====== 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?