01 April 2003
Two steps forward, one step back
Customers investigate pros and cons of wireless networking
By Matthew Lamoreaux
Vendors providing complete system solutions for decades occupied the realm of networking and communications. So choices were limited for the distributed control system or programmable logic controller (PLC) user—primarily to the network offered with the package.
Such packaging was expensive and inflexible yet dependable. Beginning with the fieldbus wars of the 1990s and followed by the popularization of Ethernet as an industrial networking alternative, open choices abounded, along with concerns over interoperability, determinism, safety, and security. Now wireless, the latest technology emerging on the industrial networking scene, takes the advantages and disadvantages of networking choices to new extremes.
Not just less wire
One of the most flaunted advantages of digital fieldbus technology has always been less wiring cost. And while this may be technically true, users say many digital fieldbus installations have not proved to offer significant total cost savings over traditional analog instrument wiring. With this reality, fieldbus promoters began to focus more on the functional advantages of digital communications, such as reduced maintenance costs and process control performance enhancements.
Now, with the advent of industrial wireless technology, less wire takes on an entirely new meaning. And the old fieldbus selling line again leads the charge. In this case, wire savings is undeniable. And on the surface, overall costs appear to be much cheaper. But with wireless technology comes an array of new issues.
Proprietary . . . again?
Ironically, after more than a decade of push toward open systems, industrial wireless technology remains largely proprietary and, when deployed on a large scale (rather than point-to-point links over relatively short distances), requires technical expertise that differs significantly from that of analog or even digital wired networks.
But isn't IEEE 802.11 an open standard? IEEE 802.11 wireless Ethernet actually refers to three different variations known as 802.11a, 802.11b, and 802.11g, said Tim Cutler of Cirronet Inc. This standard promotes interoperability among vendors' office LAN products to foster competition and ultimately result in lower costs.
While the goals of this standard are certainly worthy, progress in achieving interoperability has been limited, said Cutler. IEEE 802.3 is the standard that defines wired 10Base-T Ethernet (802.3u defines the 100Base-T standard). Wireless devices that are 802.11 compliant are actually 802.3 compliant on the connection to the network and 802.11 compliant in the over-the-air protocol.
Most industrial wireless suppliers are quick to point out that while a wireless Ethernet device must be 802.3 compliant to connect to a wired Ethernet network, it does not need to be 802.11—and in most cases, it's not. So when you want to add nodes to your industrial wireless network, you won't have to worry about running wire, but you will likely have only one supplier for those essential wireless transceivers.
Control vs. data networks
"To understand why standard wireless networks do not work well for industrial applications, it is important to distinguish between control and sensing networks vs. data networks," said Robert Poor and Brent Hodges of Boston-based Ember Corp. in a technical paper, "Reliable Wireless Networks for Industrial Applications," presented at ISA 2002 in Chicago. Wireless data networks are designed to link computers, personal digital assistants, printers, and Internet access points, where large amounts of data are sent in both directions.
In data networks, the emphasis is on speed: Faster is better. For example, IEEE 802.11b connects devices at up to 11 megabits per second (Mbps). One of the latest updates to this standard, 802.11a, will allow for data speeds up to 54 Mbps, enabling consumers to more rapidly download music and video files.
Because industrial sensors send only a few bits of data per second or minute, providing information such as temperature, pressure, and flow, data rates of 11 Mbps or even 54 Mbps are a rarity. But wireless networks for industrial control and sensing, above all, must be reliable.
Ironically, the faster data rates produce a corresponding reduction in receive sensitivity. Receive sensitivity refers to the signal strength necessary to receive data at a given bit error rate. So to get the signal strength necessary for the range most users look for in an industrial environment, the data rates must slow down significantly.
Several factors influence reliability of radio systems in industrial environments. And while we can spend a lot of time analyzing them, the key to dealing with the reliability and scalability issues is topology.
