1. X.25 Protocol

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

3. 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

4. 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

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

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
Packet Level
LAPB Hdr
Information Field

7. 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

8. 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

9. The X.25 Protocol LAPB Data Transfer Local DCE Local DTE
I-Frame Contains Packet
Seq from 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

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
Application Layer
Pkt Hdr
Data

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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

17. The X.25 Protocol
End of Presentation