X.25 Protocol
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X.25 Protocol
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The X.25 ProtocolCCITT Recommendation X.25First Published in 1976Revisions Every 4 Years -- 1980, 1984, 1988Interface Protocol for Packet Switched NetworksNetwork Providers Intended to Be Telcos
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The X.25 ProtocolThe Model
Network Has Multiple Nodes (DCEs)Host Computers (DTEs) OutsideHosts Have Addresses Like Phone NumbersVirtual Call SetupVirtual Call ClearData Transfer
DTE
DTE
DCE
DCE
DCE
DCEDCEX.25
X.25
X.25
Intra-Network Protocol
DTE
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The X.25 ProtocolThe X.25 Protocol Layers
PhysicalLink
Network
RS232, etcX.25 Frame Level (LAPB)
X.25 Packet LevelLayer Name Description
DataPkt HdrLAPB HdrFlag FlagCRC
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The X.25 ProtocolPhysical Layer – Sort of
Flag Character (01111110) at Beginning and EndData in between is LAPB FrameData Passed up to Frame LevelData Transparency 011111010 011111 10
Flag CRCData
Frame Level (LAPB)
Flag
0
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The X.25 ProtocolFrame Level – LAPBFrame Level Header Has Frame TypeSequence and Acknowledgement NumbersError Recovery ProceduresEndpoints Are DTE and Local DCE
LAPB Hdr Information Field
Packet Level
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The X.25 ProtocolLAPB Header
Address Control
N(R) 0N(S)
P/F
N(R) 1RRRNRREJ
P/FDISCSABMUADM
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
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The X.25 ProtocolLAPB 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
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The X.25 ProtocolLAPB 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
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The X.25 ProtocolThe X.25 Packet Level
Packet Header Has Packet TypeChannel Number Identifies Logical ConnectionSequence and Acknowledgement NumbersNo Error Recovery -- Data Can Be LostAddressing across Multi-Node Network
Pkt Hdr Data
Application Layer
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The X.25 ProtocolPacket Level Header
Channel Nr
Pkt Type
P(R) 0P(S)
P(R) 1RRRNR
Data Packet
Acknowledgement Packets
Other Packets
Call ReqCall AcptClr ReqClr ConfReset ReqReset ConfIntr ReqIntr Conf
Restart ReqRestart Conf
1 1
•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
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The X.25 ProtocolCall 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
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The X.25 ProtocolCall Clearing
Local DCE
Remote DCE
Local DTE
Remote DTE
Clear Request
Clear Indication
Clear Confirm
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
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The X.25 ProtocolData Transfer w/End to End Ack
Local DCE
Remote DCE
Local DTE
Remote DTE
Data Packet #1
Data Packet #1
RR P(R)=2
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
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The X.25 ProtocolData Transfer w/Local Ack
Local DCE
Remote DCE
Local DTE
Remote DTE
Data Packet #1
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)=2Data Packet #2
RR P(R)=3 Data Packet #2
RR P(R)=3
Remote DCE from Call Setup
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The X.25 ProtocolX.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
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The X.25 Protocol
End of Presentation
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