Can there be a single wireless protocol for field devices?
Wireless network security
Users and field instrumentation suppliers have great concerns over security of wireless networks. To meet this need, both ISA100 and WirelessHART encrypt every message using the built-in AES-128 encryption of the IEEE 802.15.4 radio chip. Security is managed on ISA100 wireless networks using a rotating encryption key, meaning that the security key is changed on a periodic scheduled basis. During the time it would require for an intruder to hack the network encryption key, a new key is automatically generated and distributed, preventing actual intrusions.
WirelessHART limits the encryption and checking of message integrity to only one of the eight possible choices available in the IEEE 802.15.4 standard, while ISA100 makes six possible combinations available. Both standards default to using a 32-bit message integrity code, but ISA100 also offers a configurable 64-bit or 128-bit message integrity code, providing a higher level of security that is sometimes a requirement of government and quasi-government agencies.
During development of ISA100, users declared the need for a secure method to commission or provision a new field device without a specialized handheld device or physical access to the instrument. ISA100 has a secure over-the-air method to provision a new device and allow it to join into the network using PKI (public key infrastructure). The security is based on two factors: a white-list identifying the devices to be provisioned must be installed in the network prior to provisioning, and the user must have possession of a corresponding 283-bit certificate. The white-list and certificate are installed to the network security manager using files created by the manufacturer and are supplied to the user on transportable media (CD, DVD, USB flash memory, SD card, etc.).
This eliminates the need for a device provisioning operation by the user. When a wireless instrument is installed in the field, it will automatically respond to network requests to join and then become part of the operating network. The reason this process is secure is the use of a standard 283-bit public/private key method that does not expose any unencrypted joining keys over the air, and does not rely on insecure operational procedures to distribute secret keys to a host system or through a handheld terminal. WirelessHART uses a physically attached HART handheld terminal to enter the network address and the security key. A similar interoperable mechanism is also available to ISA100 users through an infrared port.
Both WirelessHART and ISA100 support mesh networking, which is an excellent method to extend the distance a network can cover, to access devices that are shielded by buildings from direct line of sight, and to provide a resilient path for data transfer to increase reliability. However, when meshing is used in a synchronous control loop, only ISA100 can limit the mesh depth and simultaneously provide a fully resilient data path using duocast technology. Limiting network depth requires the location of field routers reachable in a single hop since multilevel meshing can cause indeterminate delays in signals reaching their intended destination. Reliable networking by using resiliency requires that the information sent on the resilient data paths be identical. The ISA100 feature that assures that the data sent on resilient data paths are identical is the duocast feature built into ISA100 wireless technology. WirelessHART has the ability to send data to multiple routes on its mesh, but not in the same slot time.
In configurations where a device needs to access data from a neighbor in a mesh network, long latencies can be involved in transmitting data through the mesh to and from a DCS. To address this, ISA100 supports direct communication between devices in proximity to each other. This peer-to-peer relationship involves direct wireless communication, operating in conjunction with an application object model that can execute control-in-the-field (CiF) logic remotely without DCS involvement, similar to the operation of CiF in FF HSE.
Resolving the dilemma
Users of process control field instrumentation face a dilemma in choosing which wireless network to install. Their choices are the following:
1. Do not install any wireless field instrumentation
2. Only install ISA100 wireless
3. Only install WirelessHART
4. Because each plant site typically standardizes on one manufacturer of DCS, install the wireless network integrated with that DCS
5. Purchase only dual-boot devices that can be configured at installation to support either standard, or
6. Install another standard such as WIA-PA, ZigBee, Bluetooth, Wi-Fi, or a proprietary network.
Each choice has its own practical considerations that users will have to sort through.
1 or 6: Avoiding wireless field instrumentation entirely, installing another standard, or installing a vendor's proprietary network is against a company's interests if it wants the benefits of standard wireless field instrumentation. Both WirelessHART and ISA100 have the field-proven, strong security needed to assure users of privacy and intrusion protection.
4: Installing both ISA100 and WirelessHART within the same company and depending on support from the DCS supplier sacrifices the experience and volume that can be obtained with a single wireless network selection.
5: Purchasing only dual-boot devices that contain both protocols or can be initialized with either meets only the need to reduce inventory. It will create much greater complexity during installation and maintenance since both networks need to be understood and maintained.
2 and 3: Installing either standard alone limits the company's ability to select the best DCS for the plant site, and may make that plant site obsolete if the network chosen does not become the wireless network norm.
Can there be a network norm?
While instrumentation suppliers and some end users might be drawn to supporting both networks via the dual-boot method, this does not achieve the long-term establishment of an industry norm, which has been at the root of requests by all end users. ISA100 can be the core of future wireless industrial networking technology in much the same way as Ethernet TCP/IP and Wi-Fi have become norms for IT networks. ISA100 is totally built upon well-established network standards. This means that ISA100 is an application-independent, Internet-based telecommunications network designed for critical and noncritical industrial automation environments, just like Ethernet TCP/IP and Wi-Fi are application-independent Internet-based telecommunications for business networks. As long as the applications are built to use common network standards based on IP protocol, they can be transported across any standard network, such as ISA100, as long as there is adequate bandwidth for the task, and it has been suitably adapted to the particular needs of the automation industry.
Moreover, like Ethernet TCP/IP, use of ISA100 is independent of the network hardware. As microprocessors and communications semiconductors evolve, and frequency assignments change, ISA100 wireless will be able to make these transitions without requiring any changes in the applications that are carried on this network technology. If history is a good predictor of future development, wireless network technology is destined to get less expensive, use less power, and become much faster. ISA100 will naturally be able to fit right onto the new wave of wireless network technology as commercial availability grows.
Manufacturers of process control equipment are concerned primarily with meeting customer needs and in reducing product cost to enable competitive pricing. Customer needs often start with solving short-term problems, but most would also like to develop a long-term architecture for their process automation needs. WirelessHART has been, due to its earlier product introduction, available to solve the short-term needs, but several end-user organizations are now recognizing the fact that ISA100 wireless alone meets the needs of a long-term wireless architecture.
Early users of wireless process control instrumentation had application requirements that centered only on obtaining process data from locations that were either too costly or impossible to wire. Most of these applications can be satisfied with either WirelessHART or ISA100 wireless, and have now been field-proven and accomplished their goals.
Users that decide to use WirelessHART as their plant wireless network are committing themselves to a control system architecture in which there is little intelligence in the field devices and no possibility of CiF with its benefits. Their DCS must do all the work of signal processing and closed loop control. ISA100 can certainly connect to those same HART field devices, but its open architecture allows synchronous two-way wireless data transfer with full function (intelligent) field devices when they are ready, without changes in the basic network. And we know that there is a strong trend toward FF with its intelligent field devices and highly synchronous data transfers.
While the early efforts have now resulted in two non-interoperable wireless standards, users still want to establish a single wireless network norm suitable for a wide range of applications well beyond acquisition of remote data points. Leading users have recognized that the wireless network norm must not only be an international standard, but must be suitable for demanding applications such as wireless FF and Internet web servers in field instrumentation. Only a highly secure IP-based protocol can do this, and only ISA100 wireless can meet these needs.
Dick Caro is CEO of CMC Associates, an industrial networking consulting company. He is the author of "Wireless Networks for Industrial Automation."
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