This cost can either
be offset or centralized by using callbacks, phone cards, or even toll-free, dial-up access
numbers.
Point-to-Point Protocol
Point-to-Point Protocol (PPP) is used for DUN. It allows hosts to communicate using
TCP/IP over serial links such as DUN or even serial cable connections. PPP actually
uses six different protocols, listed here in the order in which they are used to establish
communications:
?–? Link Control Protocol (LCP) LCP is in charge of negotiating link parameters,
maintaining those links, and then terminating it when done. You can think of
LCP as acting within the physical layer of the network stack for PPP.
?– Challenge Handshake Authentication Protocol (CHAP) CHAP is in charge
of authenticating the client using login credentials to decide whether the user is
supposed to have access or not.
?– Callback Control Protocol (CBCP) This protocol manages callback, which
allows you to configure the server to hang up the connection and call the client
back to establish communications. This is used to centralize cost since the client
is connected only for a very brief period, after which the server calls the client
back and assumes the charges for the connection. It is also used for security.
If you know exactly where the client is supposed to be calling from, hanging
up and then reconnecting to the client ensures that connections can??™t be
established from any other number.
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