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