When to choose fiber instead of twisted pair (copper)

  • These days both fibre and twisted pair are supporting 1000BASE-T and 10GBASE-T, but when do you choose one over the other? The obvious one is maximum cable length, but what are other factors which might come to mind when making this choice.

    When chosing fiber, there are electromagnetic disturbances. Edit: Of course fiber doesn't support POE.

    While you can't do poe over fiber you can run fiber close to your device and then add a fiber media converter to add poe to a shorter run of cat5/6. POE is supposedly limited to 100m distance.

  • There are a fairly large number of factors to consider and it is also important to accept that not all fiber is the same just like not all twisted pair is the same. Here are a few things that occur to me. Of course these may vary depending on environment.

    Fiber: Much longer max distances, depending on specification and throughput. No electromagnetic interference. Note however that fiber comes in a large number of grades and planning for the future may be somewhat more complex.

    Copper: Shorter max distances per link. Susceptible to electromagnetic interference. Fewer number of grades/variations (but still mind these!) meaning fewer accidents when the rollout team grabs the wrong spool (see below for a funny story). Supports power over ethernet.

    Funny story: A particular county in a Western state runs a fiber network that began as a control system for their hydroelectric dams. As time went on they found they didn't need nearly as much fiber as was laid for this and so they separated out many of the strands and used it to build a county-wide network linking homes and businesses to ISP's and telco's. Of course single mode fiber was used for longer transmissions and multi-mode for short runs.

    So when they were running the longer runs to connect towns and cities with this new network, the team that ran the fiber grabbed the wrong spool, and ran multi-mode fiber on all these long distance links. The team was quite unhappy when they found out they would have to re-run all the fiber again! Moral of the story: be very careful about ensuring that everyone knows which grades of fiber go where....

    Fiber cabling has a standard color scheme to depict the type of fiber to help prevent problems like these: - Yellow == 1 Gig SM (9 micron) - Orange == 1 Gig MM (50/62.5 micron) - Aqua == 10 Gig MM (50/62.5 micron)

    Normally I see orange as 62.5 micron MM and aqua as 50 micron MM, whether this is 1G or 10G.

    The colour code is only for single pairs, mult-pair is always coloured individually per-core to allow identification. Usually the type of fibre is printed on the jacket.

    Given the type of run I am assuming it was multi-pair.

    Yellow is generally single mode and supports 10G as well as several different 10G waves simultaneously (such as when using WDM.) saying it is 1G is misleading.

    Outside plant cable (the sort you'd be running on telephone poles and/or most outside conduit runs) is almost always black on the outside - holds up best in sunlight. So you always have to read the text that's imprinted if you have more than one kind of cable. There is no 62.5 10Gig cable. Singlemode is not only "able to do 10 gig" it's essentially unlimited - only the electronics driving it limit the speed.

    @ChrisTravers, The funny story may now end differently. There are options now to use multi-mode fiber on long distance using specific splitter. It's called Multi-Plane Light Conversion and this can be a good alternative to mutimode fiber replacement. Compared to a single-mode fiber you can increase bandwidth using independantly each mode of the multi-mode fiber.

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Content dated before 7/24/2021 11:53 AM