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OSI Model


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The OSI Model
•Open Systems Interconnection
* Standard Model for Data Communications
* Specified by International Standards Organization (ISO)
* Adopted by CCITT/ITU
* Official Model Explained in X.200 Series

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The OSI Model
•Layered Approach to Communications
* Seven Layers Altogether
* Each Layer Performs a Unique Function
* Each Layer Has Its Own Protocol
* Protocol Messages in Upper Layer is Data to Layer Below

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The OSI Model
•The Seven Layers
Physical
Link
Network
Transport
Session
Presentation
Application
Unreliable Wire, Telco Line
Reliable Across Physical Line
Unreliable Thru Multi-Node Network
Reliable End to End
Sync Points and Dialogs
Data Representation & Syntax
User Level Processing
RS232, T1, 802.x
LAPB, HDLC
X.25 Pkt, IP
TCP
ISO Session
ISO Presentation
Telnet, FTP, Mail
Layer Name
Description
Examples

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The OSI Model
•The Standards
Physical
Link
Network
Transport
Session
Presentation
Application
Unreliable Wire, Telco Line
Reliable Across Physical Line
X.25 Pkt, Q.931, IP
X.224, TCP
X.225
ASN.1, X.409
X.400, Telnet, FTP, Mail
Layer Name
Standards
Unreliable Wire, Telco Line
X.25 LAPB, Q.921 LAPD, ISO 3309 HDLC
RS232, V.35, EIA530, X.21, T1, E1

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The OSI Model
•Physical Layer
* Message Has Begin and End
* Data in between
* Data Passed up to Link Layer
Begin
End
Data
Link Layer

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The OSI Model
•Physical Layer
* Begin Can Be Flag or STX
* End Can Be Flag or ETX
* End May Include Check Sum (CRC)
* Data Transparency
Begin
End
Data
Link Layer

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The OSI Model
•Link Layer
* Link Layer Header Has Frame Type
* Sequence and Acknowledgement Numbers
* Error Recovery Procedures
* Limited to Local Wire or Circuit
LL Hdr
Data
Network Layer

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The OSI Model
•Network Layer
* Network Header Has Packet Type
* Sequence and Acknowledgement Numbers
* No Error Recovery -- Data Can Be Lost
* Addressing across Multi-Node Network
Net Hdr
Data
Transport Layer

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The OSI Model
•Transport Layer
* Transport Header Has Packet Type
* Sequence and Acknowledgement Numbers
* Error Recovery Is End to End
* Upper Layers Can Count on Reliability
Trans Hdr
Data
Upper Layers

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The OSI Model
•Complete Picture
* A Message with All Headers
* Session, Presentation, Application Omitted
Trans Hdr
Data
Net Hdr
LL Hdr
Begin
End

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The OSI Model
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•End of Presentation