Line is up, Protocol is Down

I've got several questions from some friends of what it means by the status when it shows "Line is up, Protocol is Down". From google, I found two websites explaining about the meaning of the statis, i,e:

• Troubleshooting Line Protocol is Down, from Cisco
  
• Cisco Show Interface
  

Basically, this status will be appear when one of the conditions below met:

• Local or remote router misconfigured
  
• Keep-alives not being sent by remote router
  
• Leased-line or other carrier service problems, such as noisy lines or faulty switch
  
• Timing problem on cable, possibly caused by the CSU/DSU not being set correctly.
  
• Failed local or remote CSU/DSU.
  
• Router failure.