AN2120: Connecting an M68HC08 Family Microcontroller to an ...

Application Note
The payload contains the negotiation options or the rest of the IP packet.
Finally, a 2-byte checksum or frame check sequence (FCS) which is
computed over the entire unescaped packet with the help of a lookup
table defined in RFC 1662.
In a PPP session, both peers have no distinction of who is the server and
who is the client. Both end-points can carry up a negotiation equally.
However, for practical purposes, this application note defines a PPP
server as the end-point located and handled by the ISP and a PPP client
as the end-point that initiates the connection. Another way to define a
PPP server is the end of the link that requires password authentication,
that is the authenticator.
Usually, PPP sessions are started by a client dialing up an ISP. To start
a session, the PPP client must establish, maintain, and terminate a
physical connection with the ISP.
The overall process is illustrated in Figure 15.
PPP CLIENT ESTABLISH A MODEM CONNECTION NETWORK ACCESS SERVER
ESTABLISH A PPP LINK WITH LCP
USER AUTHENTICATION
SEND USER ID AND PASSWORD WITH PAP
AUTHENTICATION ACK
EXCHANGE NETWORK DATA
DATA HEADER PPP
Figure 15. Creating a PPP Link with an ISP
AN2120
18 MOTOROLA
PPP
HEADER
DATA