Why is cable classfied in layer 1 in OSI model

Question

I am learning about the OSI model and TCP/IP Protocol.

In my understanding, they classify cables to layer 1 (Physical Layer). But, when you connect two different devices you have to use cables no matter what levels you want to connect (type varies by standards but basically, it's required). (a host -> router, a router -> switch, etc).

So, When I connect A computer to my router directly. It only reaches layer 3, I still need a cable though, right? What is the definition of cable in this case? What sort of layer can I put the cable to?

Answer 1

The physical layer (OSI L1) is the only layer where data is physically moved from one location to another. Cabling is a necessary part of that unless you're using wireless signaling.

All other layers "just" provide increasing abstraction levels that increase the overall functionality in a modular way - see OSI Model and Networking Protocols Relationship for more details.

Even devices "working on layer x" require the functionality provided by lower layers, including the physical layer - ports, cables, ... Therefore, a switch implements layers 1-2, a router implements 1-3, a host implements 1-7.