[HN Gopher] Copper is Faster than Fiber (2017) [pdf]
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Copper is Faster than Fiber (2017) [pdf]
Author : tanelpoder
Score : 55 points
Date : 2025-07-01 16:05 UTC (2 days ago)
(HTM) web link (www.arista.com)
(TXT) w3m dump (www.arista.com)
| MadVikingGod wrote:
| So the findings here do make sense. For sub 5m cables directly
| connecting two machines is going to be faster then having some
| PHY in between that has to resignal. I'm surprised that fiber is
| only 0.4ns/m worse then their direct copper cables, that is
| pretty incredible.
|
| What I would actually like to see is how this performs in a more
| real world situation. Like does this increase line error rates,
| causing the transport or application to have to resend at a
| higher rate, which would erase all savings by having lower
| latency. Also if they are really signaling these in the multi GHz
| are these passive cables acting like antenna, and having a
| cabinet full of them just killing itself on crosstalk?
| laurencerowe wrote:
| > So the findings here do make sense. For sub 5m cables
| directly connecting two machines is going to be faster then
| having some PHY in between that has to resignal. I'm surprised
| that fiber is only 0.4ns/m worse then their direct copper
| cables, that is pretty incredible.
|
| Surely resignaling should be the fixed cost they calculate at
| about 1ns? Why does it also incur a 0.4ns/m cost?
| cenamus wrote:
| Light speed is ~3ns per metre, so maybe the lowered speed
| through the fibre?
|
| Speed of electricity in wire should be pretty close to c (at
| least the front)
| b3orn wrote:
| It's c, but not the same c as in air or vacuum. The same
| applies in optic fibers. They're both around two thirds of
| the speed of light in vacuum.
| Sesse__ wrote:
| c is the speed of light in vacuum.
|
| EM signals move at about 0,66c in fiber, and about 0,98c
| in copper.
| BenjiWiebe wrote:
| More like 0.6c to 0.75c in Cat6 Ethernet cable.
|
| The insulation slows it down.
| GuB-42 wrote:
| c is constant, the speed of light is not.
|
| c is the speed of light in a vacuum, but it is not really
| about light, it is a property of spacetime itself, and
| light just happens to be carried by a massless particle,
| which, according to Einstein's equations, make it go at c
| (when undisturbed by the medium). Gravity also goes at c.
| bigfishrunning wrote:
| I've always considered C the speed of light and gravity
| goes at the speed of light, not that light and gravity
| both go C, which is a property of spacetime. This is a
| much simpler mental model, thanks for the simple
| explanation!
| Eldt wrote:
| I've always thought of c as the speed limit of causality
| myself248 wrote:
| Velocity factor in most cables is between 0.6 and 0.8 of
| what it is in a vacuum. Depends on the dielectric material
| and cable construction.
|
| This is why point-to-point microwave links took over the
| HFT market -- they're covering miles with free space, not
| fiber.
| jcims wrote:
| I always thought it was about reduced path length.
| Interesting.
| cycomanic wrote:
| It's both. Those links try to minimise deviation from the
| straight link (and invest significant money to get
| antenna locations to do that), but they also use
| copper/coax cables for connecting radios as well as
| hollow core fibre for other connections to the modems.
| laurencerowe wrote:
| I misremembered the speed of electrical signal propagation
| from high school physics. It's around 2/3rds the speed of
| light in a vacuum not 1/3rd. The speed of light in an
| optical fibre is also around 2/3rds the speed in a vacuum.
|
| It seems there is quite a wide range for different types of
| cables so some will be faster and others slower than
| optical fibre.
| https://en.wikipedia.org/wiki/Velocity_factor
|
| But the resignalling must surely be unrelated?
| Palomides wrote:
| high speed links all have forward error correction now (even
| PCIe); nothing in my small rack full of 40Gbe devices connected
| with DACs has any link level errors reported
| p_l wrote:
| DACs don't cause problems, but twisted pair at 10Gig is a PITA
| due to power and thermals
| somanyphotons wrote:
| What allows DACs to avoid the power/thermal issues that
| twisted pair has?
|
| (My naive view is that they're both 'just copper'?)