After years of debate over the best topology for wired industrial networks, much of which revolved around wiring issues, wireless might seem on the surface the end to the topology debate. But in fact, topology seems to be even more important in a wireless environment. Here's why:
Point to point. Sometimes referred to as a wireless bridge, a point-to-point link serves as a replacement for a single communication cable. A point-to-point link might connect a PLC to a remote monitoring station. Point-to-point links can communicate reliably as long as the two endpoints are located sufficiently close to each other to escape the effects of radio frequency (RF) interference and path loss. If you don't initially receive a reliable connection, you can often boost the transmit power. But you can boost your transmit power only when focusing on a single target or receiver. When you read about license-free radios with impressive up-to distances, you'll find the maximum distance typically refers to its use in a point-to-point topology.
Point to multipoint. Point-to-multipoint wireless systems, such as those based on IEEE 802.11 or Bluetooth, have one base station or access point, which controls communication with all other wireless nodes in the network. Also referred to as a hub and spoke or star topology, this architecture has similarities to wired home-run systems, in which all the signals converge on a single terminal block.Signals in point-to-multipoint networks converge at a single endpoint. Set the reliability of these networks by the quality of the RF link between the central access point and each endpoint. Industrial wireless suppliers agree it can be difficult to find a location for an access point providing dependable communication with each endpoint. Moving an access point to improve communication with one endpoint will often degrade communication with other endpoints. While it may be possible to wire together multiple access points in order to improve reliability, the cost of additional wiring can defeat the original reasons for choosing a wireless solution.
Peer to peer. Although point-to-point and point-to-multipoint systems can be useful to overcome expensive wiring problems in certain problem spots, they generally do not provide a suitable alternative to a wired industrial network. The most advanced (and most proprietary) industrial wireless systems, which in some situations provide an alternative to a wired network, offer a peer-to-peer or mesh topology in which each node not only sends and receives messages but also functions as a router, relaying messages for its neighbors. Through this relaying process, a packet of wireless data finds its way to its ultimate destination, passing through intermediate nodes with reliable communication links.
Peer-to-peer wireless network topologies offer several advantages to overcome the inherent limitations of RF systems. Because the distance between wireless nodes is generally short, link quality improves between nodes. If you reduce distance by a factor of two, the resulting signal is at least four times more powerful at the receiver. This makes links more reliable without increasing transmitter power in the individual nodes.
In an Ember Corp. system, you can add freestanding repeater nodes in the middle of the network to extend distance, add redundancy, and improve the general reliability of the network. Elpro Technologies in Stafford, Queensland, Australia, offers a similar system.
Scalability and redundancy
The actual meaning of redundancy is a matter of degree requiring careful specifications, according to Poor and Hodges. In a peer-to-peer network, the degree of redundancy is essentially a function of node density. You can deliberately overdesign a peer-to-peer network simply by adding extra nodes so that each device has two or more paths for sending data. This is a much simpler way of obtaining redundancy than is possible in most other types of systems.
Peer-to-peer networks offer the greatest degree of scalability. Because the operation of the network does not depend on a central control point, it's convenient to add multiple data collection points or gateways to other networks. Point-to-point networks can provide reliability but don't scale to handle more than one pair of endpoints. Point-to-multipoint networks can handle more endpoints, but the placement of the access point and endpoints determines reliability.
If environmental conditions result in poor reliability, it's difficult or impossible to adapt a point-to-multipoint network to increase the reliability. By contrast, peer-to-peer networks tend to be more reliable, adapt easily to environmental or architectural constraints, and scale to handle thousands of endpoints.
Most wireless networks are not suitable for closed-loop control applications, and with the ongoing security and safety issues inherent in wireless communication, critical control may never come to fruition via wireless systems.
To think of wireless industrial networks as cheaper, easier alternatives to wired networks perhaps misses the greatest advantage of wireless technology. Wireless allows users to establish digital communications in places where wired networks would never be economically feasible. The reliability and functionality of wireless networks is improving, and as the installed base of wireless equipment grows, the cost of both the equipment and its installation and maintenance should continue to come down.
Unfortunately, the biggest negative to wireless systems will likely not be the limitations of the technology but the lack of interoperability among wireless equipment from different suppliers. P
Behind the byline
Matthew Lamoreaux is a freelance writer who covers the instrumentation and automation market. Contact him at email@example.com.
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