OSI Model: physical layer
how to debug
- check if the cable is plugged in
- is the device you want to connect powered on?
- is the device on the other side powered on?
- are there any devices in between (e.g. switches) that need to be powered on?
- does any LED for the port light up?
- for network ports: figure out what the lights mean. Do they blink? How do they blink?
- can you measure a signal with an oscilloscope when you poke the data transmission lines with it?