Why Light Peak will run on copper

Why Light Peak will run on copper

Summary: A recent report that the Intel/Apple Light Peak will initially run over copper may have you scratching your head: an optical connection over wire? But its been done before. Here's the how and why.

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TOPICS: Hardware, Storage
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A recent report that the Intel/Apple Light Peak will initially run over copper may have you scratching your head: an optical connection over wire? But its been done before. Here's the how and why.

Getting the optics right Fact #1: Copper is faster and cheaper than glass. Signals propagate faster through copper than glass, but that isn't the reason for going with copper over short distances.

The real win is cost: optical connectors rely on lasers and precision optical components that aren't cheap. They'll get there, but for most early users copper links will be fine.

Fact #2: Copper is widely used today in the storage world's high-end Fibre Channel for the same cost reasons. Most FC gear has a swappable transceiver module so you can choose to use copper or glass as needed.

Fact #3: The transceiver is where the choice between copper and fiber is made. It doesn't matter to the electronics on the other side of the transceiver. Therefore it doesn't affect your apps or the rest of the system architecture.

The tradeoffs Glass fiber has some clear differences with copper, but they're only advantages if you need them.

  • Distance. Glass FC is spec'd for 10,000 meters. Twisted pair FC for 33 meters.
  • Speed. Glass FC currently supports 8 GB/sec, while copper maxes out at 2 GB/sec.
  • EMI. Electro-Magnetic Interference is a growing problem with high-speed signalling. USB 3.0 is at the ragged edge of acceptable EMI today - and standards may get stricter. Glass is cleaner and much harder to tap as well.

The Storage Bits take Given Light Peak's purported 10 Gb/sec speed, copper Light Peak will be lucky to support a 2 meter cable - plenty for notebook users - the same as copper eSATA. In a docking configuration, the dock could have optical transceivers for plugging into high-performance remote storage.

It will take time to get optical component production to the level that cuts prices. The same was true of optical Fibre Channel.

Having cheaper Light Peak components is the goal, and copper is the fastest way to achieve that. Longer term though, optical will enable faster and more flexible configurations.

Comments welcome, of course.

Topics: Hardware, Storage

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  • RE: Why Light Peak will run on copper

    "Fact #1: Copper is faster and cheaper than glass."

    "Speed. Glass FC currently supports 8 GB/sec, while copper maxes out at 2 GB/sec."

    It is like eating your cake and having it, too.
    Steve Webb
  • Has me scratching my head

    You cite as fact number 1 that "Fact #1: Copper is faster . . . than glass. Signals propagate faster through copper than glass . . ." But then you write "Glass FC currently supports 8 GB/sec, while copper maxes out at 2 GB/sec." If I assume that point 2 doesn't contradict point 1 and focus only on point 1, isn't that signal propagating through the optical fiber travelling at the speed of light?
    willhoyt
    • RE: Why Light Peak will run on copper

      @willhoyt Good questions - I should have been clearer about the difference between the 2 metrics.

      Signal propagation speed is different than the bandwidth of the medium. Glass offers more bandwidth today because of EMI issues with copper. The wires act like antennas broadcasting interference.

      Technically, the capacitance of glass is higher than that of copper, which accounts for the slower propagation. Think of it this way: in copper, the electrons are lined up like billiard balls - put in another ball and one on the end is immediately pushed out. With glass, the photons actually have to travel the length of the fiber - and they are bouncing off the walls of the fiber rather than proceeding in a straight line.

      Robin
      Robin Harris
      • succinct and clear, thanks

        @Robin Harris That is the most succinct explanation of the differences between the two media I've ever encountered.

        So I take it glass will provide "faster" transfers from something like multiplexing?
        pgit