I've noticed that upload speeds equal download speeds for fiber optic internet plans. Is it easier for Tx speeds = Rx speeds with fiber than with DOCSIS or DSL?

1 Answer 1



In general both DSL and Cable use "frequency division duplex" techniques. Consumer fibre services tend to use "wavelength division duplex". Professional fibre services often use a separate fiber for each direction.

Now you may say "wavelength" and "frequency are two sides of the same coin, and mathematically you would be correct. However practically when we are talking about electrical signals we talk in terms of frequency while when we are talking about light we talk in terms of wavelength. The frequency of light is way too high to directly interact with using electronic circuits.

With DSL, the main limitation is the usable bandwidth of a phone pair (which varies with distance, cable type, interference etc, but is normally in the tens of megahertz at best). So it's possible to increase the signal bandwidth allocated to upstream communication and hence increase the upstream data rate, but doing so comes at the cost of downstream data rate.

With Cable, there is a lot more usable bandwidth, I see figures on the order of 750MHz to 1GHz however Cable has it's own challanges. The Cable TV network was built for broadcasting TV, it was built to take a signal from a single powerful transmitter and distribute it to many receivers. The splitting inevitablly causes high path loss but that's ok for downstream traffic because the transmitter power compensates.

In the upstream direction though things are much less rosy, the splitters still have the same high loss in the upstream direction but the transmitters have to be low power (for both cost reasons and to avoid causing interference) and the receiver has to work in an environment with a high power transmitter. Yes there will be filters to try and isolate the receiver from the transmitter but filters are never perfect.

Cable is also a shared medium, so while there is more total bandwidth available than with DSL, that bandwidth has to be shared among all users on the cable segment. It also generally has to be shared with the cable TV broadcast services.

Fibre is a different ball game. The frequency bands defined for modern communications fiber cover 1260nm to 1625nm, though it's rare to use all of them. If you convert those wavelengths to frequencies and work out the bandwidth you get a ludicrously large number (something like 20 EHz).

In other words the fibre itself is rarely the limiting factor. In the case of a consumer/small buisiness fiber connection we can allocate two widely spaced wavelengths (to make filtering easier) for upstream and downstream. Indeed we can allocate more wavelengths for things like different speeds of service, or a dedicated wavelength for TV service, and none of them will come close to overlapping in frequency spectrum because the frequency of the light, is so much higher than the frequency of the modulating signals.

Hence there is no trade-off between upstream and downstream data rates.

  • Can you please explain why "Fibre is a different ball game"? How so? Is it going to phase out Cable and DSL?
    – Geremia
    Apr 29, 2023 at 22:04
  • I've updated the answer to go into a bit more detail, and yes I would expect fiber to eventually replace cable and DSL, how fast it will happen will probablly depend on where you live. Some places went for fiber to the cabinent systems which bought copper phone lines a few extra years of relavence but at least where I live the incumbent telco are now overbuilding FTTC with FTTP, I have my doubts if the FTTC was really worth it. Apr 30, 2023 at 0:34
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    So basically fiber allows for a much greater bandwidth.
    – Geremia
    Apr 30, 2023 at 2:29
  • Yes, for most practical purposes, fiber optic provides for the possibility of much greater bandwidth on a single cable link. For home and most business internet services, fiber optic will always provide the possibility of much higher bandwidth/service speed assuming the provider can support it and chooses to make it available to the customer. May 1, 2023 at 0:36
  • In a nutshell, physical bandwidth capacity of twisted-pair copper (xDSL) measures in hundreds of Mbit/s for very short range, coax (DOCSIS) in tens of Gbit/s and fiber in Pbit/s, even at long range - that's five orders of magnitude faster than anything else.
    – Zac67
    Oct 30, 2023 at 19:54

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