November/December 2010

Cover Story

Wireless standards in action

A closer look at ISA-100.11a

Fast Forward

  • Some of the unique elements in ISA-100.11a include backbone routers, standard interface for backbone routers, and duocast.
  • ISA-100.11a includes the concept of shared timeslots in addition to dedicated timeslots for communication.
  • Duocast benefits include data availability, latency, scalability, and battery life.
 
By Soroush Amidi

The term "wireless standard" may seem like an oxymoron to observers of the process manufacturing industries. After all, standard implies "the norm." And while a small handful of wireless protocols have clearly emerged as front runners, there is still quite a way to go before a single standard can truly lay claim as the preferred.

action1While specific vendors and standards-making bodies will understandably continue to promote their products as the standards of choice, the reality is plants will make their wireless decisions based on their knowledge of how technologies can address their unique needs.

So what actually makes a standard, and how does it work? This article provides a snapshot of one of the current standards, ISA-100.11a, and boils down the key elements that make it unique in addressing those needs.

The basic anatomy of ISA-100.11a

ISA-100.11a resulted from requirements defined years ago by more than 500 end users and vendors in standardization activity supported by the U.S. Department of Energy. The group wanted to ensure the standard would meet, and in some instances surpass, wired system performance. They also agreed wireless sensor systems needed to offer reliability and security required to satisfy non-critical monitoring, alerting, supervisory control, and open- and closed-loop control. They wanted a wireless sensor system to address performance needs of applications such as critical monitoring and process control, where latencies on the order of 100 ms can be tolerated, with optional behavior for shorter latency.

In short, the standard needed a rich set of elements that enabled field instrument and automation vendors to build products that could satisfy evolving needs of various industries-a future-proof standard. The committee determined certain elements best met these requirements and would enable plants to transition from a monitoring system to more critical applications as their comfort levels with wireless sensor systems increased.

The resulting infrastructure includes these components:

Gateway: An interface between the wireless and plant networks, or directly to an end application on a plant network. It marks the transition between communications compliant to the standard and other communications, and acts as a translator between ISA-100.11a and other protocols (Modbus, HART, Foundation Fieldbus, etc.).

The security manager supports provisioning devices, authenticating devices that attempt to join the wireless industrial sensor network, managing the master security keys for each device and administrating security policy for each key.

The system manager governs the wireless industrial sensor network, including devices and communications. It performs policy-based control of the network runtime configuration, monitors and reports on communication configuration, performance, and operational status, and provides time-related services.

Backbone router: A field device, which has a field network interface and a backbone interface. Backbone routers enable external backbone to carry native ISA-100.11a protocol by encapsulating the protocol data unit for transport. A backbone is a data network (preferably high data rate) that could be an industrial Ethernet, IEEE 802.11 based wireless mesh network, Wi-MAX network, or any other network within the facility interfacing to the plant's networks.

I/O device: An input or output field instrument with the minimum characteristics required to participate in the network. The I/O role provides no mechanism for forwarding messages or routing any other device. This enables the construction of the least-complex devices and the potential for low energy consumption.

Router device: These devices can provide range extension for a network and path redundancy by routing ISA-100.11a devices received from other ISA-100.11a I/O devices. The router device can also be an I/O device, in which case it routes its own data in addition to data received from other I/O devices.

Provisioning or portable device: Used to provision ISA-100.11a devices, that is, inserting the required configuration data including security key into a device to allow the device to join a specific wireless industrial sensor network.

Unique characteristics

In addition to all traditional elements found in the WirelessHART specification, ISA-100.11a's unique elements include:

  • Backbone routers
  • Standard interface for backbone routers
  • Duocast
  • Shared and dedicated time slots
  • Multiple DLL sub-network
  • Over-the-air provisioning
  • Network layer based on IPv6 over Low Power WPAN (6LoWPAN)

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Backbone routers

Backbone routers act as a transit link between the wireless field instrument network and the backbone. ISA-100.11a allows multiple backbone routers to be connected to a single backbone. Designing a wireless industrial network with multiple backbone routers provides several benefits:

  • Increased network reliability by offering the spatial and temporal diversity required to handle errors caused by radio interference and obstacles
  • Improved data latency by reducing the number of hops between the device I/O and gateway by taking advantage of a high-speed backbone
  • Eliminated single points of failure

Being able to install multiple backbone routers has been critical for plants using ISA-100.11a field instruments for critical monitoring, regulatory, or control applications.

