The introduction of wireless data transport into the industrial sector represents a convergence of the monitoring and control world, wireless sensor technology, and the IT community. Each element of this triad has its own standards and practices. The challenge for those currently using wireless technology, as well as those contemplating future deployments, is to unravel the tangled web of the technologies, standards, and operating principles.
IEEE Standards
In the wireless world, the Institute of Electrical and Electronic Engineers (IEEE) plays a crucial technical role in developing standards upon which wireless solutions can be based. The IEEE has delineated coverage zones for wireless personal area networks (WPANs), wireless local area networks (WLANs), wireless metropolitan area networks (WMANs), and wireless wide area networks (WWANs). Variants arise in each sector when increasing mobility and data-rate demands are factored into the application base. Technologies developed for a particular size network can cross into another. Topological differences, typically in terms of network design and deployment, lead to even more variations. In addition, existing wireless services, such as pagers and cellular telephony, must be accounted for, leading to significant concerns regarding the coexistence and interoperability of deployed systems.
 Figure 1. An overview of the IEEE 802.xx wireless standards shows a wide range of functionality and capability |
The most prominent IEEE standards for wireless data communications fall within the 802.xxx IEEE numbering system. The IEEE 802.11 wireless standards specify an over-the-air interface between a client and a base station or access point, as well as among wireless clients. These standards are comparable to the IEEE 802.3 standard for wired Ethernet LANs. The IEEE 802.11 specifications address both the physical (PHY) and media access control (MAC) layers and are tailored to resolve compatibility issues among manufacturers of wireless LAN equipment.
The IEEE 802.15 Working Group provides standards for low-complexity and low-power wireless connectivity. Today, there are four IEEE 802.15 standards projects in development:
- 1. IEEE Std 802.15.1-2002—1 Mbps WPAN/Bluetooth v1.x derivative work
- 2. P802.15.2—Recommended Practice for Coexistence in Unlicensed Bands
- 3. P802.15.3—20+ Mbps High Rate WPAN for Multimedia and Digital Imaging
- 4. P802.15.3a—110+ Mbps Higher Rate Alternative PHY for 802.15.3
- 5. P802.15.4—200 Kbps maximum for interactive toys, sensors, and automation needs
The IEEE 802.16 specifications support the development of fixed, broadband wireless access systems to enable rapid worldwide deployment of innovative, cost-effective, and interoperable multivendor broadband wireless access products.
In an attempt to achieve functionality across different types of networks, IEEE committees have been working for years to identify key logical elements in the delivery of data. Some of these elements reside in the description and specification of the physical channel; others reside in the control of that channel (e.g., the medium access control layer). In addition, it's frequently necessary to bridge between the two delivery methods, such as when you plug your CAT5 wire into your 802.11 router. The 802 specifications are meant to define how such interoperability can effectively perform. An overview of the 802.xx standards is shown in Figure 1.
Promoters and Supports
Trade groups have been formed to support each of these standards, or at least pieces of the standards. For example, the Wireless Fidelity (WiFi) Alliance is the trade organization affiliated with the IEEE 802.11 standard. The WiFi Alliance—a global, nonprofit industry trade association with more than 200 member companies—is devoted to promoting the growth of WLANs. The alliance's certification programs ensure the interoperability of WLAN products from different manufacturers, with the objective of enhancing the wireless user experience. Since March 2000, more than 2000 products have been WiFi certified, encouraging the use of WiFi across consumer and enterprise markets.
The market success of WiFi components has led to similar alliances or organizations promoting WPANs and WWANs. One such organization is tied to the ZigBee protocol, which can operate across (and control) an 802.15.4 wireless network. The ZigBee Alliance is an association of companies working together to enable reliable, cost-effective, low-power, wirelessly networked monitoring and control products based on an open global standard. In an effort to increase ZigBee's adoption (remember that it's a software protocol), the alliance claims that adopter companies will have a standards-based wireless platform optimized for the unique needs of remote monitoring and control applications, including simplicity, reliability, low cost, and low power. A similar trade group formed in support of Bluetooth, yet another flavor of a WPAN associated with IEEE 802.15.1.
Alternatives
Further subdivisions of network size are occurring with regard to data rate. The 802.15.4 world, with its transport data rate in the 200 Kbps range, is now being referred to as a low-rate, wireless personal-area network, or LR-WPAN. Another wireless technology based on ultra-wideband (UWB) (i.e., short time pulses imply broad frequency range; therefore really short time pulses map into ultra-broad frequency range—based on the time-frequency Fourier transform pair) works over short distances but at (scalable) high data rates, resulting in a high-rate, wireless personal-area network (HR-WPAN). Numerous data transport applications can benefit from this technology, which has led to the inevitable formation of corporate organizations/alliances. Enter the WiMedia Alliance, which represents a combination of WiMedia with the Multiband OFDM Alliance SIG (MBOA-SIG)—the two leading organizations creating UWB industry specifications and certification programs for consumer electronics, mobile, and PC applications.
