Posted by: Ron Tellas on May 19, 2017
When dealing with subpar cable and patch cords, it can be frustrating to discover what can cause dropped links – and ultimately downtime and business interruption. When cables aren’t constructed well, performance can be impacted by movement, such as being knocked or bumped, or even frequent moves, adds and changes.
In these situations, return loss of the patch cord can be changed to a point to invalidate the digital signal processing (DSP), or echo cancellation, and cause the link to go down until a new set of parameters is calculated.
As the demands for signal transmission continue to increase, and the tolerance for downtime continues to diminish, the issue of maintaining characteristic impedance for cable becomes even more important.
Keep the Eye Clean
Designers of digital systems often look at the digital signal on an oscilloscope to view its eye pattern. An eye pattern is obtained by superimposing actual waveforms for large numbers of transmitted or received symbols. Eye patterns are used to estimate the bit error rate and the signal-to-noise ratio.
A received digital signal will be a combination of the transmitted signal and any noise picked up along the transmission path. When the digital signal is transmitted on twisted-pair cable, this noise is in the form of return loss, insertion loss, near-end crosstalk, far-end crosstalk, etc. Since the digital equipment is able to predict the amount of noise on the twisted-pair cable, it can clean up the noise through digital signal processing, ultimately cleaning up the eye (also known as “establishing a link”).
The DSP parameters that clean the eye will keep the link up and running for as long as the link is needed, as long as the cabling parameters don’t change.
Ethernet signals also have eye patterns, but they are more complex due to the modulation. Pulse-amplitude modulation (PAM) encodes message information in the amplitude (magnitude) of a series of signal pulses. It’s used to support many networking protocols.
PAM modulation offers several benefits: easier transmission of a wire pair, high data rate links, simple receiver/transmitter design and the ability to transmit more bits in a symbol time. As the number of levels increases (indicated by the numeral listed after “PAM”), noise susceptibility increases proportionately.
To understand the complexity, first look at a typical eye pattern for Gigabit Ethernet, which uses a PAM modulation of five levels (PAM5). Within each symbol are four eyes for the DSP to keep clean.
For 10GBASE-T, the PAM modulation used has 16 levels (PAM16). Each symbol transmits at a much higher rate. Within each symbol are 15 eyes for the DSP to keep clean.
Maintaining Characteristic Impedance
State-of-the-art manufacturing processes do a great job at twisting the copper conductors together, called twinning, to ensure that the twist rates are consistent and stable. A stable and consistent twinning operation means that the internal cabling characteristics will not change if the cable is manipulated.
Manufacturers of lower-cost patch cables do not have this twinning expertise. As a result, when a low-cost patch cords are manipulated in the field, the internal cabling characteristics will change. This change in the levels will invalidate the DSP and cause the eye to collapse. If eye collapses too much, the link will go down until a new set of DSP parameters is calculated.
Bonded-pair technology is stable in situations like this, with the ability to maintain its characteristic impedance due to the fact that the cable’s two lines are always bonded, no matter how they move or twist – this maintains return loss levels.
As with any cable, quality manufacturing is vital to making sure that bonded-pair cable has a twist that can handle bends while maintaining its characteristic impedance. If the cable can’t handle the bend, the return loss will change. As a result, the equipment will have to decide whether to continue to send data down the line and accept the errors or re-establish with new parameters.
Belden’s Bonded-Pair cabling offers a better data transmission pathway that results in a lower number of packet resends and decreased latency in noisy conditions. Our 10GXS bonded-pair cabling not only frustrated office worker at computer maintains physical integrity during installation and throughout its lifecycle, but is also 25% smaller and lighter, making installation faster and easier. Learn more here!