ethernetratetransmission

Data rate/Line rate on the Ethernet interface

I got a question about the data rate of the ethernet interface and hope someone can give me some hints on that.

I know the calculation method of the PCIe interface, for example, PCIe Gen3 X1 lane: The data rate of single-lane should be 

8 Gb/s (Gen3 line rate) * 2 (TX/RX, full-duplex) / 8 (to Byte) = 2 GB/s

(128/130 encoding is ignored) So, how do we calculate the data rate of an ethernet interface? Take 1000base-T for example, we have 4 twisted pairs, to sum up to 1Gb data rate.

So one pair should provide a 250Mb data rate. It’s full-duplex so TX/RX provides 125Mb each at the same time. With that being said, the “line rate” of a 1000base-T interface is 125MHz (125Mb).

Do I understand it correctly about the speedrunning on the ethernet interface?

Solution

  • how do we calculate the data rate of an ethernet interface?

    Ethernet's nominal bit rate is generally defined at the top of the physical layer (L1). It includes preamble, SOF and IPG, but excludes all PHY-specific line encoding (PCS and PMA).

    This is done to make all PHY variants of the same speed 100% compatible with each other. You can convert 1000BASE-T to 1000BASE-LX to 1000BASE-SX and back to 1000BASE-T without any buffer drops.

    It’s full-duplex so TX/RX provides 125Mb each at the same time.

    No - the nominal bitrate runs each direction, simultaneously for full duplex links. Each 1000BASE-T lane transports 250 Mbit/s worth of "user" data.

    With that being said, the “line rate” of a 1000base-T interface is 125MHz (125Mb).

    Since the line rate is (usually) the PHY rate it's 1000 MBit/s, four lanes of 250 Mbit/s each.

    1000BASE-T does use a symbol rate of 125 MBaud since its PAM-5 modulation transports more than two bits per symbol. You might think that PAM-4 with exactly two bits would be sufficient, but the line code overhead eats up the rest. 1000BASE-T is already quite complex, it uses two-dimensional Trellis modulation plus scrambling to get across the wire (to produce a self-clocking signal, improve the signal/noise ratio and eliminate excess DC).

    The 1000BASE-X PHYs for fiber are much simpler. The PCS uses 8b10b to produce a binary stream of 1.25 GBd that can be directly used to modulate the laser.