3.3 Layer 2 Protocol Overview - WAN Protocols
3.3.4 PPP frame format
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.