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Can I create a distributed ethernet using just 1 x core of a single mode fiber ring ?

Update (Sep 2022):

The following is what we've implemented and works great. It's one of the options discussed in extended chat with @zac67

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Essentially there were two requirements for what I needed to do:

  1. A Bi-Directional technology such as 1000BASE-X allowing the use of the single core.
  2. A Switch that has 2 x SFP slots to allow daisy chaining.

Original Post (Dec 2021)

This is an existing installation, I can't change the following constants:

  • I have approx 100 locations (nodes) in a ring.
  • Each node is between 100m and 5km apart.
  • The total ring is around 25km.
  • Each node has 2 x single mode fibre cores, each terminated to an SC connector. One comes from the previous node, and one goes out to the next node. (The point I'm trying to make here is that from node to node is a single core, not duplex)
  • I want an ethernet outlet at each node so I can connect to all the nodes from a central control room.
  • I don't have any need for Layer 3 features

I've been researching this on and off for the last 6 months, I've spoken to fibre techs I meet on site, and tried consulting with every supplier of media converters for a solution, all of them claim it's either impossible or that they don't know how to do it.

Eventually I just decided to start buying gear and trying it in a lab.

This is my small scale setup, which I quickly discovered doesn't work as I'd like.

Doesn't work as I'd like

I've been exploring WDM converters, but that I assume would require 2x at each location with an ethernet bridge ?

I'd appreciate some advice on this.

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  • 1
    What you want is not ethernet. You can't daisy-chain ethernet like that. (well, 10base2 does, but that's not fiber, and only works to 100m) The only thing I've ever seen work like that is SONET, ATM, or vendor hacks along those lines (cisco's DPT) If this is an existing setup, what's it currently running?
    – Ricky
    Dec 1, 2021 at 6:03
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    What you want requires three ports at each location, mandating a switch. The ring mandates a spanning tree protocol, limiting the ring width to seven switches. The closest you can get is with small, managed switches featuring two SFP ports where you can fit LX (dual strand) or BX (single strand) modules.
    – Zac67 ♦
    Dec 1, 2021 at 7:46
  • Connecting two media converters back to back via a small switch would work (I'd seriously recommend a small SFP switch though). Creating a chain instead of a ring removes the need for STP. Bandwidth is one thing - what about resilience? You should really consider a star/tree topology.
    – Zac67 ♦
    Dec 1, 2021 at 13:29
  • Please tell us more about your surrounding conditions: Why are you restricted to a single strand? What kind of spacing and power conditions do those locations have? Is there a need for resilience/redundancy? Remote status monitoring? Expandibility? Is the fiber already deployed? How exactly? What are the bandwidth requirements? Is there just a single setup or is that a prototype for a series?
    – Zac67 ♦
    Dec 1, 2021 at 13:43
  • @Zac67 Sure so I could use something with 2 x SFP ports and at least one wired ethernet and do away with 'media converters' altogether ? The choice we have is this: Utilize an existing single fibre present at each node - or - don't. If we choose the latter then every time we need to connect to a node, we would have to physically drive to it and plug in to the service port locally. I was thinking maybe the return leg of the fibre could be used for redundancy (i.e. by manually plugging instead of STP if there's a break)
    – hmedia1
    Dec 1, 2021 at 13:47

1 Answer 1

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Your requirements mandate:

  • three ports at each location, requiring a switch
  • either use two media converters per location (in & out) or a switch with two SFP ports
  • you cannot use a spanning-tree protocol due to the chain length/ring diameter (xSTP has a maximum design depth of seven bridges), prohibiting a ring topology
  • single-strand fiber requires a bidirectional PHY like 1000BASE-BX10 (you cannot split a duplex fiber like in your diagram and connect the strands to different nodes) - note that there are two different modules for 1000BASE-BX: a -D and a -U need to be paired correctly on each link (transmit and receive wavelengths needs to be swapped)

From the information in chat, you don't require much bandwidth (<<100 Mbit/s) and latency isn't an issue. The installation is more or less temporary, redundancy/resilience isn't required either.

You could use a rather simple switch with SFP ports at each location instead of two media converter and a switch, reducing complexity and moving parts.

Since you cannot use a spanning-tree protocol due to the planned network depth, you'll need to keep the fiber ring open between the furthest nodes (so the chains are kept as short as possible).

At the control room you might need a larger switch, with as many SFP ports as there are chains.

If you're considering a connection to an existing network you should use a firewall to prevent intrusion (or at least a decent switch in the control room).

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    The assistance is greatly appreciated. I'm sourcing gear now for a trial setup. I've not seen a working example of what I'm attempting yet, and will post those details for the sake of completion on this thread once I am up and running
    – hmedia1
    Dec 2, 2021 at 14:52
  • Many thanks for the great advice. You gave me the confidence to try another test setup. Once I started getting specific, the supplier found a suitable 2 x SFP switch and supplied us two sets of BiDi SFP modules with the TX and RX wavelengths reversed allowing each switch to have an "IN" and an "OUT". I updated my question with what would end up as the final implementation. Cheers
    – hmedia1
    Sep 4, 2022 at 21:48

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