Ethernet bit rate

[engr_joni_ee]

The Ethernet bit rate which are common these days are 1 Gbps and 10 Gbps. The question is how do we relate Gbps to the clock frequency of the digital signal and time period. For 1 Gbps the digital signal is itself 1 GHz or 500 HMz ?

 

[KlausST]

Hi,

wikipedia?
IEEE specifications?
Internet search?

What media?
What "clock frequency" are you talking about? On an standard copper ethernet cable there is no dedicated clock signal.

Klaus

 

[engr_joni_ee]

By clock frequency I mean cycle per second measured in Hz. Is that 2 bits per Hz ? when we compare with the bit per second rate

 

[KlausST]

Hi,

again: which standard are you talking about? There are several. Please do a little research on your own to better elaborate your question.


Klaus

 

[engr_joni_ee]

Yes, there are different standards for 1 Gpbs Ethernet. I am pointing to the one that is relevant to the PCB design in which there are four differential pairs between Ethernet PHY and the Ethernet connector.

 

[FvM]

I am pointing to the one that is relevant to the PCB design in which there are four differential pairs between Ethernet PHY and the Ethernet connector.

If you are designing a PCB with a PHY you must surely know the name of the standard, isn't it?

1000BASE-T transmits 4 x 250 MBPS over 4 pairs. The respective fundamental frequency is 125 MHz.

 

[engr_joni_ee]

If you are designing a PCB with a PHY you must surely know the name of the standard, isn't it?

1000BASE-T transmits 4 x 250 MBPS over 4 pairs. The respective fundamental frequency is 125 MHz.

It look like that 1000BASE-T is 500 Mbps one way and 500 Mbps the other way, as out of four, two differential pairs belong to Tx and the other two differential pars belong to Rx. In total there are four differential pairs in RJ45 connector.

What I am getting is 1 Gbps is not both ways, right ?

 

[FvM]

It's communicating at 1GBPS full-duplex through hybrids. You should really review IEEE 802.3 Section 3, or at least ask Wikipedia.

--- Updated Apr 18, 2023 ---

 

[engr_joni_ee]

I guess 1000BASE-T is somehow clear to me. I have a look at the ZYNQ SOC development board called ZYBO Z-7 in which there is MAC included in the ZYNQ SOC and Ethernet PHY 1000 M is mounted on the board. There are four Tx and four Rx in addition to clock and control between MAC (ZYNQ) and the Ethernet PHY. I guess there is 125 Mbps on each Tx and Rx and two bits per cycle (DDR), so in actually it is 8 X 125 Mbps on Tx and same on the Rx that makes 1000 Mbps each way.

Between the Ethernet PHY and the RJ45 connector, there are four differential pairs on the development board. I guess there is phase-amplitude modulation for symbols, right ? and all four differential pairs are used for Tx and same four differential pairs used for Rx, right ?

 

[KlausST]

Hi

Now you talk about two different interfaces:
* MAC <> PHY
* PHY <> Cable
And we have to guess...

Klaus

 

[engr_joni_ee]

Yes, I am referring to both interfaces, on the PCB.
MAC and PHY
PHY and Cable
Please have a look at post #8 in which I ask my question.

 

[FvM]

Ethernet line and MAC interface are using completely different data encoding. The MAC interface is using RGMII standard, as written in the Zybo reference manual, read about it. https://en.m.wikipedia.org/wiki/Media-independent_interface#RGMII
For the 1000BASE-T line interface, you can distinguish between bit and symbol rate. Symbol rate is actually 125 Mbaud per twisted pair line, resulting in 62.5 MHz analog bandwidth.

 

[engr_joni_ee]

MAC to PHY:
ZYBO Z-7 (ZYNQ SOC) implement 1000BASE-T. MAC inside the ZYNQ SOC. There are four Tx lanes and four Rx lanes in addition to clock and control between MAC (ZYNQ) and the Ethernet PHY.

Question on the signal description on Tx and Rx lanes:
Is the data on Tx and Rx lanes sampling both edges making it DDR ?
Is that 125 Mbps x 8 on four Tx lanes ? that makes 1000 Mbps each way ?

PHY to Wire/Connector:
Ethernet PHY 1000 M is mounted on the ZYBO board.

Question on the signal description between PHY and Connetcor:
Is there phase-amplitude modulation for symbols ?
Are there four differential pairs between PHY and Connector RJ45 ?
All four differential pairs used for Tx ?
All four differential pairs used for Rx ?
If yes, then how it is full duplex 1000 Mbps ?

 

[FvM]

I believe you can read the descriptions like quoted Wikipedia entry about RGMII yourself, I don't feel a need to retell it.

As for 1000BASE-T, I quote the introducing description from IEEE 802.3 which has all key parameters.

40.1.3 Operation of 1000BASE-T

The 1000BASE-T PHY employs full duplex baseband transmission over four pairs of Category 5 balanced cabling. The aggregate data rate of 1000 Mb/s is achieved by transmission at a data rate of 250 Mb/s over each wire pair, as shown in Figure 40–2. The use of hybrids and cancellers enables full duplex transmission by allowing symbols to be transmitted and received on the same wire pairs at the same time. Baseband signaling with a modulation rate of 125 MBd is used on each of the wire pairs. The transmitted symbols are selected from a four-dimensional 5-level symbol constellation. Each four-dimensional symbol can be viewed as a 4-tuple (An, Bn, Cn, Dn) of one-dimensional quinary symbols taken from the set {2, 1, 0, –1, –2}. 1000BASE-T uses a continuous signaling system; in the absence of data, Idle symbols are transmitted.

Idle mode is a subset of code-groups in that each symbol is restricted to the set {2, 0, –2} to improve synchronization. Five-level Pulse Amplitude Modulation (PAM5) is employed for transmission over each wire pair. The modulation rate of 125 MBd matches the GMII clock rate of 125 MHz and results in a symbol period of 8 ns.