what is the OSI model?

Question

Something that has to do with the internet, I get this part. But what are those layers? Are they physical?

I also heard the term: layer 2 switch. What is that, is there a difference between this and a normal switch?

What can be achieved using the OSI model?

In: Engineering

Answer 1

The OSI reference model describes how communication in a network can be encoded in a way where each layer builds on top of the previous layer. 

The layers are not physical, they all refer to different kinds of encoding, that are stacked on top of each other. Even the first layer, the physical layer, doesn’t describe what physical medium to use to transport signals, but rather how signals can be encoded such that we can send 1s and 0s despite the actual medium sending analog signals. 

The OSI reference model doesn’t describe a specific technology or encoding standard, it only describes an idea of how encodings can be built on top of each other and what purpose each layer should fulfil. 

Roughly, it says that the job of layer 1 (physical) is go make sure you can transport 1s and 0s in a way that the receiver can read it. Layer 2 (data link layer) says to structure 1s and 0s into small units (so called Frames), put an address on them so we know which physical network device it is for, and make sure the recipient can error check it (via a checksum). Layer 3 (Network Layer) says to then combine frames into larger units called (called Packets), and put a more organized address on them such that it is clear which logical network device should be the recipient. Layer 4 (Transport layer) deals with methods to ensure packets are transported efficiently and reliably, for example by numbering packages to make sure they arrived in the same order as they were sent..
 
Layers 5-7 are very high level and deal with what kind of content is stored in these packets and how they should be first assigned to a Session (layer 5), how the actual data should be presented (layer 6), and then finally on top are the actual protocols programmers will write programs for (layer 7).