I was wondering how Ethernet equipment (for example a switch) knows that something have been plugged in and is up ?
Is there a pair of copper wire dedicated for that ? Or a bit exchange ? On which wire ?
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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 provide and accept your own answer.– Ron Maupin ♦Commented Feb 19, 2018 at 20:38
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2 Answers
It's called Autonegotiation: Wikipedia
Ports looking for a link send "link pulses" on the data lines that (mostly) convey speed(s) and half/full duplex capabilities. When both sides agree on common link parameters, the link comes up. Some Ethernet variants need to negotiate additional parameters, e.g. 1000BASE-T negotiates the clock master/slave and data pair matching. For precise details, check IEEE 802.3 Clauses 28, 37, and 73.
In the absence of autonegotiation - when it is deactivated on a port and a certain speed and duplex mode is manually set - the other side is (usually) able to sense the port speed by line behavior/encoding. However, the duplex mode cannot be sensed and is a common source of problems with autonegotiation disabled.
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1Please consider adding more details so this answer remains useful without depending on the availability of that specific link. Commented May 30, 2017 at 10:03
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1I didn't say 'copy the entire article', just the most relevant information. Pointing to an external source for a more in depth explanation is acceptable. And I'm especially fascinated here because also without autonegotiation devices are able to detect link state. Commented May 30, 2017 at 13:54
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Sorry for being terse, this isn't too hard to find once you know its name - I've added a short summary.– Zac67 ♦Commented May 30, 2017 at 16:19
Basically, for "knowing that something have been plugged in", there is a specific mechanism on the physical layer called "link pulse"
These link pulses are (now) included in the whole "autonegociation" mechanism but were existing prior (this is why your 10Mb hub was able to sensing something plugged in)
There is a really good Wikipedia article on this https://en.wikipedia.org/wiki/Autonegotiation#Electrical_signals