As mobility requirements increase, you move into the realm of a wireless wide-area network, which is easily represented by a cellular telephony network. In the IEEE data transport sphere, the standard for broadband, large-footprint wireless systems—in terms of data rate—rests with IEEE 802.16. The companion corporate alliance for this standard is WiMAX, the Wireless Microwave Access Alliance.
There are numerous other wireless data transport efforts underway, such as a wireless variant of IEEE 1394 (FireWire), IEEE 1073 (wireless transport of medical information), and a wireless form of the sensor standard IEEE 1451.5. There are associated activities under way worldwide, most in accord with their U.S.-IEEE counterparts.
Frequencies
While it's possible, and in some cases preferable, to use licensed frequencies, most wireless data transport associated with these IEEE standards and associated industry alliances operate in the unlicensed, Industrial, Scientific, and Medical (ISM) frequency bands. Although considerable attention has been paid to the 915 and 2450 MHz bands, the ISM bands have allocations in a number of spectral regions.
Considerable activity is present in the ISM frequency bands in the 5 GHz range. This activity is closely coupled with variants on WiFi, WiMAX, and (future) ZigBee applications. In some cases, questions may arise as to which frequencies in the 5 GHz range are available throughout the world. In the U.S., the applicable frequency bands are 5.15–5.35 GHz and 5.725–5.85 GHz. Operation in the 5 GHz range throughout the Americas and Asia is essentially the same as in the U.S., while European operation occurs in the 5.15–5.35 GHz and 5.47–5.725 GHz bands.
Coexistence, Interoperability, and Interworking
Given all the varieties of wireless, each backed by corporate/marketing alliances and thousands of products, is there any assurance that all of these devices can coexist in the same physical footprint and frequency space? Stated another way, if you deploy a wireless monitoring system operating at 2.4 GHz in your facility and the IT department comes along later and deploys a 2.4 GHz WiFi network, followed by the security department installing 2.4 GHz wireless surveillance cameras and the telephony crew 2.4 GHz cordless telephones, is all this going to work at the same time?
Companion questions relating to interoperability and interworking of deployed systems logically follow hot on the heels of the coexistence question. In recognition of this, at least two IEEE standards groups are trying to address the coexistence issue, namely P802.15, Recommended Practice for Coexistence in Unlicensed Bands, and 802.19, Coexistence of Wireless Data Transport. In logical fashion, the 802.15.2 group is assigned the task of enabling coexistence within the WPAN world, and 802.19's charge encompasses (and acknowledges) essentially all varieties of wireless transport.
It's at this point that wireless standards have to be reconciled with industrial implementation activities. For example, does an industrial system accept data from wireless sensors if sensor networks abide (or not) by differing standards? It's time to move from the academic world to the realistic deployment world represented by the industrial sector. Enter the Instrumentation, Systems, and Automation Society.
To clarify the situation, ISA's SP100 Committee, which is charged with delivering functional wireless technology to the industrial sector, has defined the following key terms:
- 1. Coexistence is the ability of one system to perform a task in an environment where there are other systems that may or may not be using a similar set of rules.
- 2. Interoperability is the ability of two systems to perform a given task using a single set of rules.
- 3. Interworking is the ability of two systems to perform a task where each system implements a different set of rules.
Standards and Protocols for Industrial Settings
Industrial networks operate in considerably more complicated settings than their conventional IT counterparts because of the critical nature of certain operations (e.g., close the valve in 5 ms or else an explosion occurs). In addition, complexities may arise with deployments in which IT traffic and industrial monitoring and control traffic share the same physical infrastructure, each with their own performance requirements. In the realm of the plant network, devices, switches, controllers, actuators, and computers are all connected with operational software layered throughout the deployment. The needs at each level differ, as does the connectivity between layers.
These conditions led to the industrial bus wars at the end of the twentieth century (vestiges of the wars certainly exist today), when more than 120 different protocols, some physical, some logical (data type), were developed and deployed. The question is now: Is the wireless industrial sector about to enter a similar period of frenetic activity, or will a better understanding of past experience and current technology smooth the transition to wireless networking?
Want More?
For information on SP100, contact Lois Ferson, ISA, at [email protected] or go to www.isa.org/community/sp100.
The Wireless Industrial Networking Alliance (WINA) promotes the adoption of wireless networking in industrial settings by providing neutral technical information about the various wireless options. For more information go to www.wina.org.
Peter Fuhr, PhD , can be reached at Apprion Inc., Moffett Field, CA; 650-934-3900, [email protected], www.apprion.com .
Hesh Kagan, MS , can be reached at Invensys Process Systems, Foxboro, MA; 508-549-2782, [email protected], www.invensys.com.