I'm a little confused about the "SERDES" interface between MAC and PHY chip, and I drew some figures to illustrate the connections which confuse me as shown below. The MAC controllers in 3 figures are the same, but the transceivers inside the MAC are not so straight forward to me. The 1st and 2nd figures are normal application which transmits the data through copper media with coded information (through PCS/PMD/PMA inside the PHY chip). The interface between the MAC and PHY is SGMII or XAUI for 1G and 10G base-T Ethernet. However, the 3rd figure confuses me. There are applications where the MAC is connected to the optical-electrical conversion element, and transmit the data with lasers and fiber cable.
My first question here is, where is the PHY function now (PCS/PMD/PMA) in this situation? Looks like the data is transmitting directly from the "SERDES" of MAC to the other MAC's SERDES, only the media transformed from electrical to optical, without any coding or manipulation as PHY chip does on copper application. Is there any reason why we can do that (or I should say, why we need PHY chip to transmit the data through copper media)? Is it because the 1000 Base-X follows the physical layer of FC0 (FC-PI, fibre channel physical interface) and not the OSI model? If that’s the case, why we don’t need the PCS/PMA/PMD stuff for fibre channel application? And any special design in the transceivers to achieve such goal?
The second question is, what's the difference between SGMII/XAUI and SERDES? It seems that the MAC chip can support both function on the same high-speed lanes. The SGMII/XAUI are usually used for the connection between MAC and PHY chip, where the “SERDES” is used for MAC direct connection. I used to believe that interface like SGMII or XAUI are all SERDES, but it confuses me now.
- The other question is, there's another connection called DAC (Direct Attached Copper), does it have anything to do with MAC to MAC connection? The signals now connect the MAC without PHY, why can we do that? If we can do so, why bother to add external PHY on the RJ45 scenario? Is is about the transmitting distance?
- Do we still need DC balance in fiber signaling? If not, why bother to encode the data to 8b/10b or 66b/64b? So many questions listed since I’m still new to the networking area. Hope someone can help me with this.
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So, the reason why we can use the MAC chip to directly connect to the SFP module(without PHY on the module, only optical transceivers) is that the MAC chip is supposed to have a built in PHY, is that correct?
In other words, can I assume that the data encoding, formatting, etc (all the PCS and PMA functions) are all done in the MAC chip now, instead of doing that in the external PHY chip?
If so, looks like 1000basex do need to go through PCS and PMA before the data can be sent to SFP module, is that true?
The key point which confuses me earlier is that I used to think that 1000base X didn’t require PCS and PMA, and can be connected directly to the SFP module to transfer the data from MAC logic. I grep a figure from Intel and Synopsys website and which may explain that the 1000Base-X still need PCS/PMA functions before going out the chip. If my assumption is correct, the common MAC chip may have built in two kinds of PHYs, and have a switch inside the chip to select which interface it should be running on by detecting the outside modules. For example, if it detects the SFP module with optical devices only, it’ll be configured to be a XFI interface. Please correct me if anything wrong here.
And, the reason why we need external PHY for RJ45+copper cable is because we need to transform SGMII into MDI interface to meet the requirement of the 1000Base-T requirement, right? In this case, the internal PHY may be configured as a pass-through mode, and use SGMII from MAC module to external PHY chip. Not sure if that’s correct.