I'm studying the history and technical details about fiber optics and I'm having trouble understanding which are the different layers that exist and have been used over fiber optics.

For example, I see that DWDM is used to increase bandwidth by allowing different wavelengths to coexist in the same light or laser beam. SONET/SDH is the evolution of PDH and Ethernet is also used because of its higher bandwidth. However, I read different sources and while some seem to suggest (or that's what I understand) that SONET and Ethernet were "rivals" and thus they work separately, others show layer diagrams in which Gigabit Ethernet is the layer over SONET/SDH. Also, from what I understand, DWDM works at a lower level than the other two, but often I see comparisons between the three as if they worked in the same layer, especially between SONET and DWDM.

Some links that caused confusion:


http://ecomputernotes.com/computernetworkingnotes/computer-network/physical-layer-interfaces (here, where is WDM used? I see that ehernet can work either over SONET/SDH or stright over fiber, but WDM techniques seem to be the "lower layer" (so I understand the other protocols are not its competitors)

https://community.fs.com/blog/the-relation-and-difference-between-sonetsdh-and-dwdm.html (however here are being compared as if they could work separately)

My questions aiming to clarify my confusion are:

1- Can and does GbEthernet work over SONET/SDH?

2- When GbEthernet is used, is it over DWDM or it has nothing to do with it?

3- Is SONET/SDH DWDM's competitor or do they work together but at different layers?

4- Can these protocols work completely independently? If so, is GbEthernet working directly over fiber?

5- What's it today's most common implementation?

2 Answers 2


Can and does GbEthernet work over SONET/SDH?

Not like you might be imagining. You can pass (tunnel) Ethernet frames over SONET but Ethernet has its own physical layer variants, independent of SONET. Much more common than Ethernet-over-SONET is (was) IP-over-SONET, omitting the intermediate Ethernet layer.

When GbEthernet is used, is it over DWDM or it has nothing to do with it?

Some Ethernet PHYs use WDM, most don't. When a optical transmission is split into multiple lanes to reduce each lane's data rate in the medium, you can use WDM to merge these multiple lanes into a single fiber strand. Think of each WDM stream as a color, and multiple colored signals transmitted simultaneously.

WDM is used to reduce the fiber count and cost where it matters, ie. over long range Multi-lange short-range PHYs often use multiple strands.

Is SONET/SDH DWDM's competitor or do they work together but at different layers?

SONET uses (D)WDM, and Ethernet uses DWDM or CWDM to reduce cost on long-range links. WDM can be used by any protocol that allows multiple lanes.

WDM can also be used to combine several incompatible protocols onto the same fiber. You could e.g. run SONET, Ethernet and Fibre Channel on the same fiber using different colors. Of course, you can also run multiple, independent links of the same protocol (which is a pretty common method today).

Can these protocols work completely independently? If so, is GbEthernet working directly over fiber?

SONET and Ethernet could be used to tunnel each other (with limitations), but otherwise they're completely independent.

What's it today's most common implementation?

Ethernet PHYs like 100BASE-FX, 1000BASE-LX or 10GBASE-LR run directly on fiber pairs and are very common. 1000BASE-BX10 or 10GBASE-BR (often called "BiDi" or "-BX") are common PHYs that use single-strand fiber with wavelength-division duplex WDD (same as WDM but using different colors for different directions).

SONET is a legacy protocol that's largely been phased out. It's circuit-oriented, making it wasteful of bandwidth, expensive to run and hard to scale. Ethernet is packet-oriented, making it much more efficient, easier to scale and cheaper to run.

  • thanks for the detailed answer! I have one question as a result of your answer: how can telecom operators manage different client's hired data rates on the same fiber (using ethernet) without WDM? Do they use multimode fiber or am I mixing things up? Also what is the best online resource to learn all of this? Thank you
    – Johnny
    Commented Oct 27, 2019 at 11:21
  • ISPs don't usually control client data rates apart from their downlink. WDM is a method to increase a medium's bandwidth, it has no other relation to anything further up. Multi-mode fiber is yet something completely different - it allows mixing of the light's propagation modes, limiting a fiber's effective range - it's used in short-range links (some hundred meters) while single-mode fiber allows for dozens or even hundreds of km. Wikipedia holds a large range of articles about all these topics but resource recommendations are off-topic here.
    – Zac67
    Commented Oct 27, 2019 at 11:44

You are confusing some things. Fiber optics uses a particular wavelength of light for signalling. WDM lets you use multiple wavelengths for separate signals, as if you had multiple fibers; each wavelength is a separate signal.

SONET and ethernet are methods of framing the data on a signal, telling the other end where the data starts and stops. SONET is for circuit-switched networks, where each frame in a time period represents a different circuit. SONET was developed for the traditional circuit-switch telephone system to multiplex different calls onto a single circuit as if there were multiple circuits.

Ethernet is for packet-switched networks. Ethernet frames can be encapsulated into SONET frames and de-encapsulated on the other end, or there are various ethernet on fiber standards, too, but the equipment for ethernet on fiber and SONET on fiber is very different.

Either SONET or ethernet can be run over a fiber with only one signal, or you can have multiple logical links with WDM, and each wavelength looks like a separate connection.

Circuit-switching is fading away in favor of packet switching, but remember that there is a huge installed base of expensive circuit-switching equipment that simply doesn't disappear. That is why you (can) encapsulate ethernet frames in SONET frames.

In sites, e.g. data centers, you will find one of the ethernet standards because the equipment is so much cheaper than TDM equipment, and packet switching is more flexible and efficient than circuit switching.

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