Standard interface for backbone routing

A standard interface allows users to route field instrument data through any backbone. Most backbone routers are used with Internet Protocol (IP) networks such as an IEEE 802.11 mesh network or Wi-MAX. The most common backbone routing implementation consists of encapsulated ISA-100.11a IPv6 packets inside IPv4 packets following RFC2529. As IPv6 becomes mainstream, the backbone router will be able to send ISA-100.11a IPv6 packets directly on an IpV6-enabled network.

The standard interface for backbone router offers the flexibility required to use ISA-100.11a field instruments in remote monitoring, as well as plant-wide layouts.

In remote-monitor scenarios, the backbone router, security manager, system manager, and gateway manager may be installed.

This architecture allows the wireless industrial sensor network to remain functional in case the backbone, typically a point-to-point network, is slow or unreliable. The point-to-point network could be a modem-based or a shared network with unpredictable quality of service such as a satellite connection.

In a plant-wide layout with a high-speed backbone, users install the gateway, security, and system manager in a control room, while the backbone routers are in the field.

The standard interface allows users to connect the backbone routers to high-speed backbones and send their ISA100 data without the need to have any other protocol translator between the router and gateway. The system manager, security manager, and gateway, the heart of the sensor network, is then located in a controlled environment instead of an outdoor environment. Currently, most plant-wide implementations take advantage of this feature by connecting routers to high-speed IEEE 802.11 meshing access points. 

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Shared, dedicated timeslots

Some wireless standards require each device to use dedicated timeslots for communication. ISA-100.11a includes the concept of shared timeslots in addition to dedicated timeslots. A pool of shared timeslots can be used by a collection of devices depending on their needs.

This allows the network to meet the desired level of service expected by end users when, for instance, they are changing the device configuration or when the asset management application is querying data for troubleshooting.

The shared timeslot allows the network to quickly respond to additional queries from field devices and offer wired-like performance.

Multiple sub-networks

Multiple sub-networks allow ISA-100.11a users to design sensor systems that can span from a single, small, isolated network that might be found in the vicinity of a gas or oil well or small machine shop, to integrated systems of many thousands of devices and multiple networks that cover expansive plants. There is no technical limit on the number of subnets or devices that can participate in a network. A subnet, a group of devices sharing a specific data link layer configuration, may contain up to 30,000 devices (limitation of subnet addressing space). With multiple subnets, the number of devices in the network can scale linearly.

Practically, this allows a single ISA100 system manager to manage an ISA-100.11a network with slow subnets, with devices reporting once every hour that connect infrequently and report a value using a Carrier Sense Multiple Access scheme, and faster subnets with devices that would be used in Time Division Multiple Access fashion to reliably guarantee all devices have a dedicated timeslot to transmit their measurement data.

This provides the flexibility users expect from a wireless industrial sensor system-that is, a system that allows them to scale up and mix and match different types of topologies while managing all devices through a single system manager. (An array of supported network architectures and topologies is presented in the 700+ page ISA-100.11a standard.)

Duocast

Duocast is one of the key elements that improves the "confidence in, integrity of, and availability of data sent by a wireless transmitter in an industrial environment." Duocast allows two receivers to receive and acknowledge the data sent by I/O devices. Benefits include:

  • Data availability: Duocast with backbone routers connected to a mesh backbone is the only way to obtain wall-to-wall redundancy, that is, from the field instrument to the host destination, which in most instances are redundant (redundant server, redundant controller).
  • Latency: It provides better latency, especially when a router has more children, as the device does not have to wait until the next scheduled slot to attempt a second transmit if the first transmission fails. 
  • Scalability: Duocast allows for more sensor publications from a source node than the next best option, which is graph-based route redundancy with an automatic reroute feature also supported by ISA-100.11a.
  • Batterylife: Duocast provides better battery life in case of intermittent links for the dual casting node. It provides even better battery life if the receivers are also battery powered.

Over-the-air provisioning

ISA-100.11a allows users to provision a device wirelessly with different levels of security. This feature allows end users to provision devices remotely from the ISA-100.11a security manager using digital certificates or static security keys (less secure).

Regardless of commissioning method, users can configure and make changes to field instruments from the ISA-100.11a system manager or their favorite asset maintenance application, as the ISA100 gateway supports field protocols used by today's asset maintenance applications.

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Network layer based on IPv6 over Low Power WPAN (6LoWPAN)

Instead of re-creating the wheel, ISA100 adopted specifications developed by the 6LoWPAN IETF group. IETF has defined encapsulation and header compression mechanisms that allow IPv6 packets to be sent to and received from Low-Power Wireless Personal Area Networks (LoWPANs), which are defined as networks comprised of "devices that conform to the IEEE 802.15.4-2003 standard by the IEEE." These devices are characterized by low bit rate, low power, and low cost, with typically low computational power, memory, and energy availability.

