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Industrial Ethernet Book Issue 67 / 47
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Sensor networks: wireless mesh or wireless backbone?

Automation plant operators should carefully consider their current and future needs before choosing an industrial wireless system. Some applications are well suited to a field device meshing network, while others are better served by an infrastructure meshing network. To gain the maximum benefit that meshing can offer, the selected system should support both topologies simultaneously and seamlessly in a single network. In this article, Soroush Amidi explains the characteristics of each topology to help end users decide which one best serves their needs.

THE PACE OF ADOPTION for industrial wireless by the very conservative automation and manufacturing industry testifies to the strength of wireless meshing technology. Every year, thousands of plants opt to use wireless devices, such as mobile handhelds running apps for maintenance and process monitoring, video collaboration cameras, asset location tags and wireless field instruments. This improves organisations' operational and capital expenditure performances. The attractiveness of industrial wireless lies in its mobility, flexibility and lower cost.

The use of wireless technology in the process industries is not new. Automation professionals began using wireless transmitters more than a decade ago to collect data from remote areas or equipments where the use of wired transmitters were not feasible, either because of physical or financial constraints.

What is new, however, is the development wireless meshing technology, which offers the reliability and robustness that was lacking in point-to-point wireless products used at the beginning of the 21st century.

Today, a growing number of industrial end users are implementing wireless devices, including wireless transmitters. In fact, wireless transmitters are even being installed for process monitoring in areas where wired transmitters can be used. Why? Again, it is because of the reliability offered by the wireless meshing technology.

When it comes to implementing a wireless meshing network, automation professionals are faced with choosing from several different topologies. They can implement a field device meshing topology where field devices, typically battery powered wireless field instruments or wired field instruments with wireless adapter, form a peer-to-peer meshing network. Alternatively, they can implement an infrastructure meshing topology where infrastructure nodes, i.e. lined powered industrial access points, form a peer-to-peer meshing network connecting wireless field devices, field instruments and/or Wi-Fi devices.


Wireless sensor meshing or wireless sensors with high-speed backbone? Automation plant operators should carefully consider their current and future needs before choosing an industrial wireless system.

Field device meshing topology

Field device meshing enables a wireless peer-topeer network to form among wireless field instruments. This approach does not require any lined powered wireless infrastructure to be present. Communication packets can hop between transmitters to reach the final destination. Transmitters auto-discover neighbouring transmitters and establish a communication path with each other, thus forming a mesh network.


Wireless sender for ISA100.11a: It can operate with an update rate of 1s, but at the expense of battery life.

Each transmitter acts as an independent router (i.e., the transmitter can send its own data as well as route data received from other transmitters). This allows for continuous connections and reconfiguration around broken or blocked paths by 'hopping' from transmitter to transmitter until the packet reaches the wireless sensor gateway.

Infrastructure meshing topology

In an infrastructure meshing network, field instruments do not act as routers. Instead, linepowered infrastructure nodes route the data. These infrastructure nodes auto-discover each other and establish a peer-to-peer network. This allows for continuous connections and reconfiguration around broken or blocked paths, again by 'hopping' from node to node.

A Field Device Access Points hosts an ISA100.11a backbone router board (ref ISA100.11a standards for backbone router definition) in an industrial enclosure. Data can be routed through other ISA100 field devices using the ISA100.11a standard or via a highspeed backbone router such as an IEEE 802.11 Wireless Local Area Network (WLAN) via the Ethernet port hosted on each node. Multinodes route data through other Multinodes using the IEEE 802.11 standard. They also host a backbone router board connected to a meshing access point board in the same industrial enclosure. They can route ISA100.11a data as well as Wi-Fi data.


Fig. 1. Field device meshing and infrastructure meshing: The OneWireless system supports field device meshing and infrastructure meshing topologies in either standalone mode, or in combination.

The ISA100.11a standard

So as to realise the maximum benefits offered by meshing, both field device and infrastructure meshing topologies need to be supported simultaneously and seamlessly in a single network. This is one of the core requirements of the ISA100.11a standard for wireless field instruments, which was developed by ISA100's 500+ members representing the plant automation industry. This technology was designed for use in remote applications with low frequency update rates, as well as in control and critical monitoring applications with high frequency update rates.

ISA100 members ensured that ISA100.11a transmitters could operate with both field device and infrastructure meshing topologies. Figure 1 illustrates how ISA100.11a field instruments operate in either field device meshing or infrastructure meshing mode.

A comparison

Table 1 (over page) shows the characteristics of a field device meshing topology versus that of an infrastructure meshing approach.

Latency. The ISA100.11a standard is intended to optimise the battery life of wireless transmitters. When in field device meshing/routing mode, ISA100 field instruments wake up on a periodic basis to listen to other transmitters and route data based on reporting rates. This capability allows field instruments to save power and maximise their battery life, which translates to lower latency.

In a star meshing topology, routing infrastructure nodes listen and route data in real-time. This is possible since the devices are line-powered. Field instruments simply need to transmit their data to the routers. Network timeslot allocation is much easier in an infrastructure meshing topology.

