My current understanding of the physical layer is:

I was told that when a network interface card (NIC) wants to send signal onto wire, it will low its voltage. And thus the signal will be imposed onto the wire.

NIC 1 lowers its voltage, and thus some electric potential difference will occur between itself and other NICs, and thus electric current will be intrigued. By detecting this current, signal can be detected.

But my question is:

  • If there are many other NICs connected to a shared wire, will the electric current be weakened?

  • Or do all the NICs only talks directly to a central device?
    (Such that a central device like a router, switcher or a hub is responsible to propagate the signal widely)

Could someone give some elaboration?

  • This is in many ways a rewording of your earlier question "Do all the network cards use the same frequency to send signals to wire?" superuser.com/questions/236410/…
    – Linker3000
    Jan 27, 2011 at 19:49
  • @Greg the problem is, that the most useful site is still main SO. Because it has most people there. And its not like it is buried in too many questions so that it is essential to remove them to other sites. Immediately someone is suggesting to create a site dedicated to hardware...
    – MK01
    Jan 27, 2011 at 22:21

7 Answers 7


How much time have you got? Seriously, this is a major topic but also one that's the subject of many standards and so doesn't really rely on someone here using their specific knowledge to write up something that is well documented elsewhere.

if you want to start with some serious basics, look up the OSI 7-layer model that defines the various functions performed by two entities communicating across some form of network media, and then zoom in on the physical layer where all the data transmission and reception takes place - below are some references. I expect this is the best you'll get here unless someone takes a long time to document what you need OR the question is closed as it's not really answerable here. The last link on CSMA/CD may help you understand how one form of multiple media access works.

7 layer model:



Physical layer data transmission (over twisted pair copper):




  • csma/cd carrier sensing multiple access / collision detection is a protocal that allow multiple device to communicate on the same media / cable.
    – nelaaro
    Mar 31, 2011 at 9:27

Here is an article from Rice University explaining how NICs work, specifically a 10 Gbps NIC.

  • Thanks for the link. Digging it now. Will be back later. :) Jan 27, 2011 at 17:02
  • I have briefly read the link. It's useful but maily about the interaction between the NIC and the OS, while my question is about how NIC transmit signal onto wire physically. Thanks all the same. Jan 27, 2011 at 17:12
  • @reggie, it's helpful if you provide more information than just a link without even any description text.
    – nhinkle
    Jan 27, 2011 at 21:59

But question is, if there're many other NICs connected to a shared wire, will the electric current be weakened?

I don't think there is or needs to be a lot of current: Ethernet works at radio frequencies.

But yes, I think it will be weakened. A "shared wire" might be the same thing as a "passive hub": I guess there's a limit to the number of ports in a shared hub (like there's a limit to the length of a cable).

So if you have many other NICs, you'd have an active hub or a router.

  • Yes, your "active hub" is my "central device". Thus this will be a star topology, rather than a bus topology. Jan 27, 2011 at 17:14
  • @smwikipedia I think that star and bus are the same (identical topologically) when the medium/hub is entirely passive/non-controlling. And an 'active hub' (as opposed to a 'router') might just power-boost the signal without imposing any extra control/routing/isolation.
    – ChrisW
    Jan 27, 2011 at 17:29
  • @Chris There's no such thing as a "passive hub" - hubs are basically a lot of repeaters put together. A NIC sends a signal which then gets repeated on all the other hub puorts. The reason there's a limit to the number of ports is CSMA/CD, not anything electrical.
    – user74554
    Apr 12, 2011 at 3:11

Search for information about pulse-amplitude modulation. This is modulation used in Ethernet.


..., and thus some electric potential difference will occur between itself and other NICs, ...

This assumpton misses details which thus makes this statement incorrect, add a router/hub in the equation.

If there are many other NICs connected to a shared wire, will the electric current be weakened?

A cable has two endpoints, so in normal situations there is no such thing as a lot of NICs connected to the same shared wire. So I suppose you mean you have the setting where you have a router connected to a hub and then that hub connected to some clients. The electric currents would not be weakened or else they would become susceptible to noise, if it was then my answer message would not reach your computer...

Or do all the NICs only talks directly to a central device?

Careful with words like talk (refers to a conversation) in a discussion about signals.

Data gets modulated, that signal is propagated and gets demodulated by the endpoint and stored in a buffer.

To avoid clashing one of those is used:

So, it depends on how the hub and routers handle this:

  • LAN <--> LAN is most likely CSMA.

  • LAN <--> WAN is most likely CSMA in a Hub and multiplexed in a Router.


Current flowing in CAT5 cable is negligible I believe. Data on pairs are detected differentially when the voltages are flipped indicating a bit, eight of these in a row is a byte on the wire, not necessarily a byte of data though.

To try to answer your questions: 1) No the impedance of differential inputs are typically high so low current flow - although this implies low noise thresholds. I guess the physical layer makes some compromise here. The twisted pair carrier and the differential nature of the signal detection mechanism goes a long way of negating the high impedance, I would guess.

2) Yes, typically in modern LAN's - most packet switch networks as far as I know.

  • A CAT5 cable has exactly two connectors. Obviously not the kind of connection the question was about. Nov 25, 2012 at 22:43
  • My God March 31 2011 sthysel, there's so much I need to tell you...
    – sthysel
    Nov 27, 2012 at 0:40

Linker3000 has the right idea for this - honestly, this is a large subject and it will require a lot of reading. But to (relatively) quickly answer your question...

All NICs conform to a standard for interoperability purposes. They do transmit information via voltage trops, which are translated as individual bits (0s and 1s) of data. Because they all do use a standard, if more than two NICs share a wire, there is the potential for more than one transmission over the same cable pair at one time. This led to CSMA/CD, as Linker3000 stated.

A long time ago, computers used to be connected in a bus or ring topology - multiple NICs would share a line. These topologies had a lot of problems (read the links that Linker3000 posted, I'm sure they describe a lot of them). Today, individual NICs are connected to a central device, as you speculated - groups of NICs connect to a central switch, allowing each NIC to have a line to itself.

So, to summarize:

1.) All NICs (nowadays) in a modern network topology talk to a central device. 2.) Each NIC has its own set of cable pairs that is only used by itself and the switch port on the other side. 3.) Because of this, you don't need to worry about NICs connecting to a shared wire (unless you run into a network that was built before 1995).

Hope this helps a bit, but you're still gonna have to do a lot of reading if you're looking for your NEtwork+ or CCNA certification (which, I'm guessing, is the reason for your question). Good luck.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

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