The group has already resolved issues associated with enabling IP communication between 6LoWPAN devices. Adopting the established specifications enabled ISA100 to take advantage of IETF's work as well as any further enhancements resulting from the organization.
Crosbyplant case

In 2009, Arkema's Crosby plant, located just north of Houston, Tex., agreed to be the beta site for the Wireless Compliance Institute (WCI) and vendors offering ISA-100.11a field instruments. The plant produces liquid organic peroxides used primarily to produce plastic resins, polystyrene, polyethylene, polypropylene, PVC and polyester reinforced fiberglass, and acrylic resins.

Within days of approval of the ISA-100.11a standard by the ISA Standards and Practices Board, the ISA100 WCI installed a multi-vendor user test system at the plant to demonstrate interoperability among multiple vendor devices.

Numerous applications for ISA-100.11a were identified, including temperature, pressure, contact closure, valve positioning, gas detection, corrosion detection, and others. Most of the sensing opportunities were outdoors, where cabling for sensors can be prohibitively expensive. Four applications were initially identified:

  • 500,000 gallon water tank: A firewater safety tank that must be full at all times previously relied on a simple mechanical sight gauge that "fails full," and the cost of wiring to that location for a pressure sensor was prohibitive.
  • Cold storage: Numerous cold storage warehouses operate at temperatures below 0°F. If these warehouses exceed the required temperature, product can potentially decompose and ultimately catch fire. Temperature was being reported to the control room by wire, with an audible remote alarm to indicate if a door was left open. Wireless temperature and door sensors were added to three of the warehouses, providing central reporting of exception conditions. This six-transmitter cluster provided a realistic demonstration of ISA-100.11a sensor meshing capabilities.
  • Wireless adaptor: Values from a wired level sensor on a waste water tank were fed into a nearby satellite control room, but not the central control room. An ISA-100.11a adaptor connected in series to the 4-20 mA analog output of this sensor, reporting the result wirelessly to the central control room. This demonstrated the general notion of using ISA-100.11a to provide central visibility to existing sensors scattered across the plant.
  • Gas sensor: The site included many gas sensing opportunities. A single wireless gas sensor was installed alongside an existing wired sensor. Following a successful pilot, wireless gas sensing can be expanded throughout the site when a fully certified wireless product is available.

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An infrastructure was installed including two ISA-100.11a backbone routers providing necessary site coverage. A WiFi connection between the backbone routers simplified the installation, so the remote backbone router did not need a wired connection back to the host. 

The test topology demonstrated the flexibility of ISA-100.11a standard where a backbone router would provide extensive outdoor wireless coverage, and sensor meshing would be used for uncovered areas.

Arkema was surprised by the backbone router coverage, as seven of the nine installed transmitters established strong connections directly to the backbone router. The remaining two transmitters were remotely located and connected by sensor meshing through other transmitters. All deployed transmitters had routing capability, enabling the site to add more transmitters without needing to implement additional backbone routers.

All transmitters installed at Arkema had been tested for ISA-100.11a compliance using a non-commercial version of the WCI's Device Interoperability Test Kit. This hardware/software tool uses XML scripts to emulate the operation of an ISA-100.11a system manager in a transparent and vendor-independent manner.

All transmitters were tested for ISA-100.11a compliance, and approved for use at Arkema.

To demonstrate the concept of plant-wide coverage from two access points, the initial transmitters were installed toward the periphery of the operation. The backbone routers were placed based on a visual inspection of the site, without a radio survey of any kind. Radio ranges of 100-200 meters were achieved, as expected. 

ISA-100.11a was released as an ISA standard in September 2009. Since then, this standard for wireless field instruments was followed by another standard for field instruments - IEC 62591Ed 1.0, developed by HART Foundation. Additionally, an upcoming standard (IEC 62601), developed by a consortium of Chinese instrumentation companies, is expected to receive approval.

As mentioned, it is unrealistic and impractical (and incorrect) to assume today's industry as a whole has chosen one over the other. This makes it all the more crucial to understand the ins and outs of these standards so companies can effectively evaluate which ones can meet their needs now and in the future.

ABOUT THE AUTHOR

Soroush Amidi is the product marketing manager for Honeywell Process Solutions. He manages several wireless products including OneWireless network. His career spans a variety of engineering, general management and marketing roles within Honeywell. Soroush holds a Bachelor of Engineering Sciences in Chemical Engineering from the University of Western Ontario, London, Ontario, and a Masters of Business Administration from McGill University, Montreal.

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