Reading up on how to calculate cable lengths as a follow-up to my previous question, I understand that you have to use (102-H)/(1+D) to calculated the stranded length (C) and H+C to calculate the channel length. And that the channel's max length is 100m (in my example for 1000BASE-T).

Using the standard for D which is 0.2 (for 24AWG UTP) and 90m horizontal get me C=10. Does this mean that 22AWG cables (D=0.2) will always be out of specs (since the channel lengh will be 102 regardless of the size of the horizontal length)?

Where I'm also uncertain, but I can post that into a separate questions if needed is this example:

For example, if using 60 meters of horizontal solid category 6A cable and 40 meters of stranded 24 AWG category 6A patch cable with a 0.2 de-rating factor, the total length of the channel must be reduced to 97.5 meters

How exactly would I reduce the cable? If horizontal link is permanent an can not be change I have to reduce to 37,5m stranded in order to get to 97.5?

And the last question is, what's physically behind the math/specs? Why can't I use more then 90m in my first example? With 91m horizontal I get 9.2 stranded and 100.2 for the channel length. Even if reduce the stranded to to almost zero I'm out of specs... I just can't warp my head around this. I just want to select "the right patch cable" in an existing setup (in an office building)... and thought it would be a good idea to know a bit about the limits. But this whole approach seems to be a bit complicated or did I just start out to naive?!

  • You'll be outside of the specs - the link would likely work but if it doesn't you can't blame anyone but yourself. Since this is a site about professional networking you need to use fiber when the twisted pair reach isn't sufficient. – Zac67 Aug 21 '20 at 20:55
  • @Zac67 this is to the first question, right? So my conclusion was correct that using a 22AWG cable in this scenario is out of specs? Right know I'm just trying to figure out the limits of the system for future planing and to see if there are potential problems. – Albin Aug 21 '20 at 22:08
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    You will have problems with 22 AWG because it is too big for the termination hardware. For example, trying to use Category-6 (23 AWG) cabling with Category-5 (24 AWG) termination hardware will cause flaky connections. – Ron Maupin Aug 21 '20 at 23:35

It may be that you can exceed the length specifications and get it to work, sometimes. To guarantee trouble-free networking, you need to closely adhere to all the specifications. Sometimes, the cabling seems to work, but randomly fails for unknown reasons. You may be getting a lot of frame errors and retransmissions and not even notice it until the link is used for a particular application. It can cost a lot of time and money trying to track down problems that you may not even realize are cabling problems.

Network cabling is expensive and permanent, and there are ordinances and laws about it that could cause problems if there is an inspection by a building inspector, fire marshal, etc., who can close down the building until the problems are corrected. For such an investment that should last many years, you really need to do it correctly the first time.

First, to properly install Category-6a cabling and have it actually pass the category test suite and work for an average distance, you really need a professional installer. Next, trying to install so close to the distance limits will make it difficult, even for an experienced installer to get it to pass the category test suite and consistently work correctly. Something as simple as having the blue wire on top of the blue-white wire at the termination punchdown can cause a test failure, and the proper test equipment is expensive and requires experience to use. The installer will use the equipment to test each cable run an present you with a report for the tests on each cable run.

A properly designed and implemented cable plant should not need patch cables over five meters, and if the horizontal cable is so close to the 90 meter limit, then you should really add another data closet. All the data closets should be tied back to the main data closet with fiber optic cable.

To accommodate modular furniture there is an exception called a MUTOA (Multi-User Telecommunications Outlet Assembly), but you really need professional cable designer and installers for something like that.

Modern network cabling for modern network speeds requires close adherence to the specifications. You can join BICSI or consult with BICSI RCDDs (Registered Communications Distribution Designers) and installers, but it is no longer a DIY project if you want things to work correctly and consistently.

  • Thanks for the in depth answer. Yes we used a professional installer but I wasn't in charge of the project. As far as I know cable length and probable other specifications as well weren't really discussed so unfortunately I have to do al of this "after the fact". Was it the installers responsibility to make sure it passes the category test? Should there be some kind of documentation for that? If so, would this documentation be a requirement or "just best practice"? – Albin Aug 22 '20 at 8:04
  • "Was it the installers responsibility to make sure it passes the category test? Should there be some kind of documentation for that?" Yes and yes. Ideally, the cabling company would use a designer who would oversee the installers. The installers would need to perform the test suite, and the test equipment records each test. The failed cable runs would need to be fixed and retested. The test equipment would then be connected to a PC to dump the test results, and you should get a report from the company, who would also keep a copy.. That is what a real cabling company does. – Ron Maupin Aug 22 '20 at 15:55
  • @Albin, you can still hire a cabling company to come out and test each cable run and give you the results. If the cabling was installed with a fly-by-night cabling company, you may be surprised with a lot of failures. Everything I have written here is from experience. We used a very good cabling company, and we acquired many other companies that already had cabling, and we needed to replace it all in too many sites. I used to be an RCDD, so I was the customer that was a cabling company's worst nightmare because I made them do it right. Contact BICSI. – Ron Maupin Aug 22 '20 at 16:02
  • It was a pretty big (regional) company here in Germany I wouldn't call it fly-by-night but, who know maybe you are right. I will try to get the testresults and documantation from them first. To do a (paid) re-testing I will have to convince the CEO so I need to have some kind of evidence fist. Does BICSI give recommendations for cabling companies? – Albin Aug 22 '20 at 16:36
  • I do not know if they will recommend a company, but I think you can get a list of companies with RCDDs and certified installers. You should also be able to rent the test equipment, which is pretty expensive to buy, especially for a one-time thing. – Ron Maupin Aug 22 '20 at 16:39

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