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EMAIL INTERVIEW George Thomas & Ken Sinclair
George Thomas is president and founder of Contemporary Controls which designs and manufactures industrial communication equipment for the industrial and building automation markets. Contemporary Controls' product line includes CTRLinkÔ Industrial Ethernet hubs, switches and media converters and ARC Control products for ARCNET hubs, links and repeaters. Corporate headquarters are in the western suburb of Chicago, Downers Grove, Illinois. Additional offices are located in the United Kingdom, Germany and China.
Extending Building Automation Networks
Sinclair: What are the issues with extending the length of building automation networks?
Thomas: You need to understand the rules for the technology you support. For example, the BACnet standard allows the use of five different data link technologies, each with their own rules for network expansion. Our company addresses two of those technologies: ARCNET and Ethernet and the expansion rules are different for each. ARCNET operates as a token-passing network and requires that all stations can respond to any one request within a particular time frame. Although hubs can be used to extend the network diameter, there is a limit before extended timeouts must be invoked on all the stations; otherwise, continuous reconfigurations could occur. Ethernet is a contention-based network and in order for all stations to realize that a contention has occurred, its network diameter is limited as well. If you exceed the limit, this could cause late collisions resulting in unreliable operation or an overall decrease in speed.
Sinclair: Since Ethernet is gaining in popularity, what are the rules for Ethernet?
Thomas: For Ethernet it is not a short answer. The IEEE has established a simple rule called the 5 - 4 - 3 rule but it is very conservative, and it does not address all configurations and only applies to 10 Mbps operation. The rule states there can only be a maximum of five segments, four repeater units and only three of the segments can be mixing segments. A repeating unit is either a repeating hub or repeater and a mixing segment is a bus segment such as 10BASE2 or 10BASE5. Mixing segments are no longer popular and have been replaced with either twisted-pair or fiber optic links. A twisted-pair segment can only be up to 100 meters in length so with the above rule you can have a total of five 100 meter segments for a total network length of 500 meters. A fiber optic segment can be up to 2000 meters in length, but you are not allowed to cascade five segments. However, you could have one 2000 meter fiber optic segment, two 100 meter twisted-pair segments and a total of three repeating hubs for a total network diameter of 2200 meters. The limitations at 100 Mbps are quite severe with network diameters restricted to 412 meters for fiber optics and only 205 meters for twisted-pair cabling.
Sinclair: Buildings are longer than 205 meters so how do you get around the problem?
Thomas: You need to migrate to switch technology. With switch technology, the repeating hub is replaced by a switching hub. A repeating hub is a physical layer device while a switch operates at the data link layer and is technically a bridge. With a bridge, the segments that are attached to bridge ports are each considered subnets of a much larger network. Each switch port on a switching hub functions just like an end station on an Ethernet segment; therefore, it terminates the ends of the subnet. This means that the rules for governing a network's length are only restricted to the subnet's length. By adding a switch, the effective length of the network doubles. Although the segment length restriction is still 100 meters for twisted-pair wiring, switches can be cascaded beyond the four limit in the 5 - 4 - 3 rule even at 100 Mbps. The other advantage of switch technology is that the cascading rules are so much simpler and the possibility of very long fiber optic runs exist.
Sinclair: How do you accommodate long fiber optic runs between buildings on a large campus?
Thomas: By going with switch technology, your switch can be configured for full-duplex operation. Normally Ethernet operates in half-duplex mode so that all stations on the network can detect if a collision has occurred. There are no collisions with full-duplex operation since there are separate receive and transmit paths. Therefore, the collision logic can be disabled and that is what limits distance. A single-mode fiber optic link at 100 Mbps can be as long as 15 km, but this is only possible if the link is configured as a full-duplex link.
Sinclair: There certainly are issues when extending building automation networks. How can our readers become more informed about this subject?
Thomas: For those people interested in Ethernet, our company has written a guide entitled "10 Issues to Consider Before Installing Industrial Ethernet" and it is available for free by requesting the guide from our web site http://www.ccontrols.com. Our other web site http://www.ctrlink.com has other documents describing Industrial Ethernet. For those people interested in ARCNET, our ARCNET Tutorial and Product Guide is available as well from http://www.arcnet.com.
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