4.2 PPP Session Establishment
4.2.5 Describe phase 4: link termination

LCP can terminate the link at any time. This is usually done at the request of a user but can happen because of a physical event, such as the loss of a carrier or a timeout.

Threaded Case Study
  Washington Project: Enabling PPP Encapsulation

You can enable PPP on serial lines to encapsulate IP and other network-layer protocol datagrams. To do so, enable PPP encapsulation in interface configuration mode, using the encapsulation ppp command:

Step 1 Enter interface configuration mode for the desired interface.

Step 2 Configure the interface for PPP encapsulation:

Router(config)# encapsulation ppp