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.
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.
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
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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