X 25 Protocol The X 25 Protocol CCITT

X. 25 Protocol

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 2

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

The X. 25 Protocol Layers Layer Name Description Network X. 25 Packet Level Link X. 25 Frame Level (LAPB) Physical RS 232, etc Flag LAPB Hdr Pkt Hdr Data CRC Flag 4

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 0 Frame Level (LAPB) Flag Data CRC Flag 5

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 6

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

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

The X. 25 Protocol LAPB Data Transfer Local DTE Local DCE • I-Frame Contains Packet I-Frame #1 • Seq from 0 - 7 and back to 0 RR N(R)=2 • RR Gives Next Expected I-Frame • I-Frame Can also Acknowledge I-Frame #2 RR N(R)=3 I-Frame #0 N(R)=4 9

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 10

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

The X. 25 Protocol Call Setup Local DTE Local DCE Remote DTE • Each Channel is Distinct Call Request • Select Unused Channel Locate Remote DCE • Different Channel Numbers on Each End Incoming Call • End to End is “Virtual Circuit” Internal Protocol Call Accepted • VC = Local Chnl + Network Route + Remote Chnl • Internal Network Protocol Not Specified Call Connected • Call Setup is End to End 12

The X. 25 Protocol Call Clearing Local DTE Local DCE Remote DTE Remote DCE from Call Setup Clear Request • Each Channel is Distinct • Channels Become Available • End to End is “Virtual Circuit” Clear Indication • Internal Network Protocol Not Specified Internal Protocol Clear Confirm • Clearing May be End to End or Local • Clear Packet Used to Report Procedure Errors Clear Confirm 13

The X. 25 Protocol Data Transfer w/End to End Ack Local DTE Local DCE Data Packet #1 Remote DCE Remote DTE Remote DCE from Call Setup • Each Channel is Distinct • End to End is “Virtual Circuit” • Internal Network Protocol Not Specified Data Packet #1 • Each Data Pkt Has Seq Nr Internal Protocol RR P(R)=2 • Each RR Has Next Expected Seq Nr • Example Shows End to End Acknowledgement RR P(R)=2 14

The X. 25 Protocol Data Transfer w/Local Ack Local DTE Local DCE Remote DTE Remote DCE from Call Setup Data Packet #1 RR P(R)=2 • Each Channel is Distinct • End to End is “Virtual Circuit” • Internal Network Protocol Not Specified Data Packet #1 • Each Data Pkt Has Seq Nr Internal Protocol RR P(R)=2 • Each RR Has Next Expected Seq Nr • Example Shows Local Acknowledgement Data Packet #2 RR P(R)=3 15

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

The X. 25 Protocol End of Presentation 17