WirelessHART Basics: Protocol, Physical & Data Link Layers

This page covers WirelessHART basics, including its applications, frequency band, and protocol stack, focusing on the physical layer and WirelessHART data link layer. WirelessHART is a wireless mesh network communications protocol designed specifically to meet the demands of process automation applications.

Importantly, WirelessHART is backward compatible with existing HART devices and applications. This means that existing HART applications (e.g., control systems, PLCs, maintenance tools, and asset management applications) can leverage WirelessHART without needing any software upgrades.

WirelessHART is a secure and robust mesh networking technology that operates in the 2.4GHz ISM radio band. It uses IEEE 802.15.4 compatible DSSS radios with channel hopping on a packet-by-packet basis.

WirelessHART is designed according to the IEEE STD 802.15.4-2006 compatible Physical Layer and MAC PDU, which we’ll briefly describe below. Let’s take a look at the WirelessHART protocol stack.

Figure 1: WirelessHART network architecture

Figure 1 illustrates a typical WirelessHART network architecture. The network comprises six key devices: field device, adapter, gateway, network manager, security manager, and handheld device.

Field devices equipped with routing capabilities are strategically placed to gather essential information. This information is then transmitted to the plant network for process monitoring and control.

WirelessHART PHYSICAL LAYER

The WirelessHART physical layer is primarily based on the IEEE STD 802.15.4-2006 2.4GHz DSSS physical layer. This layer defines critical radio characteristics such as the signaling method, signal strength, and device sensitivity. Similar to IEEE 802.15.4, WirelessHART supports the 2400-2483.5 MHz license-free ISM band, offering a data rate of up to 250 kbits/s.

The supported channels range from 11 to 26, with a 5MHz gap between adjacent channels.

• Data Rate: 250KBPS (62.5 KBAUD)
• Operating Frequency: 2400-2483.5 MHz
• Modulation: O-QPSK; Direct Sequence Spread Spectrum (DSSS)
• Transmit Power: About 10dBm, adjustable in discrete steps (e.g., 0dBM and others).
• PDU: IEEE compliant Physical Layer PDU, Maximum payload 127 bytes.

WirelessHART DATA LINK LAYER

Now, let’s explore the WirelessHART data link layer.

WirelessHART utilizes a strict 10ms time slot and leverages TDMA (Time Division Multiple Access) technology to ensure collision-free and deterministic communications.

The concept of a “superframe” is introduced to group a sequence of consecutive time slots. In WirelessHART, a transaction within a single time slot is defined by the following vector:

{frame id, index, type, src addr, dst addr, channel offset}

Where:

• frame id identifies the specific superframe.
• index is the index of the slot within the superframe.
• type indicates the type of slot (transmit/receive/idle).
• src addr and dst addr are the addresses of the source and destination devices, respectively.
• channel offset indicates the logical channel to be used for the transaction.

Bus Arbitration: TDMA using timeslots organized into superframes as mentioned above:

• 100 timeslots per second.
• All devices support multiple superframes with varying numbers of timeslots, allowing for a mix of fast (1 second), slow (minutes), cyclic, and acyclic network traffic. Superframes can be enabled and disabled based on bandwidth demand.
• Communication occurs in a designated timeslot and frequency channel for that specific message.
• Acknowledgment packets include timing information to continuously synchronize TDMA operation across the entire network.

WirelessHART REFERENCES

• IEEE 802.15.4-2006 DSSS: https://www.hartcomm.org/
• Product vendors: https://www.rfm.com/ , https://www.e-senza.com/