I have been told that Ethernet magnetic transformers are used for base-t Ethernet when transmission is sent over a lengthy cable. What is the purpose of the transformer? (is it for signal filtering or boosting?)

Additionally, I have a circuit that has been used for an 8 wire (1000base-t) Ethernet configuration. Would the same circuit work for a 4 wire (100base-t) configuration if I connected only pins 1, 2, 3 & 6? if so would the performance be the same as the 8 wire configuration?

I'm sorry if I may not be coherent in my question as I am not too familiar with Ethernet hardware. Thank you for reviewing my question.

P.S. I am unsure about how data is transmitted over Ethernet. Is it that all differential pairs (DA: pins 1 & 2, DB: pins 3 & 6, DC: pins 4 & 5, DD: pins 8 & 7) are busses where data can be transmitted both ways (unlike the UART where RX has to be connected to TX)? and in case I am just using 2 pairs, would I be connecting only DA and DB? Is there also an issue with connecting a 4 wire device to a network that uses 8 wires?

  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can post and accept your own answer.
    – Ron Maupin
    Jan 3, 2021 at 5:48

3 Answers 3


I have been told that Ethernet magnetic transformers are used for base-t Ethernet when transmission is sent over a lengthy cable.

They are always used, not just when "sent over a lengthy cable".

What is the purpose of the transformer?

The primary purpose is isolation. Typically they are also used as part of the signal conditioning, turning a pair of single-ended drives into a differential signal on transmit and establishing the correct common mode voltage for the receiver on receive. For this reason the device-side of the transformers is usually center-tapped.

Isolation is a very good idea in communications systems that are linking lots of hardware over a wide area. You don't want fault current/voltages in from faults in the mains wiring or devices to spread through your communications wiring.

There are basically two options for isolation, opto and transformer. Transformer isolation has a couple of major advantages. Firstly, the signal power passes through the transformer, which means you don't need to get a power supply to the "isolated" side of the barrier. Secondly, transformers are very good at generating and receiving differential signals while providing high common mode rejection; this makes them a good combination with twisted-pair wiring. Thirdly, it is easier to design transformers for high frequency (aka high speed) than optocouplers.

Transformer coupling does have some downsides; transformers don't work on DC, and small transformers that work well at high frequencies don't work well at low frequencies; but this is easily dealt with through line coding schemes that avoid low frequencies.

P.S. I am unsure about how data is transmitted over Ethernet. Is it that all differential pairs (DA: pins 1 & 2, DB: pins 3 & 6, DC: pins 4 & 5, DD: pins 8 & 7) are busses where data can be transmitted both ways (unlike the UART where RX has to be connected to TX)? and in case I am just using 2 pairs, would I be connecting only DA and DB?

It depends on the version of Ethernet. 10BASE-T and 100BASE-TX used one pair in each direction. On older hardware, you had to manually ensure that transmitter was connected to receiver (using crossover cables if needed), but more recent hardware usually has AUTO-MDIX which figures it out automatically.

1000BASE-T uses all four pairs in both directions at the same time, using echo cancellation techniques to separate received data from transmitted data.

I think 10GBASE-T also uses echo cancellation techniques, but i'm not positive on that.

Is there also an issue with connecting a 4 wire device to a network that uses 8 wires?

Most devices support lower-speed modes, but not all. In particular twisted-pair to fiber media converters usually only support a single speed on the twisted-pair side. Devices that support 10GBASE-T usually also support 1000BASE-T but only sometimes support 100BASE-TX and AFAICT never support 10BASE-T.

  • xGBASE-T uses (very much) the same echo cancellation by hybrids that 1000BASE-T introduced.
    – Zac67
    Sep 10, 2020 at 14:31

The ethernet magnetic transformer is required by the ethernet standard. You will find this question, "Why Are Ethernet/RJ45 Sockets Magnetically Coupled?" answered on the Electrical Engineering SE.

The UTP cabling specifications call for a four-pair cable. 10BASE-T and 100BASE-TX use two of the pairs, one transmit and one receive pair, while 1000BASE-T requires all four pairs, both transmitting and receiving at the same time. If you try to run 1000BASE-T on only two pairs, it will negotiate to 100BASE-TX.

