4 ms·

Theoretically yes. Though it makes 0 sense and difference in practice. The conversion between electrical and optical isn't straightforward and it isn't cheap to

by bgnn 3y ago

Theoretically yes. Though it makes 0 sense and difference in practice. The conversion between electrical and optical isn't straightforward and it isn't cheap to implement now. Let's say we've resolved this. We are still limited by the same latency of the channel, i.e. speed of light is roughly the same in fibre-optics and metal interconnects we use on chip now for electrical signals. Meaning, electrical signals we use are as fast as light. So, no delay advantage. The biggest advantage of optical is less loss, which makes a huge difference for a long channel. By this I mean really long, like meters. Till that point it has literally no advantage while it is so much more complicated.

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 bgnn 3y ago

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.