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Got a question about the voltage level modulation on ethernet interface. We use PAM3 for 100base-T, PAM5 for 1000base-T and PAM16 for 10G. However, looks like we’re using PAM4 for 100G and 200G application. Does someone know why we’re doing so? Why the PAM levels didn’t increase when the speed is growing?

Nobody
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PAM increases the information density per transfer step and thus decreases the required transfer stepping speed for a certain bandwidth. PAM-4 transfers two bits of information with each step, PAM-16 four bits and so on, halving or quartering the transmission frequency.

With copper, frequencies and stepping speeds are very limited, so even Fast Ethernet (100BASE-TX) had to use it for Cat-5 cabling, to stay inside 31.25 MHz spectral bandwidth. 1000BASE-T expanded on that so it could get away with 62.5 MHz bandwidth on the same cable type.

Fiber can run at a much higher signal frequency but there are still limits for the hardware - currently, ~50 GBd is this limit for the modulation frequency. So, anything faster either requires multi-bit transfers or multiple lanes (separate fiber pairs or wavelengths). Since the latter is more expensive (today), very fast PHYs increasingly use PAM on fiber.

Zac67
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  • Thanks Zac for your feedback. Can I understand it this way: Since the physical limitation of copper media, instead of increasing the transmission frequency, it needs more levels to transmit, so that the total transmitted data can be increased. And for Fiber, since the frequency is much hier than copper, it can use just PAM4 to reach the speed which copper can't reach even with PAM 16 or more. – Nobody May 27 '20 at 16:10