Transcript The OSI Model

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
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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
DTE
DCE
DCE
X.25
DCE
DTE
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The X.25 Protocol
The X.25 Protocol Layers
Layer Name
Description
Network
X.25 Packet Level
Link
X.25 Frame Level (LAPB)
Physical
RS232, etc
Flag
LAPB Hdr
Pkt Hdr
Data
CRC
Flag
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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
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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
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The X.25 Protocol
LAPB Header
Address
•N(R) is Ack Nr and Counts
Modulo 8
Control
N(R)
P/F
•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(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
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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
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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 #3
I-Frame #0 N(R)=4
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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
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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
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The X.25 Protocol
Call Setup
Local
DTE
Local
DCE
Remote
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
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The X.25 Protocol
Call Clearing
Local
DTE
Local
DCE
Remote
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
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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
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The X.25 Protocol
Data Transfer w/Local Ack
Local
DTE
Local
DCE
Remote
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
Data Packet #2
RR P(R)=3
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The X.25 Protocol
X.25 Without a Network
Local
DTE
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
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The X.25 Protocol
End of Presentation
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