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As the questions says, is there any specific reason why Ethernet cable is rated 100MHz?

For a 100Mbps Ethernet the data rate is 100Mbps, and the baud rate is 125Mbaud, because of the specific line coding that it uses. What is the relationship between 125Mbaud rating of data transfer and 100MHz rating of cable?

Thank you.

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    \$\begingroup\$ The most likely relationship is "good enough" \$\endgroup\$ – PlasmaHH Jul 30 '15 at 11:49
  • \$\begingroup\$ @PlasmaHH Sorry, but I did not exactly get your point. Could you please elaborate? \$\endgroup\$ – LoveEnigma Jul 30 '15 at 11:50
  • \$\begingroup\$ One day in the standarization committee someone said: "Will 90MHz work?" "No, not enough" "Hm, will 100MHz work" "Yeah, that would be just good enough" "Ok, lets standardize it as 100MHz" \$\endgroup\$ – PlasmaHH Jul 30 '15 at 11:53
  • \$\begingroup\$ @PlasmaHH Hmmm, got that! But does that mean 125Mbaud is equal to 100MHz? \$\endgroup\$ – LoveEnigma Jul 30 '15 at 12:07
  • \$\begingroup\$ no, it only means the bandwith of ethernet cable (cat5[e] actually) is rated for 100MHz). That is totally independent to whatever you actually run through it \$\endgroup\$ – PlasmaHH Jul 30 '15 at 12:17
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If you start at https://en.wikipedia.org/wiki/Ethernet_over_twisted_pair, and do some more googling, you can find this stuff out.

But start about 1991, when the first version of TIA/EIA-568 came out. This is the standard which establishes cable categories for voice/data cables for wiring buildings. At this time, 10 MHz over UTP (100Base-T)had been around for a couple of years, and the standard established 3 categories of cable for signal/data use.

Category 1 - voice/analog only, up to 1 MHz. Although it was not specified for data, it was often used at low data rates.

Category 2 - Data to 4 MHz.

Category 3 - 10BaseT, data rates of 10 Mbit/sec.

Of course, things were moving quickly, and 100BaseT (and others) soon came out. This caused the next version to be released, with

Category 4 - 20 MHz, for a standard which never really became all that popular.

Category 5 - 100BaseT, which became the standard, and which could (especially at Cat 5e) also support 1000BaseT, or Gigabit Ethernet.

Due to the way data is encoded, the data rate is actually somewhat higher than analog bandwidth, which is why Cat 5 works at gigabit rates.

And finally, in the latest release,

Category 6 - 10GBaseT, or 10 Gbit Ethernet.

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    \$\begingroup\$ Thanks for your explanation, but the most important part of the question was the relation between 100MHz with 125Mbaud. But I think I just figured it out. The measurement of Ethernet TX/RX pair outputs of PHY on oscilloscope reveals that the frequency of this data is 62.5MHz. That is, for Ethernet, this equates to baud rate being twice the frequency (yes?). This also justifies the question that bandwidth of Ethernet cable being 100MHz is sufficient to carry Ethernet signals, which are much less than 100MHz, i.e. 62.5MHz. Please correct me if I am wrong. \$\endgroup\$ – LoveEnigma Jul 31 '15 at 11:38
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    \$\begingroup\$ You are correct about the non-equal relationship between the symbol rate and the bandwidth to transmit 1000BASE-T. It is encoded with PAM and five levels. See (flukenetworks.com/knowledge-base/?nid=129248&tid=250) for more info. \$\endgroup\$ – davidmneedham Aug 10 '15 at 4:59
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100BASE-TX uses 4B/5B block encoding, this means every 4 bits of data result in 5 bits being transmitted, this is where the 125Mbaud comes from.

The actual signalling used is MLT-3, this uses 3 different voltage levels (-1,0,1) in a sequence (-1>0,>1>0). A '1' is represented by stepping through the sequence, a '0' is represented by no change in level.

One period is the time taken to go through the whole sequence, so the minimum period (corresponding with maximum frequency) of the signal on the cable occurs when there are four consecutive ones, as the level changes with each.

This means the maximum fundamental frequency of the signal is only 31.25MHz (125/4).

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1000BASE-TX, is to provide an Ethernet physical layer specification that is low complexity and can easily be implemented.

1000Mb/s operating over a 100 meters of Category 6 UTP cabling would be a benefit to users who migrate to the higher data rates.

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