I'd like to preface the main content of this post with the following: I understand this question may be equally suited to the Physics Stack Exchange, and so I have posted on there as well, but maybe a network engineer or computer science wizard may also be able to provide some illuminating knowledge.

Which medium allows for lower latency in data transfer, copper or fiber?

It is a tie (sort of). The transfer of information through electromagnetic waves, or the signal, has a velocity of approximately 200,000 km/s in both copper and glass.

A couple key advantages to optical transmission are it is immune to electromagnetic interference, and it has a much larger bandwidth, over 1000 times more than electrical transmission.

However, to achieve the lowest possible latency in a network, fiber should be used from the provider's equipment to the customer's network. Copper should be used from the customer's gateway to devices requiring a low-latency connection.

If the latency is the same in both mediums, then why not use just one medium, say copper?

The reason for this is that the distance from the customer's network to the telco's equipment is usually large enough that if the link was copper, it would require a signal repeater at some point to ensure the signal doesn't become too weak for the telco's equipment to register. Because of the additional device, the signal repeater, that data has to go through, it will take longer for the data to reach its destination than if it had traveled through just fiber. Optical transmission of data through fiber allows for much farther transmission of data due to the signal remaining stronger for longer without the need for a signal repeater to be used anywhere close to the amount needed in long distance electrical transmission (I think one would be needed for approximately every 300' in copper, but single-mode fiber can carry a signal for miles before needing to be repeated).

The reason why copper should be used in LANs is that many devices cannot yet work directly with light. Upon an optical signal entering a device, it has to be converted into an electrical signal. The opposite is true for a signal exiting a device. These conversions add time to the process. The points of conversion are essentially additional devices where the signal must pass through and be processed.

  • This Q&A might also help. – Zac67 Apr 15 '20 at 17:10
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    Or this one. Copper has less propagation delay than fiber. Also, propagation delay is not always the same as latency. – Ron Trunk Apr 15 '20 at 18:33
  • Forget about copper vs glass details... the only latency you care about is measured by maps and odometers... in short, distance covered by the hop is what matters. – Mike Pennington Apr 15 '20 at 19:51

You already provide most answers.

Propagation time: apart from (good) coax, copper and fiber are nearly equal. Decent twisted pair is slightly faster than fiber, high quality TP even more so - but nothing really relevant.

If bandwidth was a non-issue, 10BASE5 could theoretically be faster than fiber (a whooping 320 ns ahead over 500 m!) - but the ancient hardware would surely kill the competition.

Conversion: (simple) line codes need to be converted in any case, so there's no real difference - assuming integrated converters or modules (SFP). External converters cost extra, of course.

However, copper may require more elaborate line codes, increasing latency. For instance, 10GBASE-T requires some additional 0.5-1.5 μs (micro seconds) in comparison to 10GBASE-R or DAC (also running -R PCS code).

Long-haul, voice-grade copper requires very elaborate encoding, depending on the distance and speed. VDSL on short links may start at 3-5 ms but interleaved ADSL on long loops may exceed 60 ms.

Fiber in comparison runs for 100 km or so with gigabit speed or more, with simple line code, on passive fiber. Longer links require boosting but with optical amplifiers (EDFAs) that doesn't even increase latency.

In a nutshell: decent copper is good for short distance (max. 100 m) and up to 10 Gbit/s, anything else requires fiber anyway.

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