X.25 Protocol

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The X.25 Protocol

CCITT 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 Protocol

The ModelNetwork 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 Protocol

The X.25 Protocol Layers

Physical

Link

Network

RS232, etc

X.25 Frame Level (LAPB)

X.25 Packet Level

Layer Name Description

DataPkt HdrLAPB HdrFlag FlagCRC

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The X.25 Protocol

Physical Layer – Sort ofFlag Character (01111110) at Beginning and EndData in between is LAPB FrameData Passed up to Frame Level

Data Transparency 011111010 011111 10

Flag CRCData

Frame Level (LAPB)

Flag

0

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The X.25 Protocol

Frame 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 Protocol

LAPB Header

Address Control

N(R) 0N(S)P/F

N(R) 1RR

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

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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

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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

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The X.25 Protocol

The X.25 Packet LevelPacket 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 Protocol

Packet Level Header

Channel Nr Pkt Type

P(R) 0P(S)

P(R) 1RR

RNR

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 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

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The X.25 Protocol

Call 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 Protocol

Data 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 Protocol

Data Transfer w/Local Ack

Local DCE

Remote DCE

Local DTE

Remote DTE

Data Packet #1

Data Packet #1RR 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)=3Data Packet #2

RR P(R)=3

Remote DCE from Call Setup

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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

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The X.25 Protocol

End of Presentation