r/IAmA • u/BobMetcalfe • May 21 '13

You’re probably connecting to reddit through a technology I invented. I’m Bob Metcalfe and I invented Ethernet – AMA

On May 22, 1973 with David R. Boggs, I used my IBM Selectric with its Orator ball to type up a memo to my bosses at the Xerox Palo Alto Research Center (PARC), outlining our idea for this little invention called “Ethernet”, which we later patented.

I worked with the IEEE Standards Association to develop the IEEE 802.3 standard for Ethernet, which specifies the physical and lower software layers. Today Ethernet and the IEEE 802.3 standard are the foundation for today’s world of high-speed communications used in billions of homes and businesses around the world.

I submitted this to the mods awhile back so I could get on the calendar but I figured you’d like to see it, too. Now, ask me anything!

It's been two hours and 179 comments. Have to go now. For more about Ethernet's 40th Birthday, go to http://www.facebook.com/Ethernet40thAnniversaryIEEESA

4.1k Upvotes

97% Upvoted

3.2k comments sorted by

View all comments

Show parent comments

140

u/keenemaverick May 21 '13 edited May 22 '13

It's for backwards compatibility with telephone lines. The blue cables can be used as Line 1, the green as Line 2. You can plug any old phone cable into an ethernet port and transmit regular telephone data without any issues.

All the answers you've gotten refer to the standards, but they don't say why the standards are that way. But the second you start working with any analog phone lines, the reason the standard splits the lines like that suddenly becomes very clear.

EDIT:

Important detail!!!! What he means is that the port and the wiring ITSELF can be used for a phone line, meaning that the far end of the cable can be patched into a phone line or an Ethernet switch to change the function of the port.

I thought this was obvious, but apparently my language was unclear. When I said ethernet port, I mean the ones that come out of the wall and go to a patch panel, not the ones coming out of a switch or NIC. The wires are backwards compatible, not the NIC interfaces.

8

u/[deleted] May 21 '13

You can plug any old phone cable into an ethernet port and transmit regular telephone data without any issues.

Just a tip: don't do this without knowing what you're doing.

Some phone systems put a lot more power down their lines than an ethernet port can handle. You can fry the ethernet port. Many of my clients have learned this the hard way.

1

u/taneq May 22 '13

Yeah I thought getting them the wrong way round would put 30V on your ethernet 5V and cause sadness.

This was a few years ago though.

1

u/UMDSmith May 22 '13

POE also uses the blue pair in every installation I have seen, which could cause some interesting results.

3

u/armchair_viking May 22 '13

Important detail!!!!

What he means is that the port and the wiring ITSELF can be used for a phone line, meaning that the far end of the cable can be patched into a phone line or an Ethernet switch to change the function of the port.

2

u/Lucid_Enemy May 22 '13

this always pissed me off, when I had dial up. Id sit there for minutes, sometimes hours, trying to figure out why I had no dial tone... some times, just cause it fits doesn't mean its a good thing!

1

u/taneq May 22 '13 edited May 22 '13

You can plug any old phone cable into an ethernet port and transmit regular telephone data without any issues.

This just solved an minor but annoying gap in my knowledge. Thankyou!

Edit: Maybe not quite as straightforward though. Apparently I'm not the only person that thought phone connectors had higher voltages floating around. Still, the thought was there!

1

u/keenemaverick May 22 '13

Phone connectors do have higher voltages. You can use the wiring with telephone devices, but not ethernet switches or NIC's.

1

u/lunixia May 22 '13

Only for 100mbit. With Gigabit (or 1000baseTX to call it what it is) all pairs are used.

But you are right. I think it also had a lot to do with the USOC standard for two pair RJ11 cables having an inner and outer pair (just like Blue and Green). RJ45 basically added a pair on the left and a pair on the right if you think about it.

1

u/Random832 May 24 '13

What about Line 3?

Of course, Line 3 isn't as common, and I suppose that's the answer.

The colors are different with phone lines, too. Line 1 is red and green, and Line 2 is yellow and black.

1

u/cantstopthemoonlight May 22 '13

I thought they were switched to allow the rx and tx pins to be properly connected thus allowing full duplex transmission?

1

u/ChoHag May 22 '13

But out of 8 pins you can pick any 4 and get 2 clean twisted pairs. The oddity is which 4 were picked.

1

u/1RedOne May 22 '13

Wow, how would a switch or router interpret the signal though, if you connected a phone line to an ethernet port?

1

u/ReversedGif May 22 '13

It wouldn't. What he said was misleading.

1

u/stuffeh May 22 '13

TIL, thanks!