| kijiki wrote:
| DACs are usually twin-ax, which is just 2 coax cables
| bundled. The shielding matters a lot, compared to
| unshielded twisted pairs.
|
| Faster parallel DACs require more pairs of coax, and thus
| are thicker and more expensive.
| tcdent wrote:
| PHYs are going away and fiber is going straight to the chip
| now, so while the article is correct, in the near future this
| will not be the case.
| sophacles wrote:
| The chip has a phy built into it on-die you mean. This
| affects timing for getting the signal from memory to the phy,
| but not necessary the switching times of transistors in the
| phy, nor the timings of turning the light on and off.
| Hilift wrote:
| Storage over copper used to be sub optimal but not necessarily
| due to the cable. UDP QUIC is much closer to wire speed. so 10
| GB copper and 10 GB fiber are probably the same, but 40+ GB
| fiber is quite common now.
| bhaney wrote:
| > I'm surprised that fiber is only 0.4ns/m worse then their
| direct copper cables
|
| Especially since physics imposes a ~1.67ns/m penalty on fiber.
| The best-case inverse speed of light in copper is ~3.3ns/m,
| while it's ~5ns/m in fiber optics.
| jerf wrote:
| "Has lower latency than" fiber. Which is not so shocking. And,
| yes, technically a valid use of the word "faster" but I think I'm
| far from the only one who assumed they were going to make a
| bandwidth claim rather than a latency claim.
| kragen wrote:
| I assumed they were going to make a bandwidth claim and was
| prepared to reject it as nonsense.
| jcelerier wrote:
| I wonder where does the idea of "fast" beign about throughput
| comes from. For me it always, always only ever meant latency.
| switchbak wrote:
| A 9600 baud serial connection between two machines in the
| 90's would have low latency, but few would have called it
| fast.
|
| Maybe it's all about sufficient bandwidth - now that it's
| ubiquitous, latency tends to be the dominant concern?
| p_j_w wrote:
| Presumably from end users who care about how much time it
| takes to receive or send some amount of data.
| nine_k wrote:
| Latency to the first byte is one thing, latency to the _last_
| byte, quite another. A slow-starting high-throughput
| connection will bring you the entire payload faster than an
| instantaneously starting but low-throughput connection. The
| larger the payload, the more pronounced is the difference.
| mouse_ wrote:
| ehh... latency is an objective term that, for me at least,
| has always meant something like "how quickly can you turn
| on a light bulb at the other end of this system"
| rusk wrote:
| Term under discussion is "speed" which goes beyond
| latency. If you have a low latency but high bandwidth the
| link is "faster" i.e "time to last byte"
|
| Latency is well defined and nobody is quibbling on that.
| wat10000 wrote:
| Until pretty recently, throughput dominated the actual human-
| relevant latency of time-until-action-completes on most
| connections for most tasks. "Fast" means that your downloads
| complete quickly, or web pages load quickly, or your e-mail
| client gets all of your new mail quickly. In the dialup age,
| just about everything took multiple seconds if not minutes,
| so the ~200ish ms of latency imposed by the modem didn't
| really matter. Broadband brought both much greater throughput
| and much lower latency, and then web pages bloated and you
| were still waiting for data to finish downloading.
| vlovich123 wrote:
| Faster only because the distances involved are short enough that
| the PHY layer adds significant overhead. But if you somehow could
| wave a magic wand and make optical computing work, then fiber
| would be faster (& generate less heat).
| throw0101d wrote:
| > _Faster only because the distances involved are short enough
| that the PHY layer adds significant overhead._
|
| This specifically mentions the 7130 model, which is a
| specialized bit of kit, and which Arista advertises for
| (amongst other things):
|
| > _Arista 's 7130 applications simplify and transform network
| infrastructure, and are targeted for use cases including ultra-
| low latency exchange trading, accurate and lossless network
| visibility, and providing vendor or broker based shared
| services. They enable a complete lifecycle of packet
| replication, multiplexing, filtering, timestamping, aggregation
| and capture._
|
| * https://www.arista.com/en/products/7130-applications
|
| It is advertised as a "Layer 1" device and has a user-
| programmable FPGA. Some pre-built applications are: "MetaWatch:
| Market data & packet capture, Regulatory compliance (MiFID II -
| RTS 25)", "MetaMux: Market data fan-out and data aggregation
| for order entry at nanosecond levels", "MultiAccess: Supporting
| Colo deployments with multiple concurrent exchange connection",
| "ExchangeApp: Increase exchange fairness, Maintain trade order
| based on edge timestamps".