  • When you said "while 1000BASE-T requires all four pairs, both transmitting and receiving at the same time", did you mean that any of all 4 pairs can act as TX or RX at any one time, unlike 10/100base-t where tx and rx are dedicated to specific pairs? For my case I have only 4 wires (2 pairs) from my 100base-t device. Does it matter that I connect them to pins 1, 2, 3 & 6, or can I connect them to other pairs? Finally, would my transformer circuit for 1000base-tx work for 100base-tx? I have found many circuits available online and I was confused by them.
    – Benjamin
    Apr 27, 2016 at 5:48
  • I mean that 1000BASE-T uses all four pairs to simultaneously both send and receive. Both 10BASE-T and 100BASE-TX use pins 1, 2, 3, and 6, and they must be wired with one twisted pair on 1-2, and another twisted pair on 3-6. Splitting a twisted pair will not work correctly. If you didn't know the above, I seriously doubt you can build your own ethernet interfaces, so you shouldn't worry about the transformers. If you do want to build your own ethernet interfaces, you should ask your questions on Electrical Engineering.
    – Ron Maupin
    Apr 27, 2016 at 7:10

The transformer is there mainly to decouple DC common mode signals, as has been explained in other answers, and it also provides the electrical isolation needed in practical data networks (as opposed to running the entire network on your bench, all powered from the same extension strip, for example). The transformer manufacturer will state which IEEE 802.3 sub-standard it complies with. Usually, transformers for 1GBASE-T can work all the way down to 10MBASE-T speeds, and it seems that 10GBASE-T transformers retain similar performance, but it doesn't guarantee outright that a 10GBASE-T transformer will work with e.g. a 100BASE-T PHY chip, since some key parameters have changed.

But it certainly is at least theoretically possible to e.g. design a 10M/100M/1G/10G BASE-T digital signal processing chain in an FPGA (or a modern GPU, or an ASIC), attach it to an A/D-D/A front-end running at 800MS/S and using 10GBASE-T magnetics, and have a solution compliant with speeds from 10M to 10G - over 3 orders of magnitude. But in reality nobody designs such "wideband" digital processing chains, since it'd be wasted effort as 10G ports are so expensive that plugging anything slower into them is a total waste of money. Typically 10G PHY will be gigabit speeds only, often not even 1G but higher: 2.5G, 5G and 10G. Some are 10G only!

Twisted-pair Ethernet at speeds of 1GBit and beyond work like analog phone systems: a single pair is used to transmit data in both directions at once. Analog phone systems have a single differential "loop" connection: it's not that one wire transmits and another receives. It's a single closed circuit that works in two directions as the same time. It works on the principle that the sender has all the information needed to suppress its own signal from what it receives from the loop.

Each end of the connection is both a transmitter and a receiver. The transmitted signal is superimposed on the signal already existing on the circuit. The receiver then measures this summed signal, and subtracts its own transmitted signal from it. What's left is what the other transmitters on the link have been transmitting. Since such links are usually point-to-point, once you subtract your own transmission from the received signal, what's left is the other transmitted signal from the far end of the line.

The subtraction of the transmitted signal from the overall received signal has to be done whether we're talking about gigabit Ethernet or two-wire phone lines. On phone lines, the signals are slow enough that a passive device called a hybrid - essentially a transformer with a termination - can perform adequately. Such hybrids are present in analog phones and other simple phone equipment. The transformer-based approach doesn't work very well when e.g. geostationary delays are present, and is insufficient for high-speed modems, and thus even modems that work on phone lines have to implement a "hybrid" in the digital domain. They still use transformer for isolation, and may connect it in a hybrid fashion, but the performance of that transformer is inadequate by itself.

Once it became cheaper, in aggregate, to work on digital signals using chips that weigh micrograms rather than making and shipping chunks of iron and copper known as hybrid transformers (that individually weigh more than all the semiconductor chips in your PC, combined), the function of a hybrid in voice phone networks has been implemented by digital signal processing. As an example: a lowly ARM M0 microcontroller has enough computational capacity to implement an adaptive telephone hybrid in software, with excellent performance, and its manufacture uses much fewer resources than the manufacture of a telephone hybrid transformer. There are benefits to modern technology being lightweight that go way beyond mere convenience :) A lighter, simpler transformer is still there to isolate the phone line with its voltage transients from the relatively sensitive rest of the circuit.

