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Industrial Ethernet Book Issue 72 / 47
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Network media: economic and technical selection issues

For any designer of an industrial Ethernet installation, there is one important item to be included on what will certainly be a demanding project agenda: the data transport infrastructure. Les Hunt offers an independent view on design pointers for media selection and installation, including an appraisal of their technical and fiscal advantages and disadvantages.

AN INDUSTRIAL ETHERNET data transport medium will need to be selected, based on a raft of considerations, both technical and economic. It will be a long term investment, so 'futureproofing', in the context of a plant's life expectancy and future production targets, must be part of the risk assessment that will be necessary at an early stage in the media selection process. There are numerous issues that will determine the ultimate choice of data transport medium – be it copper, optical fibre, wireless, laser pointtopoint or combinations of these. However, for the purpose of this article, only those aspects of the design and selection criteria that are likely to have the greater impact on performance, reliability and safety will be addressed.

Media choice notwithstanding, it is always important to apply best practice in terms of design, installation, commissioning and troubleshooting, all of which are amply covered by the appropriate international standards to which references are made in this article. These standards will help you in your choice and be your ultimate guide to proper installation procedures, once the selection has been made.

Media choice notwithstanding, it is always important to apply best practice in terms of design, installation, commissioning and troubleshooting, all of which are amply covered by the appropriate international standards to which references are made in this article. These standards will help you in your choice and be your ultimate guide to proper installation procedures, once the selection has been made.


A typical industrial application featuring robust industrial Ethernet networking.Fibre or copper,environmental conditions will dictate the level of protection required by the installation

Back to basics

Industry standards, including ISO/IEC 11801 and EN 50173 define the requirements for data cabling and the structured cabling systems used to connect control devices to the network. Clearly, important decisions have to be made at an early stage in the design to choose the correct network medium and thus avoid future operational problems. ABB industrial Systems engineer, Nick Jones says that in the interests of future proofing an industrial Ethernet cabling system – particularly for industrial sites where the design life will, at the very least, be some ten years or more – it is important to start with the fundamentals.

Standards referred to in this article

Commercial premises:

ISO/IEC 11801     International standard for structured IT cabling
BS/EN   50173     European and British IT cabling standard

Industrial premises:

ISO/IEC 24702     International standard. Provides reference to MICE cable protection criteria
BS/EN 501733     European and British generic IT cabling standard
IEC 62012            Multicore IT cables in harsh environments
IEC 6007928        Use of fibre optic cables in explosive atmospheres
IEC 611582          (ISA/SP50) Fieldbus physical layer (copper and fibre) specification

Copper, now commonly installed in Cat 6 (or higher) formats, is frequently deployed in areas where there are low levels of electromagnetic interference (EMI). Local equipment rooms (LERs) containing servers, workstations and PLC interfaces are typical applications. But when the network extends beyond these confines, such as between LERs, the distances spanned between links, the levels of EMI the cabling may be exposed to and the ground potential between facilities where galvanic isolation may be desired, have to be considered, as they will have a significant impact on the operation of a copper network.

It is at this point, says Mr Jones, that decisions are likely to be made whether to use copper or fibre media. Fibre optics removes many of the negative aspects of copper cabling. It offers greater range than copper, it is inherently immune to EMI, it permits galvanic isolation between facilities that are at different ground potentials and it is becoming more cost competitive, given the significant rise in copper prices of recent years.

Yet the fact remains, copper is currently the most widely adopted media type for industrial Ethernet networks, according to Amplicon's data communications product specialist, Michael Whitehead. And while fibre certainly has the edge when it comes to noise immunity, the use of differential signalling with twisted wire pair configurations, while not removing it altogether, greatly reduces copper's EMI vulnerability.

Moreover, copper is still viable for large industrial Ethernet backbones operating at 10Gbps networks speeds, and copper cable can also deliver power (IEEE 802.3af/at), making it extremely useful for deploying equipment in 'hard-to-reach' areas. Power over fibre is possible, but exceedingly expensive!

Michael Whitehead also points out that the 100m range restriction of copper can be overcome by the use of Ethernet extenders, an innovative communications tool that allows transmission over legacy twisted pair or coaxial cable at distances of up to 3km. These are essentially modems that can be interfaced with Ethernet networks; the Ethernet data is modulated on an FR carrier, which is suitable for efficient transmission over legacy cable, and converted back again by a receiving modem (similar to Ethernet bridging). The trade–off when using this technique is a reduction in data rate with distance.

