Understanding Propagation and Delay Skew in Cable Testing 101 Series

February 23, 2017 / 101 learning

You may have heard that propagation delay and delay skew parameters are important for IPTV, surveillance and video conferencing, and many vendors may tout delay skew performance for these video applications.

But do you know why these parameters are important for video applications? As part of our Cable Testing 101 series, we thought you might want to know more.

What Is Propagation Delay and Delay Skew?

Delay happens for all signals across all cable types, and propagation delay is the amount of time it takes for a transmitted signal to be received at the other end of the link or channel. The propagation delay in a twisted-pair copper cabling such as Category 6 or 6A is related to the nominal velocity of propagation (NVP), as well as the length of the cable and the operating frequency. You might remember from our previous blog on reading the top of a LinkWare™ report that NVP is used by your tester to calculate the length of the cable.

Expressed as a percentage and specified by the cable manufacturer, NVP varies based on the materials used in the overall construction of the cable itself. It characterizes how fast a signal travels down the cable relative to the speed of light in a vacuum.  Since the speed of light in a vacuum is the highest speed that can possibly be achieved, the value is always below 100% with most twisted-pair cabling in the 60 to 80% range. The lower the NVP, the greater the delay.

When we look at a four-pair cable where all pairs are transmitting data, the delay may be different from one pair to another. This is what is referred to as propagation delay skew, and it is calculated by looking at the difference between the pair with the least delay and the pair with the greatest delay.

While delay in general may be a factor of the overall cable construction, delay skew is primarly caused by the overall inconsistent pair geometry and twist rates. For example, extreme differences in twist rates from pair to pair can cause a higher delay skew.

Why Does It Matter?

All twisted-pair copper cables exhibit some delay skew since twist rates are purposely varied to minimize crosstalk, but those that fail this performance parameter (expressed in nanoseconds) could significantly impact today’s digital video applications.

While network equipment and computers can typically resolve the time differentials between pairs, when the delay skew is too high, it can result in increased bit error rates and jitter. For high resolution RGB video signals where each color is sent down a separate pair, too much delay skew will have your customers complaining about a jittery picture on their video displays.

So while industry standards require less than 50ns for delay skew, cables that exhibit less than 25ns like shown here are better for video applications. And given the ever increasing use of digital signage in commercial enterprise environments, many cabling vendors are now offering “low skew” cables with delay skew values closer to 2 or 3ns. If video is the primary application, these cables might be worth considering.