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I am asking this because after doing some research I found some conflicting information.
First, on Wikipedia page on Ethernet physical layer I found that the voltage on the lines is 2.5V. https://en.wikipedia.org/wiki/Ethernet_physical_layer
But in more trainings and course that I take in Uni the professors said that it is 9V or 12 V.
Are there more standards for the physical connections and voltages on ethernet lines?
There's many different voltage, and it varies depending of frequencies
From the IEEE 802.3-2008 document borrowed from the official IEE 802.3 get page - but it seems it is no more freely available.
7.4.1.3 AC common-mode output voltage
The magnitude of the ac component of the common-mode output voltage of the driver, measured between the midpoint of a test load consisting of a pair of matched 39 > Ω ± 1% resistors and circuit VC, as shown in Figure 7–13, shall not exceed 2.5 V peak from 30 Hz to 40 kHz and 160 mV peak from 40 kHz to BR.7.4.1.4 Differential output voltage, open circuit
The differential output voltage into an open circuit, measured at the interface connector of the driving unit, shall not exceed 13 V peak.7.4.1.5 DC common-mode output voltage
The magnitude of the dc component of the common-mode output voltage of the driver, measured between the midpoint of a test load consisting of a pair of matched 39 Ω ± 1% resistors and circuit VC, as shown in Figure 7–13, shall not exceed 5.5 V.
Newer standards may have different limits, but since they are usually retro-compatible, they must not have changed much. I dug trough a few of them but didn't found specifications that contradict those above.
The 802.3bq-2016: Physical Layer and Management Parameters for 25 Gb/s and 40 Gb/s Operation, Types 25GBASE-T and 40GBASE-T document has the table:
in which you can find a Differential-mode voltage of < 2.4 + 19.68 (f / 30) mVpp close enough to 2.5V.
(Vpp stands for the voltage differential peak-to-peak, i.e. difference between the lowest and highest voltage in a period.)
There's another voltage involved, when using Power Over Ethernet (POE), which range from 37 to 57V.
Some answers can be found in IEEE 802.3, however care is needed when reading that document, there were a lot of variants of Ethernet and if you aren't careful it's easy to end up reading the wrong section.
The electrical specifications for 10BASE-T can be found in 14.3.1.2.1
The peak differential voltage on the TD circuit when terminated with a 100 Ω resistive load shall be between 2.2 V and 2.8 V for all data sequences for a type 10BASE-T MAU that is not a type 10BASE-Te MAU.
For 100BASE-TX it appears that 802.3 does not directly specify PHY electrical characteristics, instead clause 25.2 references another standard.
The 100BASE-TX PMD (and MDI) is specified by incorporating the FDDI TP-PMD standard, ANSI INCITS 263-1995 (TP-PMD), by reference, with the modifications noted below. This standard provides support for Category 5 twisted pair cabling. For improved legibility in this clause, ANSI INCITS 263-1995 (TP-PMD), will henceforth be referred to as TP-PMD.
Unfortunately that standard doesn't seem to be freely available and I'm not paying $60 to answer a stack exchange question.
Looking at the section on 1000BASE-T i'm struggling to find a clear statement of output voltage, but Figure Figure 40–20 shows a scope trace with a signal varying up to around ±1.5V when transmitting a test waveform.
10BASE-2 and 10BASE-5 are specified in terms of current drive instead of voltages. e.g From clause 10.4.1.3 for 10BASE-2
the signal has an offset com- ponent (average dc current including the effects of timing distortion) of from –37 mA min to –45 mA max and an ac component from ± 28 mA up to the offset value.
So potentially you could have an offset of –45 mA and an AC current of ±45ma leading to a total max current of -90 mA. This drives into a 25 ohm load, so that works out to a maximum of around -2.5V, into a properly terminated cable. However the voltages may be higher if multiple stations transmit at the same time.
Either way though, your professors mentions of 9V and 12V do not seem to be in any way reflective of reality.
