This seems to be a day for odd questions, and I just thought of one.
The Ethernet plugs we use at work are wired for the "568B" standard, with the orange wires on pins 1 and 2, and the green wires on pins 3 and 6.
Here's the question: _why_? I've been told that the whole idea with Ethernet is to avoid "Near end crosstalk", so it seems to me that the best way to do that would be to put one pair on pins one and two, and the other on pairs seven and eight. How did we wind up with 568B?
Bill
Early on, there was an idea that you might want to put a voice pair on
4/5. Then an eight-position modular plug wired for single-line voice would work if plugged in to a data line. I suppose if you were running the original AT&T StarLAN over category-3 wiring, this might have worked. AIUI, MIT's internal phone wiring is still done very much like that: all the home runs end up at a distribution frame in the closet, and then are cross-connected to analog voice, ISDN, or an Ethernet switch. (We run our own network in my building, and we use VoIP, so all of that wiring lies fallow in the nearly-empty phone closets.)Gigabit Ethernet (and higher speeds) require all four pairs, and in fact use them bidirectionally. (That's why "crossover" cables are no longer useful.)
-GAWollman
Disclaimer reinformcement: I may work for MIT, but they don't pay me enough to speak for them.
568B supersedes 568A. 568B is also known as "AT&T Configuration" and is common in wired installations. 568A, when still found, is typically in jumper cables. 568B is essentially mandatory for 100 Mbps and faster.
Both 568A and 568B are now superseded by 568C as of February 2009. :-)
Brief details of that can be found in this document:
Search for 568 in the above PDF for some background. Here's a brief clip copy'n'pasted from page 21 of that PDF: " " In February 2009, TR-42 published a new " standard: ANSI/TIA-568C.0, Generic Telecommunications " Cabling for Customer Premises. " This publication created a foundation for three " types of documents (common standards, " premises standards and component standards) " and now becomes the TR-42 standard covering " cabling topologies, design, distances and " outlet configurations, as well as specifics for " cabling infrastructure in all locations. Later In " 2009, TR-42 published the ANSI/TIA-568-C.2, " Balanced Twisted-Pair Telecommunications " Cabling and Components Standard, covering " the component and performance requirements " for balanced twisted-pair cabling. " [...] " TIA-568-C.1 was published with " TIA-568-C.0. The publication of ANSI/TIA-568-C.2, " Balanced Twisted-Pair Telecommunications " Cabling and Components Standard, completed " the third revision of the 568 standard, since " ANSI/TIA-568-C.3, Optical Fiber Cabling Components " Standard, was published in 2008.
A simple diagram comparing 568A and 568B is here:
Unfortunately, copies of ANSI standards cost big bucks; the (expletive deleted) standard for tripod camera attachments cost me US$35 IIRC and that was only ~15 pages.
We 'wound up' with 568B because people didn't like 568A.
The 8P8C pin-out is 'upward compatible' with the RJ-11/-14, so 'pair 1' is on the center pins (4 & 5), 'pair 2' is the 'surrounding' pair (3 & 6), and the remaining 2 'pairs' were put on sets of adjacent pins (1&2 , 7&8).
10-Base-T Ethernet was designed to _share_ a 4-pair cable with a phone line. the phone is on pair 1, for compatibility, and 'data' went on pairs 2 and 3.Now, -if- the two Ethernet pairs were on 1&2, 7&8, the separation between 'tip' in the two pairs would have been the same as the separation between 'ring' in the two pairs. Meaning the inductive coupling would have been roughly _equal_, effectively _maximizing_ the induced signal in the differential circuit. By placing the conductors ad -different- distances, one gets different amounts of induced signal in each wire -- this leads to the 'excess' amount on one wire being 'ignored' by the differential receiver, so that, effectively, only the _lesser_ of the two induced signals is 'seen' by the attached equipment.
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