Topic PPP (Point to Point Protocol)

Syed Ali Jafar Zaidi

Point to Point Protocol• It is a protocol of data link layer. • PPP is used to established a direct connection between two nodes.• It provides connection authentication transcription and compression• PPP is use over many types of physical networks including serial cable and phone cable.

Frame Structure

PPP Establishment PPP goes through four distinct

phases to provide communications over point-to-point links1. Link establishment & configuration

negotiation2. Link quality determination3. Network-layer protocol configuration

negotiation4. Link termination

PPP Encapsulation Flag - Set to 0x7E (bit sequence 011111110) to

signify the start and end of a PPP frame. In successive PPP frames only a single Flag character is used.

Address - In HDLC environments, the Address field is used to address the frame to the destination node. On a point-to-point link, the destination node does not need to be addressed. Therefore, for PPP, the Address field is set to 0xFF, the broadcast address. If both PPP peers agree to perform address and control field compression during LCP negotiation, the

PPP Encapsulation Address field is not included. Control - In HDLC environments, the Control field is used

for data-link layer sequencing and acknowledgments. PPP does not provided link-to-link reliable data transfer. Therefore, for all PPP frames, the Control field is set to 0x03 to indicate an unnumbered information (UI) frame. If both PPP peers agree to perform address and control field compression during LCP negotiation, the Control field is not included.

Protocol ID - The 2-byte Protocol ID field identifies the protocol of the PPP payload. If both PPP peers agree to perform protocol field compression during LCP negotiation, the Protocol ID field is one byte for Protocol IDs in the range 0x00-00 to 0x00-FF.

PPP Encapsulation Frame Check Sequence (FCS) - A

16-bit checksum that is used to check for bit level errors in the PPP frame. If the receiver's calculation of the FCS does not match the FCS in the PPP frame, the PPP frame is silently discarded.