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Questions about Ethernet..

Discussion in 'Networking' started by Book, Sep 29, 2008.

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  1. Book

    Book Thread Starter

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    I was trying to figure out some stuff about Ethernet. So ethernet is a "standard" that defines certain things on the physical and data link layer.

    1) Why is it not regarded as a protocol, since that's what a protocol does, it defines the way that machines can communicate on a certain layer. In that respect, ethernet is a protocol both on the data link and the physical layer. So I don't see any difference with higher level protocols like IP for example.
    2) How is it possible that there exist multiple "versions" of ethernet, since the protocol on the above layer (layer 3 for example) would require precise information on what packets it is going to accept. So at least the format of the frames and the packets should be consistent with all the verisons of ethernet.
    3) Why is ethernet 100BASE-TX and 10BASE-T defined over 4-pair twisted pair cable since they DON'T require the last 2 pairs? (BTW, could I run a second connection over the remaining pair?

    4) This is a somewhat different topic, but using the data link layer alone, is it impossible to contact a machine that is not directly connected to the wire you're transmitting the signals to?

    Thanks a lot for any clarification.
     
  2. JohnWill

    JohnWill Retired Moderator

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    Ethernet description: Ethernet Physical Layer

    Open Systems Interconnection Basic Reference Model

    Yes, you can run a second connection over the other two pairs of a standard CAT5 cable, adapters are readily available to provide the proper connections. Note that gigabit over copper uses all 8 wires, so doing this precludes gigabit connections.


    Sounds like you're catching up on your homework, you don't want us to do all of it, do you? ;)
     
  3. Book

    Book Thread Starter

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    Yeah, I read all about those links, but still, these are the conclusions that I have come to and have these questions. I'm just looking for brief answers, so I know that I am correct

    Is Cat5 cabling defined independently of ethernet? Cause if that's the case, then it makes slightly more sense that they chose it for the standard. but if it was defined with the standard or for the purpose of serving it, then they added 2 more pairs for nothing! Plus, wikipedia says, that it's possible to use the other 2 pairs for other things but its' not recommended (doesn't say why, though).
     
  4. JohnWill

    JohnWill Retired Moderator

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    I think the questions you are asking probably defy "brief" answers. ;)


    Re the extra wires, people sometimes use them for telephone connections. However, mistakenly crossing the 90V ringing voltage, or even the 50VDC open line voltage with your Ethernet NIC will probably kill it, so it's not a good idea. I've personally used them for a second 100mbit Ethernet link, and it's never been a problem. Since adapters are commercially available to split out the connections, I can only presume that I'm not the only one that does this.

    I honestly don't know if CAT5 was specified independently, never needed to know, and it wasn't important enough to research. :)
     
  5. Book

    Book Thread Starter

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    I think they were developed separately, because the CatX cables are defined by another committee of some sort. Check this: http://en.wikipedia.org/wiki/TIA/EIA-568-B

    So I guess they picked this cable because no other had at least 2 pairs and met their specifications? Don't know..
    Anyway, for my other questions, I really need little responses. If any other member can comment on them please add a reply. I'll reform them so you can answer with a simple yes or no, if you want leave extra comments:

    1) Can "Ethernet" be regarded as a protocol of the first and second layers of the OSI model since it defines how communication is performed at that level? (In that respect, the various versions of it, for example 10BASE-T and 100BASE-TX and the so many others that are not over twisted pair cables, are just different protocols for the same layers of the OSI model).
    2) Using protocols in the data link layer alone, is it impossible to contact a machine that is not directly connected to the wire you're transmitting the signals to?

    Thanks
     
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