Flag Fields
The flag field begins and ends each frame with a unique pattern of
01111110. Generally a single flag ends one frame and begins the next. The
receiver attached to the link continuously search for the flag sequence to
synchronize the start of the next frame.
Address Field
The address field is a single octet (8 bits) and contains the binary sequence
11111111 (0xff hexadecimal). This is known as theAll-Station Address. PPP
does not assign individual station addresses.
Control Field
The control field is a single octet and contains the binary sequence
00000011 (0x03 hexadecimal).This sequence identifies the Unnumbered
Information (UI) command with the P/F bit set to zero.
Protocol Field
The protocol field is defined by PPP.The field is 2 octets (16 bits) and its
value identifies the protocol datagram encapsulated in the Information field
of the frame.
Protocol field values in the range ’0xC000’–’0xFFFF’ indicate Layer 3 data
(protocol datagrams) such as LCP,PAP, CHAP,SPAP, and CCP.Values in
the range ’8000’–’BFFF’ indicate that the datagrams belong to the Network
Control Protocols (NCP). Values in the range ’0’–’3FFF’identify the network
protocol of specific datagrams.
Information Field
The information field contains the datagram for the protocol specified in the
protocol field. This is zero or more octets.
When the protocol type is LCP,exactly one LCP packet is encapsulated in
the information field of PPP Data Link Layer frames.
Frame Check Sequence (FCS) Field
The frame check sequence field is a 16-bit cyclic redundancy check (CRC).
PPP links can negotiate the use of various options which may modify the basic
frame format; the description below applies to the frame format prior to any such
modifications. PPP LCP packets are always sent in this format as well, regardless
of negotiated options, so that LCP packets can be recognized even when there is a
loss of synchronization on the line.
The router supports two such options: Address and Control Field Compression
(ACFC) and Protocol Field Compression (PFC). These are described in detail in a
later section.
The PPP Link Control Protocol (LCP)
PPP’s Link Control Protocol (LCP) establishes, configures, maintains, and
terminates the point-to-point link. This process is carried out in four phases:
┌──────┬─────────┬─────────┬──────────┬─────────────┬────────┬───────┐
│Flag │ Address │ Control │ Protocol │ Information │ FCS │ Flag │
└──────┴─────────┴─────────┴──────────┴─────────────┴────────┴───────┘
8 bits 8 bits 8 bits 16 bits variable 16 bits 8 bits
Figure 27. PPPFrame Structure
Using PPP
Chapter32. Using Point-to-Point Protocol Interfaces 451