As just described, there is a significant difference in data latency between a field device meshing network and an infrastructure meshing network. That is why Honeywell recommends field device meshing topologies for applications requiring slow reporting rates from the field, and infrastructure meshing topologies for applications requiring fast reporting rates from the field.


Fig. 2. An ISA100.11a-compatible flexible network: The infrastructure meshing network will cover most of the site's area and the field device meshing network will connect remote devices to the high-speed infrastructure meshing network via one or two battery-powered routing field devices.

Battery life. In a field device meshing topology, routing transmitters have to actively listen and send data to neighbouring transmitters. The battery life of a transmitter is a function of the number of transmitters meshed with the device. It decreases as the number of meshed transmitters increases. In an infrastructure topology, transmitters send data to routing infrastructure devices. This translates into a significant difference between the battery life of a transmitter acting as a routing node in a field device meshing topology, and a transmitter in an infrastructure meshing topology.

Honeywell recommends field device meshing topologies for applications requiring slow reporting rates, and for plants not planning to scale up to tens of transmitters. Infrastructure meshing topologies are suitable for applications requiring fast reporting rates and millisecond latency, and for plants planning to scale up to hundreds of transmitters.

Reporting rate. In order to avoid frequent replacement of batteries, routing transmitters are only used for applications requiring slow reporting rates. A non-routing transmitter in an infrastructure meshing topology can be configured at the highest reporting rate. Some transmitters can report as fast as a 1-second update rate.

Honeywell recommends field device meshing topologies with ISA100.11a field instruments for users with applications requiring slow update rates (30 seconds or more). For applications requiring fast update rates, it recommends an infrastructure meshing topology with either Multinodes or Field Device Access Points (see box).

Scalability. Routing transmitters can route a limited number of field instruments. This limitation is primarily because of the 802.15.4 radio. This has a 250 kps throughput, which is just sufficient to route data from a handful of transmitters. The other limitation is because of the components selected for use with wireless field instruments. Power consumption and management are key requirements driven by customers' need to have wireless field instruments with multi-year (10 years) battery life.

The components are selected based on their power consumption and power management capability. This typically means a small memory and computing footprint, which differs from line-powered routing infrastructure devices where power consumption and power management are not an issue. Wireless field instruments can route up to four transmitters efficiently, while routing infrastructure nodes can route up to 80 transmitters.


Table 1: Characteristics of a field device meshing topology versus an infrastructure meshing topology

Flexibility. Field device meshing topology limits users to an ISA100.11a field instrumentsbased application. An infrastructure meshing topology having industrial meshing access points designed to extend the process control network into the field, can support ISA100.11a field instruments and Wi-Fi devices, and therefore enable a multitude of applications. Such devices (Multinodes) can simultaneously route data between Wi-Fi (IEEE 802.11 b/g) clients, ISA100.11a field instruments and Ethernet/IP devices and host applications.

It is possible to start small with a field device meshing network and scale up to an infrastructure meshing network. Most installations will be a combination of infrastructure meshing and field device meshing networks. The infrastructure meshing network will cover most of the site's area and the field device meshing network will be used to connect remote devices to the highspeed infrastructure meshing network via one or two battery-powered routing field devices. Figure 2 shows the Honeywell implementation of an ISA100.11a-compatible network.

And finallyˇ­

Automation professionals should carefully consider their current and future needs prior to selecting an industrial wireless system. Some applications are well suited to a field device meshing network, while others are better served by an infrastructure meshing network. To gain the maximum benefit that meshing can offer, the selected system should support both topologies simultaneously and seamlessly in a single network.

Current market trends indicate that most installed wireless systems will use infrastructure meshing complemented by field device meshing when infrastructure meshing is installed at the core of the plant where power is readily available. Field device meshing is used in remote areas of the plant where power is not readily available.

ISA100.11a: The Honeywell view

The Honeywell OneWireless R200 network supports field device and infrastructure meshing topologies by making full use of ISA100.11a characteristics. This system provides the flexibility to scale a wireless network up or down depending on requirements.

ISA100.11a transmitters can operate with both field device and infrastructure meshing topologies - one of the main reasons why ISA100 is at the core of the OneWireless infrastructure and all of its wireless devices.The system therefore supports field device meshing and infrastructure meshing topologies in either standalone mode, or in combination.

Honeywell's Field Device Access Points are industrial APs for ISA100.11a field instruments while Multinodes are industrial APs for ISA100.11a field instruments andWi-Fi devices. OneWireless networks having Multinodes are suitable for applications requiring fast reporting rates and millisecond latency, and for plants planning to scale up to 100s of transmitters.

The company's XYR6000 transmitters can report as fast as a 1-second update rate. Thirty percent of OneWireless users have set their transmitters at a 1s update rate while 60% of users have set their transmitters at a 5s update rate. The remaining 10% of users have selected an update rate longer than 5s.

Soroush Amidi works for Honeywell Process Solutions.


www.honeywell.com/ps/wireless


Source: Industrial Ethernet Book Issue 67 / 47
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