1. X.25 Protocol

2. 2 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

3. 3 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 DTE DCE X.25 Intra-Network Protocol DTE

4. 4 The X.25 Protocol The X.25 Protocol Layers Physical Link Network RS232, etc X.25 Frame Level (LAPB) X.25 Packet Level Layer NameDescription DataPkt HdrLAPB HdrFlag CRC

5. 5 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 FlagCRCData Frame Level (LAPB) Flag 0

6. 6 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 LAPB HdrInformation Field Packet Level

7. 7 The X.25 Protocol LAPB Header AddressControl N(R)0N(S)P/F N(R)1 RR RNR REJ P/F DISC SABM UA DM FRMR 11P/F Information Frame Supervisory Frames Unnumbered Frames 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

8. 8 The X.25 Protocol LAPB Link Setup and Disconnect Local DCE Local DTE SABM UA SABM = Set Asynchronous Balanced Mode UA Acknowledges SABM DISC Requests Disconnect UA Acknowledges DISC Exchange on Local Link Only DISC UA Now in Data Transfer Mode Now in Disconnected Mode

9. 9 The X.25 Protocol LAPB Data Transfer Local DCE Local DTE I-Frame #1 RR N(R)=2 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 #2 RR N(R)=3 I-Frame #3 I-Frame #0 N(R)=4

10. 10 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 Pkt HdrData Application Layer

11. 11 The X.25 Protocol Packet Level Header Channel NrPkt TypeP(R)0P(S) P(R)1 RR RNR Data Packet Acknowledgement Packets Other Packets Call Req Call Acpt Clr Req Clr Conf Reset Req Reset Conf Intr Req Intr Conf Restart Req Restart Conf 11 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

12. 12 The X.25 Protocol Call Setup Local DCE Remote DCE Local DTE Remote DTE Call Request Incoming Call Call Accepted Call Connected Internal Protocol 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

13. 13 The X.25 Protocol Call Clearing Local DCE Remote DCE Local DTE Remote DTE Clear Request Clear Indication Clear Confirm Internal Protocol 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 Remote DCE from Call Setup

14. 14 The X.25 Protocol Data Transfer w/End to End Ack Local DCE Remote DCE Local DTE Remote DTE Data Packet #1 RR P(R)=2 Internal Protocol 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 Remote DCE from Call Setup

15. 15 The X.25 Protocol Data Transfer w/Local Ack Local DCE Remote DCE Local DTE Remote DTE Data Packet #1 RR P(R)=2 Internal Protocol 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 #2 RR P(R)=3 Data Packet #2 RR P(R)=3 Remote DCE from Call Setup

16. 16 The X.25 Protocol X.25 Without a Network Two DTEs Communicating Directly No Intervening Network One DTE Plays the Role of DCE LAPB Ensures Reliability Acknowledgements “End to End” DTE Addressing Immaterial Local DTE Data Packet #1 RR P(R)=2 Data Packet #2 RR P(R)=3 Local DTE

17. 17 The X.25 Protocol End of Presentation