background image
102 Chapter 5: Configuring PPP and Controlling Network Access
PPP supports these functions by providing an extensible LCP and a family of NCPs to negotiate
optional configuration parameters and facilities. The protocols to be transported, the optional
capabilities, and the user authentication type are all communicated during the initial exchange
of information when a link between two points is set up.
PPP Components
PPP can operate across any DTE/DCE interface. The only absolute requirement imposed by
PPP is the provision of a duplex circuit, either dedicated or switched, that can operate in either
an asynchronous or synchronous bit-serial mode, transparent to PPP link layer frames. Other
than those imposed by the particular DTE/DCE interface in use, PPP does not impose any
restrictions regarding transmission rates.
In just about every type of WAN technology in internetworking, a layered model is shown to
provide a point of reference to the OSI model and to illustrate where each particular technology
operates. PPP is not much different from other technologies. It too has its own layered model
to define form and function. Figure 5-2 depicts the PPP layered model.
Figure 5-2
PPP Layered Model
As with most technologies, PPP has its own framing structure. This structure enables the
encapsulation of virtually any Layer 3 protocol. Because PPP is, by nature, point-to-point, no
mapping of protocol addresses is necessary. Figure 5-3 shows the PPP frame format.
Upper-layer protocols
(such as IP, IPX, AppleTalk)
Network Control Protocol (NCP)
(specific to each network-layer protocol)
Link Control Protocol (LCP)
High-Level Data Link Control (HDLC)
Physical Layer
(such as EIA/TIA-232, V.24, V.35, ISDN)
3
2
1
OSI layer