Cat6 Ethernet Cable Untwist Performance Compared - Fluke DSX-8000 Test Results
Written by Don Schultz, trueCABLE Technical Manager, Fluke Networks Copper/Fiber CCTT, BICSI INST1, INSTC, INSTF Certified
One of the cool things about having a Fluke DSX-8000 CableAnalyzer around is the ability to do “what-if” testing. Although expensive at approximately $15,000 when fully kitted out for copper, the insight it can provide has taught me to identify how well a particular cable run is performing. On the flip side, the DSX-8000 can also tell me if a cable is a failure and if so, how and how badly it fails. Those nuances of “how well” and “how badly” are extremely important to anyone running down problems during an Ethernet LAN cable installation.
Realizing that the average installer will not have access to such an advanced testing instrument nor the required training and Certification to make use of it, I typically go above and beyond to explain Certification test results in relation to real world and on-the-ground issues. For any installation the majority of issues encountered will be at the terminations. Although any number of termination mistakes can cause a failure (like crossed conductors) there are more subtle termination mistakes that I can talk about and write about. In fact, I have talked to the point of blue in the face about them but wouldn’t it be nice if we had some hard data and example pictures to show that illustrates this? Today we will take a dip into Ethernet untwist and why it can make or break your installation. I have some pretty pictures and a short video too! I strongly recommend you watch the video AND read the blog that follows to get the full story.
The ANSI/TIA 568 series of specifications for copper twisted pair Ethernet cable makes it clear that the maximum permissible amount of untwist for any conductor pair is 1/2”. The reason why there is a limit has to do with how Ethernet cable works. Without pair twists, you have a wire suitable for clothesline. Seriously! I give a quick explanation about why conductors are twisted at all in Why Are Wires Twisted Inside an Ethernet Cable? This 1/2” untwist limit applies to Cat5e through Cat8 Ethernet cable. 1/2” is the limit because humans can still work with it comfortably and the amount of NEXT (near end crosstalk) induced by 1/2” untwist is accounted for in the specification. You can go tighter if you wish, but don’t go looser!
- Fluke DSX-8000 CableAnalyzer setup for PCA (patch cord testing) Cat6 test limits, using Cat6 PCA adapters
- One factory terminated prototype Cat6 U/UTP patch cord, to be offered in the future by trueCABLE
- Three hand terminated solid copper Cat6 U/UTP Riser rated cable segments terminated at both ends using our Cat6/6A Unshielded Pass-Through RJ45 Plugs
- Large Cut-To-Fit strain relief boots used where practical to mechanically (therefore electrically) stabilize the termination
The terminations at each cable were varied from 3/16” all the way to 3/4” untwist (both ends). The factory terminated Cat6 patch cable is at 3/16” by default. The Fluke instrument was then used to measure the performance of each cable. Pictures of the terminations were taken to present next to the Fluke DSX-8000 test result.
First up, let’s line up the test results obtained and quickly talk about them. We will then go test by test.
The above test results are a summary of the four tests I performed. Here is the breakdown of the columns:
- Cable ID: What I typed in to identify the particular cable I was testing
- Summary: Given as PASS or FAIL
- Test Limit: This is the ANSI/TIA 568-2.D test limit applied. In this case, TIA Cat6 Patch Cord, with varying lengths since I was using some varying lengths of cable. The test presets are based on meters (m).
- Length: This is the electrically measured length of the cable based upon NVP or Nominal Velocity of Propagation
- Headroom: The all important number expressed in dB (decibel) for NEXT performance. NEXT is near-end crosstalk. The more negative the number, the noisier the crosstalk and therefore the performance is lower. This is the number we are looking at for this blog.
- Date / Time: Self explanatory
The factory terminated prototype patch cable and the standard 1/2” untwist hand terminated patch cable scored PASS. It should be noted that the factory terminated prototype patch cable passed better. What that means is it had more “headroom” to approach failure. There are practical implications to this. First, this indicates a tighter termination gets you more headroom and it (might) help with a cable run that would otherwise be “on the edge”. Another implication is factory terminated patch cords will do their job better than patch cables constructed using hand terminated RJ45 8P8C plugs--especially onto solid copper Ethernet cable. Oftentimes, a hand terminated solid copper patch cord will FAIL this test. I got lucky with a pass that was barely a pass.
The two out of specification terminations scored FAIL. One failed worse than the other, and this is an important point. The more untwist = bigger failure. This has practical implications as well. The worse the failure, the more packets a patch cable is likely to drop in the real world.
Let’s take a closer look, shall we?
Factory terminated Cat6 patch cable with test result. Looks pretty good!
Hand terminated solid copper Cat6 patch cable with standard 1/2” termination. Not bad, it passed. Barely.
Yikes. Not good. This termination is just 1/8” out specification and the NEXT signals are going well below the red line. Can it get worse? Oh yes…
A full 3/4” untwist termination. It failed even worse.
Real World Implications & Wrapping Up
So, now we can see with some clarity just how increasing untwist you get worse performance. That said, this is with a Fluke certification device running some stricts tests. How would this show up in the field? In other words, will your Ethernet switch still connect up 1G? The short answer is yes, it likely will, even with the worst test result shown above. Why would that be?
- The Ethernet protocol is forgiving. It has error handling built-in, and will retransmit badly formed (mangled) packets caused by excessive crosstalk.
- Ethernet switches, if all eight conductors are making electrical connection (and even in the worst case scenario they are) will do everything it can to maintain 1Gb/s speed
HOWEVER, the important thing to understand is while the protocol and switch are compensating for some seriously bad terminations and maintaining a 1G speed, a portion of that bandwidth is now being consumed by retransmitted (bad) packets. You would not notice the phenomenon if your Internet connection happens to be 300Mb/s, for example. You have 700Mb/s additional headroom for garbage data to pass unnoticed. You will notice on high speed (1 Gigabit) Internet speeds or high speed computer to computer file transfers! How much packet drops will consume your real transfer speeds vary with how bad your cable is. It takes a special packet analyzer using BER (Bit Error Rate) testing to find out the practical effect.
So, there you have it. Finally, a photographic and data driven explanation as to why you want to have good terminations. With that said, I will say HAPPY NETWORKING!!
trueCABLE presents the information on our website, including the “Cable Academy” blog and live chat support, as a service to our customers and other visitors to our website subject to our website terms and conditions. While the information on this website is about data networking and electrical issues, it is not professional advice and any reliance on such material is at your own risk.