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PPP uses the principles, terminology, and
frame structure of the International Organization for Standardization
(ISO) HDLC procedures (ISO 3309-1979), as modified by ISO
3309:1984/PDAD1, "Addendum 1: Start/stop transmission." ISO
3309-1979 specifies the HDLC frame structure for use in synchronous
environments. ISO 3309:1984/PDAD1 specifies proposed modifications to
ISO 3309-1979 to allow its use in asynchronous environments. The PPP
control procedures use the definitions and control field encodings
standardized in ISO 4335-1979 and ISO 4335-1979/Addendum 1-1979.
The flag sequence is a single byte and
indicates the beginning or end of a frame. The flag sequence consists
of the binary sequence 01111110.
The address field is a single byte and
contains the binary sequence 11111111, the standard broadcast address.
PPP does not assign individual station addresses.
The control field is a single byte and
contains the binary sequence 00000011, which calls for transmission of
user data in an unsequenced frame. A connectionless link service
similar to that of Logical Link Control (LLC) Type 1 is provided.
The protocol field is 2 bytes and its
value identifies the protocol encapsulated in the information field of
the frame. The most up-to-date values of the protocol field are
specified in the most recent Assigned Numbers Request For Comments (RFC).
The data field is zero or more bytes
long, and contains the datagram for the protocol specified in the
protocol field. The end of the information field is found by locating
the closing flag sequence and allowing 2 bytes for the FCS field. The
default maximum length of the information field is 1,500 bytes. By
prior agreement, consenting PPP implementations can use other values
for the maximum information field length.
The frame check sequence (FCS) field is
normally 16 bits (2 bytes). By prior agreement, consenting PPP
implementations can use a 32-bit (4-byte) FCS for improved error
detection.
PPP is a relatively simple protocol to
configure and troubleshoot. PPP troubleshooting on a network typically
involves one of two things - encapsulation or authentication.
Encapsulation mismatches are easily remedied by configuring the PPP
encapsulation on both ends of the link. PPP authentication issues will
be discussed in Chapter 9.
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