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Isn't OSI (Open Systems Interconnection) seven layers?

Yes it is. In the standard, seven layers have been defined. Just let's remember the seven layers of OSI.

Layer 7: The application layer.
Layer 6: The presentation layer.
Layer 5: The session layer.
Layer 4: The transport layer.
Layer 3: The network layer.
Layer 2: The data-link layer.
Layer 1: The physical layer.

If you are working in transport networking, you already know that Layer 0 is the photonics layer, more specifically, DWDM is considered as the Layer 0 of the OSI layers.

DWDM – Dense Wavelength Division Multiplexing is an Optical Transmission technology.

DWDM allows you to transmit multiple wavelengths on a single fiber, as the name suggests it utilizes wave division multiplexing.

DWDM can be deployed in ultra-long haul, long-haul, regional, and metro areas.

DWDM also uses amplification to reach long distances and can be an expensive technology to deploy.

On the other hand CWDM which is a metro technology multiplexing a maximum of 8 wavelengths per fiber and does not require amplification i.e. much cheaper than DWDM

WDM is used to transport SDH/Ethernet/IP between regions or cities or to aggregate traffic where large bandwidth is required

The reason, SONET/SDH can be a client of DWDM but not vice vers

What is Layer 0 in the OSI Layers ?

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