What makes up WirelessHART? There are four key elements to be aware of:
- A flat mesh network
- Network management
- Frequency hopping
- Time Division Multiple Access (TDMA)
Security and reliability are hallmarks of Wireless HART. Since it's used for industrial applications in process automation, it needs to be absolutely secure, and reliability is also a given. You wouldn't expect any less from this type of communication system. In most cases, if you are using a mobile device, SmartPhone, iPad, etc., your requests for information and the resulting transmissions back to your device are:
a. Wireless communication
b. Going through a router
The router can be a little dicey, as it may not be secure. Thus, anyone in range can listen in on your incoming and outgoing transmissions. Also, the router can be a single point of failure, and it can also be slow. The more devices going through the router with their information requests and data, the slower it gets. This scenario is not desirable if you are planning to use wireless communication for your plant processes.
Wireless HART solves these concerns by using the above four elements to improve and enhance how information is handled. Since HART and WirelessHART are primarily diagnostic in scope and use, you'll be able to capture and evaluate all the data from your plant processes, installed devices, etc., so that diagnostics and predictive maintenance can be performed, and it will be a secure and reliable system.
Flat mesh network
Let's take a look at the flat mesh network. It may also be referred to as a mesh network topology. In a mesh network, all the network participants form a mesh. All participants in the network act as repeaters and have multiple ways to communicate with each other. All the devices know who their immediate neighbors are, where the gateway is, who they should communicate with, and when they should do it. This allows information to be passed reliably. It also has the positive effect of lengthening the data transmission. Since the devices can act as repeaters, the range can be up to 200 meters. This dramatically increases the attractiveness of WirelessHART in a plant setting, as many plant processes can cover wide areas, or even move to other buildings on the grounds. Further, the mesh network is reliable. Think "neighbors helping neighbors." Since there is no single point of communication, if a device goes down or cannot communicate, the mesh network topology will not leave you stranded. You can simply reroute your communication to another functional neighbor, and the communication is not disrupted. The mesh network allows you to build reliability and distance from the control center into your network communications.
The network is organized and controlled by the Network Manager, which is found in the gateway. Consider it the central brains of the entire wireless network. The Network Manager forms the network by using the mesh network, frequency hopping, and TDMA. The Network Manager also determines the best communication paths, both primary and alternate, between all network participants, and the time, channel, sender, and receiver for the messages. If you add a new device or participant to the network, they are added automatically by the Network Manager. All of these system optimizations are done without human intervention, which helps to simplify the setup and installation tasks for WirelessHART. Now, the role of a Network Manager is not new, and, in fact, it has a job history from wired networks. Wired networks had additional considerations like power consumption coming from the mains and limited alternative communication pathways (restricted to actual wiring connecting devices).
WirelessHART has the benefit of being battery powered where energy consumption is an issue. The mesh network and the fact that all devices can talk to each other, as well as behave as a repeater, means that the number of theoretical communication paths increases dramatically with each participant added to the network.
If you've ever seen the sci-fi movie "Signs," there's a scene where a baby monitor is used to listen in on the communications between aliens lurking in the farmer’s cornfield. In a plant environment, you want to protect your communications and not share the communication frequency with other devices. Frequency hopping corrects this problem, using the standard IEEE802.15.4. This standard identifies 16 channels, each with a bandwidth of 2 MHz.
WirelessHART uses 15 of these channels. Channel allocation occurs between 2405 GHz and 2480 GHz. The distance of channel center frequencies to each other is 5 MHz. The single channels do not overlap, and can be used simultaneously. The whole WirelessHART network uses all the channels in parallel. If a channel is in use, the Network Manager can utilize frequency hopping to move quickly to another, open channel to continue communications without interruption.
Time Division Mutliple Access (TDMA)
If the role of frequency hopping is to find open communication slots across the 15 channels used by Wireless HART, then TDMA is the manager of communications within these slots. TDMA divides a second into 100 slots, 10 ms each. The entire network is synchronized in these slots. TDMA basically works as a flow chart or control list. We recommend using no more than 30% of the total communication capability to keep communications running optimally.