I have heard from a few people that cables less than 1 meter are prone to transmission problems. Is there any minimum copper cabling length?

Please explain if this is true or not and share any available RFCs or standard documents. Thanks.

  • I have never seen transmission problems caused by short cable when connecting devices manufactured by the same vendor. In contrary, connecting devices of different vendors sometimes gives communication problems ever on 'normal' cable length. – user36844 Jun 27 '17 at 5:10
up vote 7 down vote accepted

I once made a crossover adaptor using a coupler and 2 tips butted almost against one another with probably less than an inch of cable between each connector point. Worked great!

Point is, you won't find any standard spec on a short cable length. All you have to go off of here is user experience. There are tons of threads where people say their 6" patches are fine. Some report dropped packets. Some people like me have used 4" ones for trunks between switches without issues. Others have replaced short cables with long ones and resolved network issues.

Do what works for you in this regard. If you implement short cable lengths, you would be wise to watch the port for errors to ensure the operation, but after it's fine for a while, I would figure it would be fine in general.

Here's another thread with a link that was considered uncreditable here. https://serverfault.com/questions/108480/what-is-the-minimum-ethernet-cable-length-for-a-cat6-gigabit-connection

There is no minimum cable length when talking about standard copper-cables. When it comes to fiber, there is a minimum length depending on technology, diodes and so on.

  • 2
    note, fibre minimums are a function of power. longer reach (ie. higher power) expects higher attenuation; at short lengths the signal can blind (and even damage) the receiver. – Ricky Beam May 10 '14 at 16:54

As I currently do not have the full standards available to me, the best answer I have seen on this topic is from the forums at the BICSI website. Based on this post by an employee of Fluke Networks (manufacturer of a number of popular cabling test units), there appears to be no minimum length in the standard.

However, there are both an implied minimum length and recommendations for minimum lengths when using minimally compliant components for Cat5e/6 and Cat6A.

Below is the applicable quote:

For Category 5e and 6, there is no minimum length requirement. ANSI/TIA/EIA-568-B.2-1 in Annex K does give a warning about reflected FEXT on shorter links with minimally compliant components. The obvious solution is not to purchase minimally compliant components. In the early days of Cat 6 when vendors were struggling to do better than marginally compliant, short links were an issue. Today, this is not an issue if you stay with a main stream vendor.

Within this same standard, there is also advice on distance when using a consolidation point. It advises a minimum distance of 5 m between the CP and TO. In ISO/IEC they are a little more clearer is specifying 15 m between the DP and CP. This is all for Category 6/Class E.

With regards to Category 6A, there is a minimum length requirement - kind of. In Annex J of ANSI/TIA-568-B.2-10 is describes worst case modeling using a 10 m link. The suggestion therefore is that you should not go less than 10 m. But again, that is with minimally compliant components. As with Category 6 stated above, there are now components available that will give you passing field tests below 10 m. HOWEVER, even vendors with good components may still have a minimum length requirement in their design specifications. The only way to know where you stand is to talk to the vendor AND test it to see.

If you are talking specifically about patch cords, then 0.5 m is the implied minimum length in ANSI/TIA/EIA-568-B.2-1 for a certified patch cord. That's because the math for the limit lines really does not work below this. Infact, getting a certified patch cord of 0.5 is going to be tricky. Many vendors only offer a certified patch cord of 1.0 m or longer. I suspect that this may be the most useful information with regards to your question.

In addition, you need to keep in mind the capabilities of the tools you have on hand. There are field testers that may not support cables under a minimum length, but this is a limitation of the tester and not based on the standard. Obviously if you are testing with a unit that has a minimum length of 1 meter, you would want to avoid cables less than 1 meter as you would not be able to get accurate results.

I'm not sure if the minimum cable length was ever listed in an RFC. Whatever the minimum is depends on several factors.

A basic description of the problem caused by cables being too short is that the electrical signal, when transmitted will bounce off the end of the wire and reflect back in the other direction. Just like throwing a ball at a wall if you're close enough to the wall when the ball bounces back it may reach you.

For a networking signal, if the signal bounces back and is strong enough to reach the sender, then the sender has to figure out if the signal is new, or a reflection of its own transmission.

There are many factors that go into whether or not bounce back will happen; transmission signal strength, wire resistance, cable length, etc.

So, whatever the minimum is, it's long enough to not receive bounce back. In the early days of Ethernet networking, for twisted pair it was one meter. Today equipment is usually manufactured with a number of counter measures to reduce or eliminate the effects of bounce back. I've reliably used cables as small as one foot without problems. But again, that will depend on whether the rest of the physical equipment and software can compensate adequately for the short length.

  • 4
    Reflection was only an issue for 10base-2 (coax). XXXbase-T doesn't have that problem -- solved by twisted pair. There has never been a minimum distance for any "T" standards. The primary failure mode of short twisted pair cables has always been substandard crimping followed by pulling too much twist out of the cable. – Ricky Beam May 10 '14 at 16:51
  • Like I said, I don't know if it's in an RFC, but I seem to remember there being a 0.5m minimum. That being said, I personally have encountered issues with CAT6 pre-fab cables at 1' length. Changing to 3' cables resolved the issue. So it happens. – bahamat Dec 18 '14 at 19:57

Somebody needs to read the IEEE spec on ethernet cables. When you run Cat5/6 UTP the minimum is 3'. You cannot (should not) use cables shorter than 3'. One cable between a router and a patch panel won't likely exhibit a problem, but multiple will. The spec is clear. No shorter than 3' or you're out of spec.

  • 2
    Exactly where is that in IEEE 802.3? Looked all over and couldn't find it. – Zac67 Jun 26 '17 at 18:26

Check IEEE 802.3 40.7 Link segment characteristics: no minimum link distance is specified for 1000BASE-T.

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