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The OSI Layers Surasak Sanguanpong

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Presentation on theme: "The OSI Layers Surasak Sanguanpong"— Presentation transcript:

1 The OSI Layers Surasak Sanguanpong nguan@ku.ac.th
Last updated: 27 June 2002

2 OSI Reference Model Layer 1 Physical Layer 2 Data Link Layer 3 Network Layer 4 Transport Layer 5 Session Layer 6 Presentation Layer 7 Application The Open Systems Interconnection (OSI) model was developed by the ISO as a model for computer communications architecture a framework for developing protocols standards OSI model consists of seven layers

3 Layering Packet Frame Bit Application Programs Layer 7 Application
Physical Layer 2 Data Link Layer 3 Network Layer 4 Transport Layer 5 Session Layer 6 Presentation Layer 7 Application User-oriented functions Communications functions Packet Frame Bit

4 Why Seven Layers? Applications Applications Applications Transport
Physical Transport Presentation Network Need at least Physical and Application layers Session Datalink Transport Physical Network Applications Both end-to-end and hop-by-hop actions are required Transport layer handles end-to-end functions such as flow control Network layer handles routing in hop-by hop fashion Datalink Datalink Physical Physical Separate some application functions to Presentation and Session Layers Datalink concerns all the link independent details for different link technology

5 Service Access Point (SAP)
Service #M Each layer has its own functionality When a layer needs functionality of another layer, it must issue a request. At each layer boundary, there is at least one point of services called SAP Layer N Layer Boundary Layer N-1 = Service Access Point

6 Virtual Communications
Application protocol Applications Applications Presentation protocol Presentation Presentation Session protocol Session Session Transport protocol Transport Transport Network Access Protocol Network Network Datalink protocol Datalink Datalink Physical protocol Physical Physical Physical Media

7 Protocol Data Unit Encapsulation
protocol data unit (PDU) = combination of data from the next higher and control information Data Applications Encapsulation Application PDU Data AH Presentation Presentation PDU Data AH PH Session Session PDU Data AH PH SH Transport Transport PDU Data AH PH SH TH Network Network PDU Data AH PH SH TH NH Datalink Datalink PDU Data AH PH SH TH NH DH DT Physical Data AH PH SH TH NH DH DT Additional of control information to data is referred to encapsulation

8 Decapsulation Decapsulation Applications Presentation Session
Data Decapsulation Applications Data AH Presentation Data AH PH Session Data AH PH SH In each layer, corresponding header/trailer has been removed Transport Data AH PH SH TH Network Data AH PH SH TH NH Datalink Data AH PH SH TH NH DH DT Physical Data AH PH SH TH NH DH DT

9 OSI and TCP/IP OSI TCP/IP Application Presentation Session Transport
Network Data Link Physical Application Transport Internet Network Access Physical

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