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The answer in this post suggests that it has two reasons.

While the impedance characteristics of a transmission line become less favorable for high speed data as the line gets longer (resistance goes up, capacitance goes up, the line is more susceptible to noise, etc); however, the main reason Ethernet limits cable lengths to 300m is because of time. ... (Explanation of CSMA/CD)

However, modern Ethernet is operated with a switch, which eliminates collision between frames, and a need of CSMA/CD (CSMA/CD is no longer used recently)
For this reason, I think that more general reason is that it is because of impedance of a cable, which makes the signal weaker.
Which answer is more appropriate between two?

  • There are many ethernet standards, and each has its own maximum cable length. See this answer for a list of some of the ethernet standards and the maximum cable length for each. – Ron Maupin Dec 19 '18 at 19:11
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Which answer is more appropriate between two?

Certainly impedance, resistance and other factors are a consideration. However I would side with time.

Newer Ethernet standards include the capability for backwards compatibility. Devices can still operate in half-duplex mode and use CSMA/CD, and you can still buy devices today that only operate at 10Base-T (while these have become much rarer, I still see them installed from time to time - mainly building automation or industrial devices).

The infrastructure cabling also needs to continue to support all speeds, even down to 10/half.

There is a growing trend for some higher end network gear to no longer support 10Mbps data rates, but we are still quite a long way from dropping 100Mbps from the mix, and half-half duplex support is still required at that speed. Maybe some day time won't be a consideration, but for now it will continue to be to provide backward compatibility.

  • Newer Ethernet standards have dumped CSMA/CD and HDX, the fastest existing one being Fast Ethernet. GbE officially supported repeaters and HDX but no hardware is available for that. – Zac67 Dec 19 '18 at 7:30
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    @Zac67, I don't understand the point you are trying to make. The "newer Ethernet standards" still provide for backwards compatibility and infrastructure cabling still needs to support older devices. Just because newer/faster standards don't use CSMA/CD or half-duplex doesn't mean the infrastructure can stop supporting it. Copper infrastructure is agnostic of the Ethernet standard that is used on it, up to the performance specified by the cabling class/category. – YLearn Dec 19 '18 at 7:51
  • Newer/faster Ethernet standards (PHYs) are not backward compatible - the ports using those standards often are backward compatible as it's supported by autonegotation and fairly easy to do. Essentially, 1000BASE-T doesn't support CSMA/CD / HDX in practice and 10GBASE doesn't support it at all. A 10GBASE-T port supporting 100BASE-TX very like supports HDX for that speed as well. – Zac67 Dec 20 '18 at 1:22
  • @Zac67, seems you want to split hairs. Yes, you are correct the standards are not backwards compatible. I did not claim the standards were backwards compatible, but rather that they "provide" or "include the capability" to be. The reason they were written in the way they were was to provide easy backwards compatibility in the devices that were engineered based on them. So when you first deployed 100Base-TX, you didn't need to forklift your entire 10Base-T IT infrastructure, and so on. I didn't think the distinction was germane to the question asked, but apparently you do. – YLearn Dec 20 '18 at 3:41
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The question in the link is actually about the length of 300 feet (really 100 meters, or 328 feet), but the answer discusses 300 meters, and I am unaware of any ethernet standard that had a limit of 300 meters.

For the common ethernet standards that run on UTP (10Base-T, 100Base-TX, 1000Base-T, and 10GBase-T), the distance limitations are for 100 meters, assuming up to 90 meters of solid-core horizontal cable (permanent link), and a maximum of 10 meters of stranded cable (split between both ends) for a total channel length of 100 meters.

The problem arises as you increase the frequency of the signal to get faster speeds. To do that, you must change the cable to continue to meet the 100 meter standard. ANSI/TIA/EIA has recognized cable categories (Category 5e, Category-6, and Category-6a; other cable categories have come and gone), and has a set of tests* that must be matched for each category to be certified for that category.

Understand that a long wire is really an antenna, both radiating and receiving extraneous electromagnetic signals. If the noise on the cable becomes too much, the signal cannot be distinguished from the noise. Twisting the differential pairs of wires within the cable gives some immunity to noise, but there are four pairs of wires within the cable sheath, and they each will send out and pick up signals from the other pairs in the sheath, along the entire cable length, sending the signals in both directions, and to and from adjacent cables. This is called crosstalk. The longer the cable, and the higher the frequency of the signal, the harder it is to control the noise on the wire. Cable vendors will take steps, e.g. more twists per inch, bigger wire gauge, or even adding a shield around the individual pairs, to try to meet the new requirements for a new cable category.

