What is Layer 0 in the OSI Layers ?

What is Layer 0 in the OSI layers ? Isn’t OSI (Open Systems Interconnection) seven layers? 


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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 the 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 area’s.


  • 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



layer 0

DWDM as Layer 0 



Above picture shows DWDM as Layer 0, SONET/SDH as Layer 1.


The reason, SONET/SDH can be a client of DWDM but not vice versa. Which mean, SONET/SDH signals can be carried over DWDM but DWDM cannot be carried over SONET/SDH or any other transport mechanism. It is photonic layer.


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4 Replies to “What is Layer 0 in the OSI Layers ?”

  1. There is nothing like a layer 0 in the OSI model, and this is by design. Layer 1 is the “network access layer”. It includes all(!) to ‘access the bits’ – from pulse coding to voltage, from plugs to physics (optical, electronical). Just as with many other layers, you may (and must at a certain level of details) part it into different interlayers.
    And if you talk about the IP, OSI is the wrong layering at all. The TCP/IP model has different, and not necessarily clean, layers. A lot of IP-protocols do not relate to exact 1 OSI layer.

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