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It seems that I do understand something about Direct Attach Copper (DAC) Cable and SFP/SFP+ terms.

According to the wikipedia 10G DAC cable also referred as 10GSFP+Cu, 10GBASE-CR or 10GBASE-CX1, however none of these terms are IEEE standard and I cannot find more information about it. So is there anyone who can answer for following questions:

  1. Am I right that all DAC cables use one twinaxial cable as media?
  2. Am I right that there cannot be any autonegotiation between two transceivers in one DAC cable, they just work at one fixed speed?
  3. Where can I find description about how data are modulated and demodulated before sending to the twinaxial cable and after receiving signal from cable for different speeds (1G, 10G, 100G, ...)?
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Am I right that all DAC cables uses one twinaxial cable as media?

Practically yes, but there's no way to be sure - and that's nothing to care about.

Am I right that there cannot be any autonegotiation between two transceivers in one DAC cable, they just work at one fixed speed?

Autonegotiation over DAC is uncommon but possible. Yet, it happens in PCS, 'above' the PMA and PMD sublayers involved in DAC.

Where can I find description about how data are modulated and demodulated before sending to the twinaxial cable and after receiving signal from cable for different speeds (1G, 10G, 1000G, ...)?

Theoretically, you can't. How the actual data is transported between DAC ends is solely at the manufacturer's discretion.

Practically however, a DAC needs to be cheap, so most use the SFP ports native encoding from PCS (e.g. 10GBASE-R with 64b66b) and transmit that over twinax (one pair for single-lane PHYs; quad-lane PHYs e.g. with 40G QSFP+ require four twinax pairs). The base PCS modulation is also used with most fiber PHYs and defined in IEEE 802.3.

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  • And "SFP ports native encoding" is the encoding used in to transmit data between MAC and SFP (over the TD+, TD-, RD+, RD- pins)? Commented Jun 8 at 18:11
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    The high-speed serial interface uses the underlying PCS encoding, just without the medium attachment inside the transceiver - 1000BASE-X, 10GBASE-R and so on.
    – Zac67
    Commented Jun 8 at 18:22
  • "Am I right that all DAC cables use one twinaxial cable as media?" No - I have 100Gbs DAC of 10 meters from Mellanox. Example at fs.com (no affiliation)
    – JFL
    Commented Jun 9 at 12:31
  • How it can be Direct Attach Copper if it made from fiber? Commented Jun 9 at 14:54
  • Direct-attach cable based on fiber is called active optical cable (AOC),. Sometimes "DAC" is applied as well, as direct-attach cable.
    – Zac67
    Commented Jun 9 at 16:23

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