As the data rates increase, so does the computational capacity needed to perform this separation of transmitted and received data, and of adapting to the imperfections of the cable plant (e.g. each connector/jack or kink causes echoes!). So, by the time you look at a 10GBASE-T PHY chip, it has about as many transistors as the Pentium III (Katmai): ~10 million. Of course only a part of this transistor budget is relegated to near- and far-echo cancellation and equalization, but it's still not an insignificant chunk. Guesstimating from block diagrams provided by e.g. Broadcom, a good 1/10th-1/3rd of the chip deals with just that (I'm not dealing with such chip design and don't have any rules of thumb handy to provide a narrower estimate). The digital filters used for echo cancellation and path equalization have lengths on the order of 100-1000T (taps). That means that the PHY has to do a few thousand 10-12bit multiplies and additions per each sampling period just to separate the transmitted signals from received signals, and to equalize away the imperfections of the cable system; and there's 800 MSamples/s needed to run 10GBASE-T. So we're talking on the order of 1TMACs/s (that is one Tera MACs/s, or one million million multiplies-and-additions per second).

You could emulate this using a modern GPU. For perspective: an early Intel HD Graphics platform from 2010 could perform maybe 0.03TMAC/s. Intel Iris Graphics introduced in 2013 could handle about 1TMAC/s at half precision - and would just about have the capacity needed to perform the equalization and echo cancellation for a single 10GBASE-T transceiver. And this is just to get "clean" digitized signals that you still have to demodulate, descramble, decode, and error-correct (and do the reverse when transmitting). That's easily another equivalent of a few hundred GFLOPs/s, I imagine, although quite a bit of it is not floating point but parallel bit operations and fast memory lookups.

An aside: you could have more than two transmitters distributed along the line, as long as their signals could be otherwise separated. Satellite positioning systems deal with this: all GPS satellites send at the same frequency, but their signals are each coded differently, so that if you use a code that decodes one satellite's data, all the other satellites's signals get turned into noise that can be separated from useful data. This is called code-division multiplexing. But this was truly an aside, since GPS is one-way-only: the transmitters are the satellites, and the "GPS devices" like the phones are receivers only. Attempting to transmit at GPS frequencies will get you in legal hot water, and these days fairly quickly at that (never mind that it's totally pointless: the satellites aren't listening to you!).

But you could in principle use code-division multiplexing to e.g. drop several 10GBit/s equivalent PHYs on a single "ether" - e.g. use baluns to couple those PHYs to a good old thinnet 75 Ohm coax. Then, let's say each of the PHYs would be limited to transmitting at 10MBit/s. You could have dozens of them, all transmitting at once, using different code parameters, and each one of them could instantly hear all the others and produce the 10MBit/s received stream from any other - in fact, with the resources of a custom 10GBASE-T PHY, it could decode many of those transmissions at once. So, with modern technology, a "good old" 75-Ohm coax 10MBit Ethernet could be switched from TDMA to CodeDMA (not CDMA!!) and allow collision-free network segments having the number of nodes typically present on those segments back-when (from a few to a few dozen). It would not be possible to get a full 10GBit/s bandwidth from just one coax, but I imagine that 1-3GBit/s would be possible... with each network card using computational resources of a thousand Cray-1 machines. Now you all know why they didn't have code division multiplex Ethernet in the 80s - it's quite elementary: Cray-1 had a production run of about 100 units. Had they made about 2000 of them, you could use each 1000 to put together a CodeDMA 10BASE-T node to demonstrate it all. Also - back then the necessary ADCs and DACs were mostly fiction, so the implementation would need to be done using slower converters with intermediate frequency translation, and the digital processing would have to re-translate the sub-bands into the wideband baseband time-series format, and then out of it on the transmit side. But of course, the limiting factor was the poor availability of supercomputers, mind you :) Yes, optical FFT maybe could be leveraged to implement some of those FIR segments. But back in the 80s that was fairly secret stuff :)

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.