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You can con?¬?gure PPP on the following types of physical interfaces:
?– Asynchronous serial: Plain old telephone service (POTS) dialup
?– Synchronous serial: ISDN or point-to-point leased lines
The Link Control Protocol (LCP) portion of PPP is used to negotiate and set up control
options on the WAN data link. PPP offers a rich set of services. These services are options
in LCP and are primarily used for negotiation and checking frames to implement the pointto-
point controls that an administrator speci?¬?es for a connection.
318 Chapter 8: Extending the Network into the WAN
With its higher-level functions, PPP can carry packets from several network layer protocols
by using network control protocols (NCP) . The NCPs include functional ?¬?elds that contain
standardized codes to indicate the network layer protocol type that is encapsulated in the
PPP frame.
Figure 8-17 shows how NCP and LCP provide these functions for PPP.
Figure 8-17 PPP Components
Three phases of a PPP session establishment are described in the following list:
1. Link establishment phase
In this phase, each PPP device sends LCP packets to con?¬?gure and test the data link.
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