to be unlicensed according to FC (Federal Communications Commission) standards.

  • O-QPSK modulation (offset quadrature phase-shift keying)

  • Direct-sequence spread-spectrum (DSSS) with frequency-hopping between 15 channels within that band for security and interference reduction

  • TDMA (Time-Division Multiple Access) bus arbitration, with 10-millisecond timeslots allocated for device transmission. TDMA bus arbitration means that the Network Manager plans and schedules the transmission times of all field devices, giving each one its own dedicated time to “speak.”

Data Link Layer:

  • Network ID number uniquely identifies each WirelessHART network, allowing multiple networks to overlap the same physical area

  • Channel “blacklisting” – automatically avoids hopping to noisy channels

Network Layer:

  • Wireless HART uses Mesh networking

  • Four levels of data message priority: Command, Process data, Normal ( all messages other than command), Alarm (messages reporting device alarms and events).

Application layer:

  • 128-bit encryption of data

  • Backward-compatibility with wired-HART command structure and DDL (Device Description Language)

How does a wireless HART works?

Each of the transmitters in the network is connected to a wireless transmitter. The transmitters have to communicate with a common Gateway at the controller end.

The network of transmitters are connected in Mesh network, in which the transmitters are also connected one another. The connection between transmitter provides the transmitter with another duty, as repeater. If one device can’t communicate with the gateway because of barriers, the adjacent device can work as a repeater and takes the data from the device and transmit to the gateway.

If there is a blockage in the network to some transmitters adjacent transmitter can provide the communication link with the gate. When the blockage is frequently moving (for examples vehicles), providing link will be difficult.

In such situation, there is an advanced algorithm provided in the Gateway processor called as Network manager.The purpose of the network manager is to provide the network tunning. Network manager assign time schedule to each transmitter to communicate. It determines the frequency-hopping schedule, detects and authenticates new devices added to the network, dynamically adjusts device transmission power, and selects alternative routes between devices.

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