Architecture

A WirelessHART Network consists of WirelessHART field devices, at least one WirelessHART gateway, and a WirelessHART network manager. These components are connected into a wireless mesh network supporting bi-directional communication from HART host to field device and back.

Network Manager
The Network Manager is an application that manages the mesh network and Network Devices. The Network Manager performs the following functions:

  • Forms the mesh network
  • Allows new devices to connect to the network
  • Sets the communication schedule of the devices
  • Establishes the redundant data paths for all communications
  • Monitors the network

The mesh network architecture does not restrict where the Network Manager may reside in the plant automation network. The Network Manager may be located on a Gateway Device or on another system device. There is one Network Manager per mesh network. The Network Manager also communicates with the Security Manager to get encryption and authentication data to secure the network.
 

Gateway
The Gateway Device connects the mesh network with a plant automation network, allowing data to flow between the two. The Gateway Device provides access to the WirelessHART devices by a system or other host application.

Network Devices
A Network Device is a node in the mesh network. It can transmit and receive WirelessHART data and perform the basic functions necessary to support network formation and maintenance. Network devices include Field Devices, Router Devices, Gateway Devices and mesh Handheld Devices.

Field Devices
The Field Device may be a process connected instrument, a router or Hand Held device. The WirelessHART network connects these devices together.

Router Device
A device to improve network coverage (to extend a network) capable of forwarding messages from other Network Devices.
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Process Connected Instrument
Typically a measuring or positioning device used for process monitoring and control. It is also capable of forwarding messages from other Network Devices.

WirelessHART Adapter
A device that allows a HART instrument without wireless capability to be connected to a WirelessHART network.

Hand Held Support Device
Hand held devices are used in the commissioning, monitoring, and maintenance of Network Devices, they are portable and operated by the plant personnel.

There are three types of hand held devices:

  1. Traditional loop connected HART hand held device
    Used for configuration and initial commissioning of wireless instruments.
  2. WirelessHART (mesh) connected hand held device
    The wireless handheld device is used for local configuration and support of maintenance procedures. This hand held device will typically join an existing mesh network, and therefore pass through the WirelessHART security procedure, and allow the user to interact with other network device (usually a process connected device).
    The hand held device may also create its own WirelessHART network to connect to a stand alone device in a workshop environment.
  3. Plant automation network-connected hand held device
    This hand held device connects to the plant automation network through some other networking technology such as WiFi. This type of hand held device talks to WirelessHART devices through the plant automation network and the Gateway Device in the same fashion as the control system does. This is a host supported application and outside of the scope of this document.
Network Types
WirelessHART networks can be configured in a number of different topologies to support various application requirements including the following:

  • Star network—Star networks have just one router device that communicates with several end devices. This is one of the simplest network topologies. A star network may be appropriate for small applications.
  • Mesh network—Mesh networks are formed by network devices that are all router devices. Mesh networks provide a robust network with redundant data paths which is able to adapt to changing RF environments.
  • Star mesh network—Star mesh networks are is a combination of the star network and mesh network
Network Formation
A key attribute of a WirelessHART network is its ability to self-organize. There are three components of network formation: advertising, joining, and communication scheduling.

Advertising - As part of advertising, devices that are already part of the network may send packets announcing the presence of the network. Advertisement packets include time synchronization information and a unique Network Id. Devices trying to join listen for these packets and try to match the advertised Network Id with their own; once at least one advertisement is heard, the new device can attempt to join the network.

Joining - A new device joins the network by sending a join request packet through an advertising node to the Network Manager. If authenticated the Network Manager will respond with an activation packet, accepting the new device into the network and setting up links between the new device and other existing nodes.

Scheduling - The Network Manager then requests information from the new device on how often it requires that data be sent through the network. The Network Manager will then send out a schedule to the new device and any intermediate routing devices with the new schedule information.

A new network starts when a Gateway Device (typically at Network Manager’s request), starts to advertise. Being the first device in the network, the gateway starts its own schedule, to which other nodes later synchronize.

Data Reliability
Frequency Hopping
WirelessHART uses the unlicensed part of the radio spectrum in the 2.4GHz ISM band. As such, it could be subject to interference from several other sources such as other wireless networks. To solve this problem WirelessHART switches the frequency used based upon a pseudorandom sequence to avoid problematic, individual frequencies.

Blacklisting
A further refinement to Frequency Hopping is Blacklisting. Using this technique a frequency band (channel) that is known to provide persistent interference can be permanently skipped. Blacklisting is a user configurable feature of the Network Manager based on the proximity of other wireless networks that are in the same physical environment of the WirelessHART network.
Path Redundancy
One way a mesh network like WirelessHART obtains very high end-to-end data reliability is by providing redundant communication paths from the field devices to the receiving device such as a gateway or system interface. To obtain path redundancy each field device should be within communication range of at least two other devices that can receive its messages and forward them. Path redundancy is not necessary for a WirelessHART network to operate but is strongly recommended.

In figure on the right, two distinct communication paths can be defined for each field device so that if any device or communication path is disrupted the communications will be successful using the redundant path.
 
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