If the cable passes all tests after being properly installed, it is guaranteed to work for up to 100 meters. That is not to say that some people do not fudge that and install longer lengths, which may or may not actually work (possibly with lots of retransmissions). Also, poor installation could severely limit the length of cable that will work, but the cable would not pass the test suite for the category. Small things in the installation, e.g. untwisting a pair slightly too far on termination, can cause a cable to fail the category test suite.


*The primary tests are:

  • Wire Map - Checks for proper pin to pin termination, and for each of the 8 conductors the wire map checks for: Continuity to the far end, Shorts between any two or more conductors, Reversed Pairs, Split Pairs, Transposed Pairs, Any other miswiring.

  • Length - The physical length of the cable is the actual length derived by measurement of the cable(s) between the two end points. The electrical length is the length derived from the propagation delay of the signal and depends on the construction of the cable. The maximum physical length of the horizontal cable (permanent link) one end of the cable to the other is 90 meters. The maximum length of the channel model is 100 meters.

  • Insertion Loss - Insertion loss is the loss derived from inserting a device into a transmission line. The insertion loss for both the permanent link and the channel models are the total insertion losses of all the components.

  • Near End Cross Talk (NEXT) - Pair to pair NEXT loss is the measurement of signal coupling from one pair to another. The result is based on the worst pair to pair measurement.

  • Power Sum Near End Cross Talk (PSNEXT) - Power sum NEXT takes into account the statistical crosstalk between all pairs while energized. This is a calculated amount derived by adding up the crosstalk results between all pair combinations.

  • Equal Level Far End Cross Talk (ELFEXT) - FEXT is the unwanted coupling of a signal induced by a transmitter at the near end, measured on the disturbed pair at the far end. ELFEXT is the same measurement of FEXT, less the effect of attenuation.

  • Power Sum Equal Level Far End Crosstalk (PSELFEXT) - As in Power Sum NEXT, these are computed values based on the sum of all the possible pair combinations under the respective tests.

  • Return Loss - Return loss is the value of energy reflected by impedance variations when devices are inserted into the cabling system.

  • Propagation Delay - Is the time it takes the signal to travel from one end of the cable/system to the other. The maximum channel propagation delay is 555ns (nanoseconds) and for the link it is 498 ns, both measured at 10 MHz.

  • Delay Skew - Delay skew is the signalling delay difference in time (nanoseconds) between the fastest pair and the slowest pair. The maximum channel delay skew is 50 ns, and in the permanent link it is 44 ns.

Any tests that are out of specification will fail the test, and the condition must be corrected and the test suite performed again until the cable passes or is replaced.

You can also permanently damage a cable by exceeding the pulling tension or minimum bend radius when installing the cable run.

Also, splices, taps, couplers, etc., are not allowed in a cable run.


There is a newly recognized category (Category-8), but it is shielded cable (vs. the UTP cabling for the other categories), and the distance limitation is 30 meters (24 meters for the solid-core horizontal cable, and a total of 6 meters of stranded cable, split between both ends). This category was recognized to support 25GBase-T and 40GBaset-T.

  • ... just for completeness, 10base5 had a limit of 500 metres, and 10base2 had a limit of 185 metres. I haven't seen either of these this century. – jonathanjo Dec 19 '18 at 10:59
  • Yes, I considered those, but I do not see anything that with 300 meters for copper cabling. See this answer I posted on Super User. – Ron Maupin Dec 19 '18 at 18:16
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NE has been over this a few times... yes, the reason the many decades old standard says 100m (328ft), is for collision detection. Because of backwards compatibility, it has never been removed or changed. It was a compromise based on frame size and propagation speed. (it's also why the minimum length is 64 bytes.) Signaling capability is an increasing concern for modern high speed networks as higher frequencies are more sensitive to impedance, and lower voltages naturally can't go as far, but this wasn't much concern to 10base-X. (if your transmitter ignores current limits [and, well, most of the 802.3 spec], a 10base-2 signal can be pushed much farther than 100m.)

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