If you are using PPP encapsulation, you can also use authentication. Authentication enables you
to verify who is connected to a service. Note that this is optional and not required in any ISDN
configurations.
The two authentication choices are Password Authentication Protocol (PAP) and Challenge
Handshake Authentication Protocol (CHAP). CHAP is preferred over PAP because of its superior
security features. CHAP and PAP are covered in greater detail in Chapter 24, “Point-to-
Point Protocol,” of this book.