Top 20 Practical Cabling Tips for Network Engineers – Part Two

Here are 20 things about network cabling that I think that most Network Engineers have forgotten or don’t know about network cabling. Things that I have learned, the hard way, in twenty years of looking after networks.

Most engineers think that cable is simply plug and play. And, mostly, that is true. In recent years, Ethernet standards and manufacturing have developed into reliable and “good enough” but the realities are still in place. Cabling needs some care and attention to detail to be reliable.

Degrading Signals

11. Neat Cabling Causes Signal Interference

Neat cabling creates cross talk by electromagnetic induction and therefore signal degradation by virtue of induction. Cabling should be untidy, loosely bundled, and randomly mixed to avoid signal induction.

This is not common practice, and, to be fair, is an extreme approach since the signal leakage from your copper plant should be minimal. But technically, it’s correct that induction might occur.

12. Keep Away from Power Cables

When copper cables run parallel to electrical cables, they will act like transformers or inductors and induce 50/60Hz currents and noise spikes from the electrical cable. Put as much distance between data and power cables as possible.

13. Keep Away from Electrical Interference Sources

There are many sources of 50/60hz interference and you should also consider them. Don’t use flouro lights in your data center, use LEDs or some other low power lighting that will use less power and generate less interference. Keep motors in air conditioning room completely away from data cabling. Of course, your coolers should be on different power infrastructure to reduce power ripples.

14. Keep Your Cable Dry

Moisture changes the dielectric coefficient in copper and significantly impacts the signal performance. Keep your cable dry during the installation process. Using copper cable in conduiots or trenche sbetween buildings should include special considerations such as waterproof conduits and capped ends to prevent moisture creep.

15. Cables need to be cool

Cables, especially cables with PoE, can overheat in large bundles. This change in thermal property changes the electrical performance and impact the signal propagation.

16. Star Pass is Fail at Installation Time

A “STAR PASS” is not good enough. As shown in earlier points, your cable plant WILL degrade over time. When a cable tester shows the cable has barely passed (often called a Star Pass by cabling installers) the signal performance check it means that your cable will work today but eventually it will fall below specifications. In other words, a star pass is failed test and starred for attention (not praise). Make sure that your cabling contract demands rectification of “star pass” cable tests.

Fibre Optic Cables

17. OTDR testing is not enough

Fibre Optic cabling must be tested with an OTDR and Power Meter. OTDR testing is easily fudged and is really just shining a torch as a simple light test. What you really want to know is that the acceptable amount of light power is being lost in the installed and terminated cable run.

18. Fibre Optic Loss is Power Sum

The length of fibre cabling is less important than quality connectors and proper terminations. Each connector and splice causes a small amount of signal loss. Therefore, power levels are the important test factor for fibre optic cables.

19. Dust Caps have a Purpose

Dust caps on fibre optics connectors are used to prevent dust buildup inside the connector. A single mode fibre is 9nm wide and about the same size as dust. The laser signal can be seriously attenuated and thus reduce the run length or cause signal problems.

20. Don’t Kink Fibre Cables

Fibre optic patch leads are flexible but the fibre core can break, or worse, can fracture. This causes weak laser signal by creating power loss in the cable. Weak signals may not be decoded by the laser reciver. Be nice to your fibre cable.

The EtherealMind View

The weakest link in the cabling infrastructure defines the performance of your entire cable system. A bad patch lead means the entire cable run is faulty. It’s surprising how much impact a bad cable install can have on a facility (experienced it a couple times).

A faulty cable can hard to detect. Many cable faults are simple “up / down” or errors will show on the physical interface in the router or switch. But marginal cabling may only be detected by replacing the cable or performing a signal test.

Cable can degrade over time because it’s made of actual stuff. A cable that was working yesterday can be intermittently faulty next week.

Other Posts in A Series On The Same Topic

  1. Top 20 Practical Cabling Tips for Network Engineers - Part Two (31st July 2012)
  2. Top 20 Practical Cabling Tips for Network Engineers - Part One (19th July 2012)
  • Pingback: Top 20 Practical Cabling Tips for Network Engineers - Part One — EtherealMind()

  • John P Graham

    Nice pair of articles, which I have now forwarded to everyone in my NOC team. Thanks for all your effort with packet pushers, Greg. It has been immensely useful for keeping me plugged in to networking while I was trying to gain a toe hold in the industry.

  • Simon Naughton

    Seen first hand. Cisco CTS 3210 connected to a self made crappy cat5 (not cat5e) cable that caused all kinds of problems. QoS policy was checked, codec’s RMA’d, system reconfigured a number of times…. in the end, it was the cable.

    Yep, a contractor tried to save a couple of dollars on a AUD$200K + device…. just crazy.

  • http://pulse.yahoo.com/_NX4XT3HPJLNWRT57CJEETDVDDI danp

    I just love having justification to not have a nice tidy cable plant, thanks!

    that said, here’s my favorite cabling idea for building wiring closets: put 1U switches next to 1U patch panels, and use short patch cables (1 foot is nice). cuts down on a TON of mess.

  • elias84

    Greg, a small correction (after 2 years!) I think you must have meant 9um (or 9μm) for the single mode fibre. Excellent guide though.

  • http://www.storix.com/blog.html Rick Chatham

    Hey Greg. Really appreciated this article. I had to do some British -> American translation (such as cooler and torch) lol But I appreciated all the reminders (including the one on messy cabling, since I have a boss who wanted us to create “cable porn”.