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I think this original question was between devices within a computer, rather than within a single chip/package. Optical interconnects within high-end servers/r

by nsteel 3y ago

I think this original question was between devices within a computer, rather than within a single chip/package. Optical interconnects within high-end servers/routers are being designed today for use in the next generation. We previously reached the limits of copper pcb traces, coax has lots of problems, optical is next. It's not about latency, it's about signal integrity. Ironically, the heat produced by the optical transceivers is one of the biggest problems.

bgnn 3y ago
Yeah for within a computer, maybe. But then we're still using inefficient standards like DDR for memory access. There's sooooo much more we can do for copper interconnects while more viable optical tech is being developed. Think about denser modulation PAM4 is already in, PAM8 and PAM16 are being looked at. 10Gbit Ethernet uses a symbol density of 128 over a 100m long channel. We have the digital horsepower for encoding/decoding the data on-chip. I also see people are implementing full-duplex channels. This will double the density. Again, Ethernet is doing this for 30 years already but no chip-to-chip standard like PCIe, DDR etc are doing these yet. I'd think optical will replace back-plane side but not anything within a motherboard itself in the coming 10 years. Copper has a lot of juice left in it.
- nsteel 3y ago
  I'm not aware of anyone actually going to use pam8 or 16 with high-frequency links. The signal integrity is a mess and the corresponding FEC requirements undo all the benefits. I suppose a relatively high-speed chip-to-chip standard in use within consumer computers today is GDDR. If you want something with serdes then it's obviously PCIe, which has been using NRZ and now PAM4. Then there's Interlaken which has been around for yonks in HPC and networking. More recently NVLink also. None of these point-to-point serdes solutions can give you latency anywhere near DDR. And when they get to 200Gbps, copper backplanes are dead. It's cable now, and optical is coming. On the PCB, copper channels are thankfully holding out.

by nsteel 3y ago

bgnn 3y ago

Yeah for within a computer, maybe. But then we're still using inefficient standards like DDR for memory access. There's sooooo much more we can do for copper interconnects while more viable optical tech is being developed. Think about denser modulation PAM4 is already in, PAM8 and PAM16 are being looked at. 10Gbit Ethernet uses a symbol density of 128 over a 100m long channel. We have the digital horsepower for encoding/decoding the data on-chip. I also see people are implementing full-duplex channels. This will double the density. Again, Ethernet is doing this for 30 years already but no chip-to-chip standard like PCIe, DDR etc are doing these yet. I'd think optical will replace back-plane side but not anything within a motherboard itself in the coming 10 years. Copper has a lot of juice left in it.

nsteel 3y ago
I'm not aware of anyone actually going to use pam8 or 16 with high-frequency links. The signal integrity is a mess and the corresponding FEC requirements undo all the benefits. I suppose a relatively high-speed chip-to-chip standard in use within consumer computers today is GDDR. If you want something with serdes then it's obviously PCIe, which has been using NRZ and now PAM4. Then there's Interlaken which has been around for yonks in HPC and networking. More recently NVLink also. None of these point-to-point serdes solutions can give you latency anywhere near DDR. And when they get to 200Gbps, copper backplanes are dead. It's cable now, and optical is coming. On the PCB, copper channels are thankfully holding out.