|
| Latency matters (and may even be regulated) in some of these
| use cases.
| zokier wrote:
| The PHY contributes only 1ns difference, but the results also
| show 400ps/m advantage for copper which I can only assume to
| come from difference in EM propagation speed in the medium.
| myself248 wrote:
| No. Look at the graph -- the offset when extrapolated back to
| zero length is the PHY's contribution.
|
| The differing slope of the lines is due to velocity factor in
| the cable. The speed of light in vacuum is much faster than in
| other media. And the lines _diverge_ the longer you make them.
| MadVikingGod wrote:
| It's true, but also if you go look at their product catalog
| you will see none of their direct attach cables are longer
| then 5m, and the high bandwidth ones are 2m. So, again, it's
| true, but also limiting in other ways.
| throw0101d wrote:
| This coming from Arista is unsurprising because their original
| niche was low-latency, and the first industries that they made
| in-roads in against the 'incumbents' was finance:
|
| > _The low-latency of Arista switches has made them prevalent in
| high-frequency trading environments, such as the Chicago Board
| Options Exchange[50] (largest U.S. options exchange) and RBC
| Capital Markets.[51] As of October 2009, one third of its
| customers were big Wall Street firms.[52]_
|
| * https://en.wikipedia.org/wiki/Arista_Networks
|
| They've since expanded into more areas, and are said to be fairly
| popular with hyper-scalers. Often recommended in forums like
| /r/networking (support is well-regarded).
|
| One of the co-founders is Andy Bechtolsheim, also a co-founder of
| Sun, and who wrote Brin and Page one of the earliest cheques to
| fund Google:
|
| * https://en.wikipedia.org/wiki/Andy_Bechtolsheim
| zokier wrote:
| What are applications where 5ns latency improvement is
| significant?
| thanhhaimai wrote:
| High Frequency Trading is one.
| Loughla wrote:
| Anything else? Because that's the only one I can think of.
| smj-edison wrote:
| I'd expect HPC would be another, since a lot of algorithms
| that run on those clusters are bottlenecked by latency or
| throughput in communication.
| williamdclt wrote:
| HPC?
| Tijdreiziger wrote:
| HPC = High-Performance Computing
|
| https://en.wikipedia.org/wiki/High-performance_computing
| ezekiel68 wrote:
| For the parent: and not only bottlenecked at single hops
| but also hampered by the propagation of latency as the
| hops increase, depending on the complexity of the
| distributed system design.
| empaone wrote:
| any high-utilization workload with a chatty protocol dominated
| by small IOs such as: * distributed filesystems such as
| MooseFS, Ceph, Gluster used for hyperconverged infrastructure.
| * SANs hosting VMs with busy OLTP databases * OLTP replication
| * CXL memory expansion where remote memory needs to be as close
| to inter-NUMA node latency as possible
| nimos wrote:
| This isn't really surprising. Fiber isn't better because of
| signal propagation speed, it's all about signal integrity.
|
| https://en.wikipedia.org/wiki/Velocity_factor
| exabrial wrote:
| IIRC, the passive copper SFP Direct Attach cables are basically
| just a fancy "crossover cable" (for those old enough to remember
| those days). Essentially there is no medium conversion.
| citizenpaul wrote:
| Its been long known that Direct Attach Copper (DAC's) are faster
| for short runs. It makes sense since there does not need to be an
| analog-digital conversion.
| ezekiel68 wrote:
| I suppose you are right, but we may not say "it has been widely
| known". Lots of us who read HN come from the the software side
| and we coders often hand wave on these topics when shooting the
| breeze -- much like how a casual car enthusiast might not
| imagine it was possible for a 6-cylinder engine to have more
| more horsepower than a V8.
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(page generated 2025-07-03 23:00 UTC)