Michael Whitehead also points out that the 100m range restriction of copper can be overcome by the use of Ethernet extenders, an innovative communications tool that allows transmission over legacy twisted pair or coaxial cable at distances of up to 3km. These are essentially modems that can be interfaced with Ethernet networks; the Ethernet data is modulated on an FR carrier, which is suitable for efficient transmission over legacy cable, and converted back again by a receiving modem (similar to Ethernet bridging). The tradeoff when using this technique is a reduction in data rate with distance.


The outer cable armour matters in the presence of acidic atmosphere and UV  radiation:Ethernet cables like this,including abrasion–and chemical–resistant outer insulation and braided shielding,are design for harsh industrial environments.This example is a new Cat5e offering from Alpha Wire's Xtra–Guard range
[www.alphawire.com/XGIE]

What am I automating?

Fibre may well be the medium of choice for many designers because it is perceived to offer a high degree of future–proofing – its huge bandwidth promising support for any future hardware upgrade. However, as a first step, it is important to consider what is being automated and what devices are to be supported in the field, prior to making this assumption.

A continuous or batch processing operation will have very different requirements to those of a fast moving manufacturing operation. Events are likely to be measured in seconds or even minutes for the former and milliseconds, possibly microseconds, for the latter. What devices are to be deployed, and what media do these devices support? A process analyser is one thing – a precision servo drive quite another! The devices and the type of automation environment will, to a large extent, define the bandwidth requirement and thus the connection technology.

The speed of response of the network and the distances involved will pose questions for the designer in making media choices. As a rule, if devices (servo drives, controllers, I/O interfaces, etc) are assembled close to one another and EMI issues are not significant, then copper would adequately cover the network requirements. However, if these assemblies are to be linked or synchronised with other, distant automation groups across a widely dispersed facility, then fibre must be considered as the appropriate medium for this task. This mixed media topology is not uncommon and while it may seem a compromise, it is both a technically and financially competent solution.

Further considerations

When selecting a media type and topology, apart from those characteristics already mentioned, the designer must take further considerations into account. Determine whether the application is fixed or moving, the environmental conditions are harsh or possibly hazardous; take into account the number and type of connections and whether or not redundancy is mandated.

Among his twelve basics for reliable networks, Professor Karl Heinz Niemann (the author responsible for the preparation of planning, installation and commissioning guidelines for Profinet and Profibus) places the selection of cables and connectors high on this list. Primarily, these must match the demands of the environmental conditions on the plant floor, he says, but where media selection is concerned, the wide variation in connection methods and their relative costs, should not be ignored.

There are, of course, different types of fibre available to the network designer – multi–mode, singlemode, silica and plastic – and the choice will be dictated by such factors as distance and installation limitations. Multimode fibres have a high transmission rate capacity over relatively short (less than 1km) distances and their interconnection and associated electronics are much simpler compared with the specialist requirements of singlemode fibre installations, which offer much greater transmission distance (many kilometres). Plastic fibre optic cables provide a good, relatively easytoconnect alternative for relatively short transmission distances.


An example of a mixed media installation using CC–Link IE,including CC–Link IE Control: 1000base–SX fibre (1Gbit/s); 'Field'–Cat5E copper (1Gbit/s), and (currently in the planning stages) wireless.   [CLPAEurope]

Media cost vs. installation cost

Harting's Gavin Stoppel offers some interesting comparisons for those about to set a budget for their installations. The cost of good–quality Cat 6a 8–core 10Gbit/s Ethernet cable, he says, can add up to US$6 per metre, excluding termination.And for termination, users need to allow four to five minutes of an unskilled operator's time, plus the cost of two rugged RJ45 connections for each end, he adds.

There are, however, quick–connect methods available for copper, the Variosub RJ – 45 connector from Phoenix Contact being one example for Cat 5e 8–core installation, which also maintains continuity of shielding.

For a fibre network, the cost of a goodquality multimode OM3 tight–buffered 4–core cable can start at US$1.20 per metre, excluding termination. However, Mr Stoppel warns that it can take up to an hour of a highly skilled operative's time to terminate and install the fibre connections to the device.

Another useful tip offered by Mr Stoppel is that the cost of implementing a fibre network works out the opposite way round to copper – that is, fibre is more expensive over short distances and very cost effective over longer distances. In addition, fibreoptic cables can take up less space and can be easier to install if treated correctly.

