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I was reading CCNA R&S book by Todd Lammle, I saw this in the chapter TCP/IP Basics under Network Access/Link Layer
Advantage of this layer over Layer 1 and Layer 2 of OSI model is that there were no set physical layer specifications, so it could run on any existing or future network
what does this mean? I am not getting any idea it. Somebody please explain!
Advantage of this layer over Layer 1 and Layer 2 of OSI model is that there were no set physical layer specifications, so it could run on any existing or future network
This is nonsense. Whether you call it link layer or make a distinction between data link layer and physical layer, the functionalities are exactly the same.
The OSI model doesn't define a specific physical layer it just defines its functionality. ARPA/DoD/IETF just don't care about the hardware side and throw everything together.
The OSI approach is very helpful for providing a good structure for the lower four layers (which actually need some sublayers as well). The upper layers are pretty much mashed together in practice, so the OSI model can't be easily applied - however, it may still be helpful for modelling a new protocol or application.
A good layering structure opens the door to flexible enhancements and replacements in an isolated layer while preserving all the other layers.
As with any model, you have to keep in mind that it's a model and you don't have to stick to the letter at all times.
