Continuing our look back at content we've created since 2002, here's a mesh networks piece as it appeared in Q3 2004. Since Ian is departing Industrial Networking as a regular contributor, we chose one of his. Note the companies in the table that are gone or have been swallowed up since then.
Wireless networking and associated technology certainly has changed the way people work and interact with each other. WiFi networks, cellular phones and national personal radio networks are examples of this explosion during the past five years. Wireless technology also is moving into the industrial environment as well, getting significant buzz in industry and trade journals.
Of course, wireless is not entirely new to industry, since licensed band radios have been in use for years in SCADA systems and for plant operator radios. What is new is the increasing adoption of ISM (industrial, scientific, medical) or unlicensed bands in an industrial setting.
|MESH TECHNOLOGY PROVIDERS|
|Elpro Technologies (www.elpro.com)|
|Ember Technologies (www.ember.com)|
|Kiyon Inc. (www.kiyon.com)|
|Moteran Networks (www.moteran.com)|
|Nortel Networks (www.nortel.com)|
These installations can range from simple single-point readings to multiple-transmitter installations to complete networks. Similarly, the type of network or connection can be point-to-point (master/slave) or mesh (peer-to-peer) communications.
Traditional installation practices are point-to-point or point-to-multipoint (i.e., master/slave) transmissions that are restricted to a rigid structure with a maximum of two "hops," device-access point-device and are also more susceptible to signal loss in the event a large object such as a crane moves into a signal path. The point-to-multipoint case also has the risk of data flow restrictions through the access point, since this point also is the network master.
All access points should be OSI Layer 3 switch-capable so they can detect and handle priority messages in the switch buffer.
One option to remedy the point-to-point system's problems is to use a mesh network. These networks differ in that each device is "aware" of the other devices within its broadcast range. Therefore, much like the Internet, if one path from point A to point B is out of service, at least one alternate path remains available. In effect, the network is self-healing. Similarly, if the signal between devices is too weak, installation of another device along the path of the low signal strength can remedy the situation.
Additional considerations for wireless networks in industrial settings that require consideration are:
Security — Because ISM wireless signals are typically broadcast omni-directionally, and also are used by other commercial-off-the-shelf technologies, it is possible they can be captured and potentially altered by outside agents.
Bandwidth — Protocols such as TCP are optimized for networks that drop 0.1% of all packets, while radio networks can drop as much as 5-10% of packets sent.
Distance — Outdoors, line-of-site transmission signals drop off in proportion to the square of the path distance. Because of factors such as polarization and destructive interference, indoor signals can decrease following a cube law (or higher) function thus restricting signal distance to as little as 15-30 m.
Data Rate — To insure timely receipt of messages and minimize collisions, the effective data throughput in a wireless network should be kept to 30% of the maximum attainable radio data rate.
Mesh technology overcomes many of the limitations of traditional master/slave networks while its peer-to-peer communications ability will enable new innovative sensing technologies such as "motes" and "swarms."
Mesh technology is not the magic arrow of network or system design but it certainly is another important addition to our quiver of possible solutions.
The Instrumentation Systems and Automation Society's (ISA) Standards & Practices department is having a meeting on the use of wireless technology in automation and control environments during this month's ISA 2004 conference and exhibition. This session is open to all parties interested in working identifying and working on standards to insure reliable wireless communications in industrial settings. There are a number of prominent mesh technology providers. For a partial list see the accompanying table.