Optimize signal clarity in an industrial network

Physical layer issues account for more than half of network installation problems—noise, ground loops, signal reflection, and cross-talk among them. Improve signal clarity with this advice.


OSI Layer One (the physical layer) issues comprise more than 50% of reported problems in network installations, emphasizing that signal clarity is an important consideration when planning a network infrastructure of an industrial application. Twisted-pair cabling is a staple in industrial network infrastructure. More flexible and less expensive than coaxial cable, Cat5e and Cat6 cables are recommended for many installations; therefore, the structure of the twisted-pair cable is critical to ensure high-quality transmission. 

Differential Ethernet

How does differential Ethernet work? Cat5e and Cat6 cables transmit signals using differential mode signals, which are opposite polarity and equal amplitude from each wire in the twisted pair. At the receive end of the channel, the equal but opposite polarity signals are evaluated for the "difference" potential. If we had a 2 V signal, the difference in magnitude between the signals would be 4 V. Noise acts as a common mode signal when it is coupled into the balanced pair channel. It is "common" to both wires and is the same amplitude and phase. This interference signal does not propagate separately from the data signal but is integrated into the overall voltage signal to become a composite waveform.

The beauty of differential mode transmission is that it elegantly removes the noise from the source signal when the "difference" between the two wires is calculated. (See diagram.) With a 4 V differential signal (-2 V to +2 V) and a 1 V common mode disturber (+1 V on each wire), the voltage magnitude differential between the two wires is still 4 V (3 V to -1 V). In a perfectly balanced cabling system, the induced common mode signal would appear as two equal voltages that are simply subtracted out by the transceiver, thereby resulting in perfect noise immunity.

Differential mode transmission removes the noise from the source signal when the

In reality twisted-pair cables are not perfectly balanced, and the TIA (Telecommunications Industrial Association) has limits on cable specification intended to make sure at least a base level of twisted-pair balance is maintained. DCR unbalance, capacitance unbalance (CUB), and transverse conversion loss (TCL) are examples of specifications designed to ensure pair balance. 

Balancing twisted pairs

Balancing twisted pairs involves numerous strategies, and the better balanced the twisted-pair cabling, the more reliable the signals. Some of the most common challenges when using twisted-pair cabling are:

  • Susceptibility to noise: Twisted pairs tend to separate due to movement during installation, flexing, or handling. Each pair can be pictured as an antenna that can receive or transmit signals; thus variations in conductor-to-conductor spacing are cumulative and result in susceptibility to EMI and RFI that degrades signal transmission and network performance.
  • Signal reflections: When twisted pairs separate, they create impedance irregularities that can cause signal reflections (return loss). Impedance variations are also cumulative.
  • Pair-to-pair crosstalk: All twisted-pair Ethernet cables have crosstalk or pair-to-pair coupling, which is caused because each pair has different twists/in. (lay length). Lay length variation can increase the crosstalk that is cumulative down the length of the cable.
  • Connector issues: Twisted pairs can separate inside an improperly terminated connector, which can lead to lack of signal integrity. Tracing degradation inside connectors is a time-consuming and expensive process.
  • Lack of mechanical robustness: Like other cable types, twisted pairs are subject to degradation due to stretching and flexing during installation. When tension is applied with unequal force from one conductor to another, issues can occur.

Noise includes ground loops

If a twisted pair is not perfectly balanced, modal conversion of balanced to unbalanced signals is going to occur at RF frequencies. Differential mode signals at 20-30 MHz or higher can convert to common mode signals and vice-versa. The conversion artifacts adversely impact noise immunity from the environment as well as contribute to crosstalk between pairs and between other balanced cables. Only highly balanced twisted pairs can mitigate the modal conversion artifacts.

Signal phase in a balanced pair also is important because each signal must arrive at the end of the pair in proper phase. Both wires must be the same electrical length so the differential process acts on the "equal" but opposite signal.

Three types of noise can be coupled onto twisted-pair cabling: differential, environmental, and ground loop. Differential noise, that is, noise from nearby pairs in a cable, is called NEXT (internal to the cable), while noise from nearby cables is called ANEXT (from adjacent cables). Environmental noise is capacitive or inductively coupled to balanced pairs from external electromagnetic fields from disturbers such as electric motor noise, fluorescent light ballasts, and radio-frequency (RF) sources (in increasing order of severity). Ground loop noise is induced by a difference in potential between conductor ends or physical ground point locations in a building or between buildings. 


Shielding can decrease the potential for modal conversion by limiting noise coupled onto the twisted pair from the environment. Shielding acts as a noise attenuator so that disturber signals are as small as possible before they impact the twisted pairs underneath the shield. A shield cannot remove noise; it can only attenuate noise. Balance twisted pairs utilizing differential signals remove noise. The more perfectly balanced they are, the more noise the balanced pairs can remove. The greater the noise source, the more critical it becomes to have good pair balances. 

Bonded-pair technology

Bonded-pair technology can alleviate many of the challenges of twisted-pair cable, exhibiting consistencies that traditionally have only been possible with coax and twin-lead designs. When conductors are bonded (that is, adjoined along their longitudinal axis), they can provide uniform conductor-to-conductor spacing, uniform twisting of insulated conductors into pairs, and a robustness that assures that the twists of the pair will not loosen up or separate during manufacturing or installation. 

Don’t cut corners

Twisted-pair cabling provides multiple benefits in an industrial networking application. However, cutting corners in cable quality can lead to downtime and costly maintenance. It is important to choose balanced-pair data cabling that can protect against noise, signal reflections, crosstalk, and other variables that can impact signal clarity.

Gareis is principal product engineer, Belden Inc. Edited by Mark T. Hoske, content manager, Control Engineering, CFE Media.


Belden offers a cable finder guide:




No comments
The Top Plant program honors outstanding manufacturing facilities in North America. View the 2015 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
Safety for 18 years, warehouse maintenance tips, Ethernet and the IIoT, GAMS 2016 recap
2016 Engineering Leaders Under 40; Future vision: Where is manufacturing headed?; Electrical distribution, redefined
Strategic outsourcing delivers efficiency; Sleeve bearing clearance; Causes of water hammer; Improve air quality; Maintenance safety; GAMS preview
SCADA at the junction, Managing risk through maintenance, Moving at the speed of data
Safety at every angle, Big Data's impact on operations, bridging the skills gap
The digital oilfield: Utilizing Big Data can yield big savings; Virtualization a real solution; Tracking SIS performance
Applying network redundancy; Overcoming loop tuning challenges; PID control and networks
Driving motor efficiency; Preventing arc flash in mission critical facilities; Integrating alternative power and existing electrical systems
Package boilers; Natural gas infrared heating; Thermal treasure; Standby generation; Natural gas supports green efforts

Annual Salary Survey

Before the calendar turned, 2016 already had the makings of a pivotal year for manufacturing, and for the world.

There were the big events for the year, including the United States as Partner Country at Hannover Messe in April and the 2016 International Manufacturing Technology Show in Chicago in September. There's also the matter of the U.S. presidential elections in November, which promise to shape policy in manufacturing for years to come.

But the year started with global economic turmoil, as a slowdown in Chinese manufacturing triggered a worldwide stock hiccup that sent values plummeting. The continued plunge in world oil prices has resulted in a slowdown in exploration and, by extension, the manufacture of exploration equipment.

Read more: 2015 Salary Survey

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.
This article collection contains several articles on the vital role of plant safety and offers advice on best practices.
This article collection contains several articles on the Industrial Internet of Things (IIoT) and how it is transforming manufacturing.
This article collection contains several articles on strategic maintenance and understanding all the parts of your plant.
click me