Comment by direwolf20

16 days ago

Gigabit Ethernet uses four pairs per direction. It uses the same four pairs in both directions at the same time.

7 comments

direwolf20

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estimator7292  16 days ago

1000Base-T uses two pairs per direction, actually. It's full duplex. Each port sees two TX and two RX pair.

There are four pair of wires in the cable. If you use all of them for TX, you can't receive.

  • cpgxiii  16 days ago

    > There are four pair of wires in the cable. If you use all of them for TX, you can't receive.

    No, you absolutely can use them all for transmit and receive at the same time. The device at each end knows what signal it is transmitting, and can remove that from the received signal to identify what has been transmitted by the other end.

    This is the magic that made 1000Base-T win out among the candidates for Gige over copper, since it required the lowest signaling frequencies and thus would run better over existing cables.

  • anonymousiam  16 days ago

    1000Base-T uses four pairs in both directions at the same time. It does this through the use of a hybrid in the PHY that subtracts what is being transmitted from what is received on the wires. 802.3ab is a fairly complicated specification with many layers of abstraction. I spent a few months studying it for a project about a decade ago.

  • direwolf20  16 days ago

    That's not true. https://en.wikipedia.org/wiki/Gigabit_Ethernet#1000BASE%E2%8...

    • robocat  16 days ago

      Relevant section:

        Autonegotiation is a requirement for 1000BASE-T implementations as minimally the clock source for the link has to be negotiated, as one endpoint must be master and the other endpoint must be slave.

  1000BASE-T uses four cable pairs for simultaneous transmission in both directions through the use of echo cancellation with adaptive equalization. Line coding is five-level pulse-amplitude modulation (PAM-5).

  Since autonegotiation takes place on only two pairs, if two 1000BASE-T interfaces are connected through a cable with only two pairs, the interfaces will complete negotiation and choose gigabit as the best common operating mode, but the link will never come up because all four pairs are required for data communications.

  Each 1000BASE-T network segment is recommended to be a maximum length of 100 meters and must use Category 5 cable or better.

  Automatic MDI/MDI-X configuration is specified as an optional feature in the standard that is commonly implemented. This feature makes it safe to incorrectly mix straight-through and crossover-cables, plus mix MDI and MDI-X devices.

      (Slight edits)

  • eigen  15 days ago

    > 1000Base-T uses two pairs per direction, actually. It's full duplex. Each port sees two TX and two RX pair.

    you may be thinking of 1000Base-TX (TIA‐854) which uses 2 pairs in each direction, similar to 100Base-TX (IEEE 802.3u). whereas 1000Base-T (IEEE 802.3ab) uses all 4 pairs in both directions.

    basically, the -TX are dual simplex with a set of wires for each direction and -T are full-duplex with the same wires used in both directions at the same time.