X.25 Protocol

CCITT Recommendation X.25 The X.25 Protocol CCITT Recommendation X.25 First Published in 1976 Revisions Every 4 Years -- 1980, 1984, 1988 Interface Protocol for Packet Switched Networks Network Providers Intended to Be Telcos

Intra-Network Protocol The X.25 Protocol The Model Network Has Multiple Nodes (DCEs) Host Computers (DTEs) Outside Hosts Have Addresses Like Phone Numbers Virtual Call Setup Virtual Call Clear Data Transfer DCE DTE X.25 Intra-Network Protocol DCE X.25 DCE DTE DCE X.25 DTE DCE

The X.25 Protocol The X.25 Protocol Layers Layer Name Description Data Physical Link Network RS232, etc X.25 Frame Level (LAPB) X.25 Packet Level Data Pkt Hdr LAPB Hdr Flag CRC

Physical Layer – Sort of The X.25 Protocol Physical Layer – Sort of Flag Character (01111110) at Beginning and End Data in between is LAPB Frame Data Passed up to Frame Level Data Transparency 011111010  011111 10 Frame Level (LAPB) Flag Data CRC Flag

The X.25 Protocol Frame Level – LAPB Frame Level Header Has Frame Type Sequence and Acknowledgement Numbers Error Recovery Procedures Endpoints Are DTE and Local DCE Packet Level LAPB Hdr Information Field

The X.25 Protocol LAPB Header Address Control N(R) N(S) P/F N(R) is Ack Nr and Counts Modulo 8 N(S) is Seq Nr and Counts Modulo 8 RR = Receiver Ready RNR = Receiver Not Ready REJ = Reject SABM = Link Setup Req UA = Unnumbered Ack DM = Disconnected Mode FRMR = Frame Reject N(R) N(S) P/F Information Frame N(R) 1 RR RNR REJ P/F Supervisory Frames DISC SABM UA DM FRMR 1 P/F Unnumbered Frames

Now in Data Transfer Mode Now in Disconnected Mode The X.25 Protocol LAPB Link Setup and Disconnect Local DCE Local DTE SABM = Set Asynchronous Balanced Mode UA Acknowledges SABM DISC Requests Disconnect UA Acknowledges DISC Exchange on Local Link Only SABM UA Now in Data Transfer Mode DISC UA Now in Disconnected Mode

The X.25 Protocol LAPB Data Transfer Local DCE Local DTE I-Frame Contains Packet Seq from 0 - 7 and back to 0 RR Gives Next Expected I-Frame I-Frame Can also Acknowledge I-Frame #1 RR N(R)=2 I-Frame #2 RR N(R)=3 I-Frame #3 I-Frame #0 N(R)=4

The X.25 Protocol The X.25 Packet Level Packet Header Has Packet Type Channel Number Identifies Logical Connection Sequence and Acknowledgement Numbers No Error Recovery -- Data Can Be Lost Addressing across Multi-Node Network Application Layer Pkt Hdr Data

The X.25 Protocol Packet Level Header Channel Nr Pkt Type P(R) P(S) P(S) Data Packet Channel Nr Selects which Connection P(R) is Ack Nr and Counts Modulo 8 P(S) is Seq Nr and Counts Modulo 8 RR = Receiver Ready RNR = Receiver Not Ready P(R) 1 RR RNR Acknowledgement Packets Call Req Call Acpt Clr Req Clr Conf Reset Req Reset Conf Intr Req Intr Conf Restart Req Restart Conf 1 Other Packets

The X.25 Protocol Call Setup Local DCE Remote DCE Local DTE Remote DTE Call Request Each Channel is Distinct Select Unused Channel Different Channel Numbers on Each End End to End is “Virtual Circuit” VC = Local Chnl + Network Route + Remote Chnl Internal Network Protocol Not Specified Call Setup is End to End Locate Remote DCE Incoming Call Internal Protocol Call Accepted Call Connected

Remote DCE from Call Setup The X.25 Protocol Call Clearing Local DCE Remote DCE Local DTE Remote DTE Remote DCE from Call Setup Clear Request Each Channel is Distinct Channels Become Available End to End is “Virtual Circuit” Internal Network Protocol Not Specified Clearing May be End to End or Local Clear Packet Used to Report Procedure Errors Clear Indication Internal Protocol Clear Confirm Clear Confirm

Remote DCE from Call Setup The X.25 Protocol Data Transfer w/End to End Ack Local DCE Remote DCE Local DTE Remote DTE Remote DCE from Call Setup Data Packet #1 Each Channel is Distinct End to End is “Virtual Circuit” Internal Network Protocol Not Specified Each Data Pkt Has Seq Nr Each RR Has Next Expected Seq Nr Example Shows End to End Acknowledgement Data Packet #1 Internal Protocol RR P(R)=2 RR P(R)=2

Remote DCE from Call Setup The X.25 Protocol Data Transfer w/Local Ack Local DCE Remote DCE Local DTE Remote DTE Remote DCE from Call Setup Data Packet #1 Each Channel is Distinct End to End is “Virtual Circuit” Internal Network Protocol Not Specified Each Data Pkt Has Seq Nr Each RR Has Next Expected Seq Nr Example Shows Local Acknowledgement RR P(R)=2 Data Packet #1 Internal Protocol RR P(R)=2 Data Packet #2 Data Packet #2 RR P(R)=3 RR P(R)=3

The X.25 Protocol X.25 Without a Network Local DTE Data Packet #1 RR P(R)=2 Data Packet #2 RR P(R)=3 Two DTEs Communicating Directly No Intervening Network One DTE Plays the Role of DCE LAPB Ensures Reliability Acknowledgements “End to End” DTE Addressing Immaterial

The X.25 Protocol End of Presentation