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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. The X.25 Protocol. The Model Network Has Multiple Nodes (DCEs)

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the 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
the x 25 protocol1
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

the x 25 protocol2
Flag

LAPB Hdr

Pkt Hdr

Data

CRC

Flag

Network

X.25 Packet Level

Link

X.25 Frame Level (LAPB)

Physical

RS232, etc

The X.25 Protocol

The X.25 Protocol Layers

Layer Name

Description

the x 25 protocol3
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

the x 25 protocol4
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

the x 25 protocol5
N(R)

P/F

N(S)

0

RR

RNR

REJ

N(R)

P/F

1

DISC

SABM

UA

DM

FRMR

P/F

1

1

The X.25 Protocol

LAPB Header

Address

Control

  • 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

Information Frame

Supervisory Frames

Unnumbered Frames

the x 25 protocol6
Local DTE

Local DCE

The X.25 Protocol

LAPB Link Setup and Disconnect

  • 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

the x 25 protocol7
Local DTE

Local DCE

The X.25 Protocol

LAPB Data Transfer

  • 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 #1

RR N(R)=2

I-Frame #2

RR N(R)=3

I-Frame #3

I-Frame #0 N(R)=4

the x 25 protocol8
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

the x 25 protocol9
Channel Nr

Pkt Type

P(R)

P(S)

0

RR

RNR

P(R)

1

Call Req

Call Acpt

Clr Req

Clr Conf

Reset Req

Reset Conf

Intr Req

Intr Conf

Restart Req

Restart Conf

1

1

The X.25 Protocol

Packet Level Header

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

Acknowledgement Packets

Other Packets

the x 25 protocol10
Local DTE

Local DCE

Remote DCE

Remote DTE

The X.25 Protocol

Call Setup

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

the x 25 protocol11
Local DTE

Local DCE

Remote DCE

Remote DTE

The X.25 Protocol

Call Clearing

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

the x 25 protocol12
Local DTE

Local DCE

Remote DCE

Remote DTE

The X.25 Protocol

Data Transfer w/End to End Ack

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

the x 25 protocol13
Local DTE

Local DCE

Remote DCE

Remote DTE

The X.25 Protocol

Data Transfer w/Local Ack

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

the x 25 protocol14
Local DTE

Local DTE

Data Packet #1

RR P(R)=2

Data Packet #2

RR P(R)=3

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
the x 25 protocol15
The X.25 Protocol

End of Presentation

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