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What is Layer 0 in the OSI Layers ?

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 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.

To have a great understanding of SP Networks, you can check my new published: Service Provider Networks Design Book

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Orhan Ergun, CCIE/CCDE Trainer, Author of Many Networking Books, Network Design Advisor, and Cisco Champion 2019/2020/2021

He created OrhanErgun.Net 10 years ago and has been serving the IT industry with his renowned and awarded training.

Wrote many books, mostly on Network Design, joined many IETF RFCs, gave Public talks at many Forums, and mentored thousands of his students.  

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