Minimum Ethernet Cable Length?

Question

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.

Answer 1

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

Answer 2

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.

Answer 3

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.

Answer 4

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.

Answer 5

Check IEEE 802.3:

For 10BASE-T, Clause 14.1.1.1 Medium Attachment Unit (MAU) explicitly states "Provides for operating over 0 m to at least 100 m of twisted pair without the use of a repeater."

The 100BASE-TX Clause 25.4.9.2 Link transmission parameters also fails to specify a minimum distance.

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

Table 55–11—Power backoff schedule for 10GBASE-T shows a length of 0 m.

Table 113–15—Power backoff schedule for 25/40GBASE-T shows a length of 0 m.

=> There is no minimum cable length for twisted-pair Ethernet. If a device or a cable doesn't work with a very short link it can be considered broken.

("Copper" also includes ancient coax cabling which does have minimum spacing of 2.5 m (10BASE5, Clause 8.6.2.2) or 0.5 m (10BASE2, Clause 10.7.2.1) - but I don't think that was the question. For completeness sake, 1000BASE-CX states 0.1 m minimum length (Clause 39.1). Fiber might require a minimum attenuation on a link to avoid blinding the receiver, especially for long-reach variants.)

Answer 6

International standards - ISO 11801 and EN 50173 / 50174 are your friend. Typically 1m is the minimum for a direct link, but if you’re connecting to patch panels (i.e. the end to end cabling has 3 or more connections) the minimum patch cable length is 2 metres, with a minimum length of 15m for structured cabling, and an extra 5 metres for any structured cable run from an intermediate point, such as area distribution.

This avoids the interference / reflections / cross talk caused by having multiple connectors close together in series.

This is reflected into the warranty/installation requirements for most patching and cabling systems, and can be referenced by local building codes, codes of practice, and legislation, for example BS7671 and BS6701 in the UK.

Example structured cabling guides (assuming you don’t design networks for a living, and thus need to buy the standards) can be found here: http://www.lavancom.com/portal/download/pdf/catalog/Installation%20and%20Testing%20Guideline.pdf

https://www.nexans.co.uk/eservice/UK-en_GB/fileLibrary/Download_540209803/UK/files/General+Installation+guide+2014_V2_1.0+DWI_LR.pdf

Answer 7

Somebody needs to read the IEEE specification on Ethernet cables. When you run Cat 5/6 UTP the minimum is 3' (90 cm). You cannot (should not) use cables shorter than 3' (90 cm). One cable between a router and a patch panel won't likely exhibit a problem, but multiple will. The specification is clear. No shorter than 3' (90 cm) or you're out of specification.