But whether it is fibre or copper, environmental conditions will dictate the level of protection required by the installation. The industry has devised an approach to cable environmental protection, dubbed 'MICE', an acronym of 'Mechanical', 'Ingress', 'Climatic' and 'Electromagnetic', each graded from one to three according to the intensity level. Hence M3I3C3E3 would represent a heavy industrial application and a cable thus specified would need to be fully protected with adequate armouring, shielding and ingress protection – extending also to the connection method. Cable environmental considerations are covered by a number of standards, including ISO/IEC 24702/EN501733 and IEC 62012 (cables installed in harsh environments).


A mixed media topology incorporating a fibre based backbone ring network comprising Moxa industrial Ethemet managed switches, with a copper extension using Patton 2174 Ethemet extenders capable of extending a copper link up to 3km to a remote Moxa RTU controller.
[www.amplicon.com]

Hazardous area applications

There are instances where fibre installation – for all its advantages – must be viewed with caution. Light radiation from connectors, for example, is an issue for hazardous area configurations; more esoterically, ionising radiation and highenergy particles will affect the performance of fibre based networks. CERN, for one, has carried out extensive work in this area, as it relies implicitly on high–speed Ethernet links to transport huge quantities of data between experimental stations.


Industrial Ethernet installations generally use toughened connectors, allowing installation in adverse environments, including temperature extremes, high humidity and conditions of high shock and vibration. These Han 'PushPull' cabletocable housings from Harting enable flexible IP 65/67 protected cabling configurations for industrial Ethernet, achieving a bandwidth of up to 10 Gbit/s with a reliable AWG 2227 conductor cross section.     [www.harting.com]

So, why may light radiating from a fibre connector pose a hazard in an environment where explosive mixtures of gases may be present? Light absorption by surfaces or particles may cause localised heat build–up; the optical wavelength of the light transmitted via the fibre may match the absorption band of the gas; there may be photo–chemical ignition due to photo–disassociation of oxygen molecules by radiation in the UV spectrum; laser induced breakdown of the gas at the focus of a strong beam producing plasma and a shock wave – both eventually acting as the ignition source – these are issues that must be addressed.

Copper Ethernet cable categories

Cat 5/5e    Supports 100Mbps (possibly 1000Mbps over short distance); 5e offers improved crosstalk performance.
Cat 6         Supports 1000Mbps, possibly 10Gbps over short distances
Cat 6a       Supports up to 10Gbps over distances of up to 100m
Cat 7/7a    Socalled 'Class F' cables are new and relatively untried. Are likely to support 100Gbps

For guidance on this topic – avoiding light radiation leakage, choosing 'inherently safe' optical frequencies, fibre break detection and the like – readers might wish to refer to the Fieldbus Intrinsically Safe Concept (FISCO) standard IEC 60079 – 28, which deals with optical devices deployed in hazardous areas, including optical fibre installations. The physical layer standard IEC 61158 – 2 provides additional guidance for fieldbus networks serving hazardous process manufacturing operations.

Wireless media

Wireless has a considerable role to play in modern plant and factory automation, but, like all other media, it does have certain limitations. Wireless cannot be considered truly deterministic for certain automation applications, particularly those requiring fast, dynamic response such as motion control where the cycle times may be as short as 100¦Ìs.

Wireless is perhaps more widely appreciated by the process manufacturing sector, where large sites see physical media installation costs soar. Foundation Fieldbus (FF) is currently developing profiles for operation of its High Speed Ethernet (HSE) over various wireless backbone technologies such as WiFi, WiMax, cellular, satellite, and so on, using the architecture model. These profiles will be the subject of FF's Remote Operations Management enduser demonstration programme, which is to be rolled out over the next two years. The wireless backbone architecture model supporting FF HSE has now been approved by the ISA100.15 working group.

Meanwhile, back in the factory environment, B&R Automation's Stephane Potier says wireless might be feasible for cycle times as short as 100ms, but latency issues can arise as a result of difficult plant layouts causing collisions and subsequent transmission delays. Where it is permissible, however, he suggests that laser pointtopoint systems might provide a very robust alternative to wireless – as well as providing big savings in media installation costs.

The installation of a rugged network in an industrial environment can be a little daunting. However, by carefully examining the different media available and taking a step–by–step approach, users will be able to achieve an optimum solution that will also allow for future extensions and production demands.

Les Hunt is Editor of the UK-based magazine, Design Products & Applications


Source: Industrial Ethernet Book Issue 72 / 47
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