4.1 PPP
4.1.4 The six fields of a PPP frame
As shown in the Figure,  the fields of a PPP frame are as follows:
  • Flag-Indicates the beginning or end of a frame and consists of the binary sequence 01111110.
  • Address - Consists of the standard broadcast address, which is the binary sequence 11111111. PPP does not assign individual station addresses.
  • Control - 1 byte that consists of the binary sequence 00000011, which calls for transmission of user data in an unsequenced frame. A connection-less link service similar to that of Logical Link Control (LLC) Type 1 is provided.
  • Protocol - 2 bytes that identify the protocol encapsulated in the data field of the frame.
  • Data - 0 or more bytes that contain the datagram for the protocol specified in the protocol field. The end of the data field is found by locating the closing flag sequence and allowing 2 bytes for the frame check sequence (FCS) field. The default maximum length of the data field is 1,500 bytes.
  • FCS - Normally 16 bits (2 bytes). Refers to the extra characters added to a frame for error control purposes.
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