Point-to-Point Protocol (PPP) Frame Format

Point-to-Point Protocol (PPP) is generally the default RAS protocol in Windows and is most commonly used protocol of data link layer that is required to encapsulate higher network-layer protocols simply to pass over synchronous and asynchronous communication lines. In PPP, link establishment is controlled and handled mainly by Link Control Protocol (LCP). It is also required to connect the Home PC to server of ISP through a modem. It was also adopted by ISPs to simply provide dial-up Internet Access. PPP Frame Format : PPP frame is generally required to encapsulate packets of information or data that simply includes either configuration information or data. PPP basically uses the same basic format as that of HDLC. PPP usually contains one additional field i.e. protocol field. This protocol field is present just after control field and before information or data field. 
 
Various fields of Frame are given below :

1. Flag field - PPP frame similar to HDLC frame, always begins and ends with standard HDLC flag. It always has a value of 1 byte i.e., 01111110 binary value.
2. Address field - Address field is basically broadcast address. In this, all 1’s simply indicates that all of the stations are ready to accept frame. It has the value of 1 byte i.e., 11111111 binary value. PPP on the other hand, does not provide or assign individual station addresses.
3. Control field - This field basically uses format of U-frame i.e., Unnumbered frame in HDLC. In HDLC, control field is required for various purposes but in PPP, this field is set to 1 byte i.e., 00000011 binary value. This 1 byte is used for a connection-less data link.
4. Protocol field - This field basically identifies network protocol of the datagram. It usually identifies the kind of packet in the data field i.e., what exactly is being carried in data field. This field is of 1 or 2 bytes and helps in identifies the PDU (Protocol Data Unit) that is being encapsulated by PPP frame.
5. Data field - It usually contains the upper layer datagram. Network layer datagram is particularly encapsulated in this field for regular PPP data frames. Length of this field is not constant rather it varies.
6. FCS field - This field usually contains checksum simply for identification of errors. It can be either 16 bits or 32 bits in size. It is also calculated over address, control, protocol, and even information fields. Characters are added to frame for control and handling of errors.

Notes :

• It has the ability to negotiate IP Addresses dynamically.
• It contains Link Control Protocol (LCP) simply to develop link options.
• It contains error checking for each PPP frame.
• It also has the ability to transport several protocols on a single serial connection.