The OSI Layers Surasak Sanguanpong nguan@ku.ac.th - PDF document

1/9 The OSI Layers Surasak Sanguanpong nguan@ku.ac.th http://www.cpe.ku.ac.th/~nguan Last updated: 27 June 2002 Applied Network Research Group Department of Computer Engineering, Kasetsart University 2/9 OSI Reference Model Layer 7 The Open Systems � Application Interconnection (OSI) model was developed by Layer 6 Presentation the ISO as Layer 5 a model for computer � Session communications Layer 4 architecture Transport a framework for developing � Layer 3 protocols standards Network OSI model consists of � Layer 2 seven layers Data Link Layer 1 Physical Applied Network Research Group Department of Computer Engineering, Kasetsart University

3/9 Layering Application Programs Layer 7 Application User-oriented Layer 6 functions Presentation Layer 5 Session Layer 4 Transport Layer 3 Communications Packet Network functions Layer 2 Frame Data Link Layer 1 Bit Physical Applied Network Research Group Department of Computer Engineering, Kasetsart University 4/9 Why Seven Layers? Applications Applications Applications Transport Presentation Physical Network Need at least Physical Session and Application layers Datalink Transport Physical Network Applications Both end-to-end and Datalink hop-by-hop actions are Datalink Physical required Transport layer handles Physical Separate some end-to-end functions such application functions to as flow control Datalink concerns all the Presentation and link independent details for Session Layers Network layer handles different link technology routing in hop-by hop fashion Applied Network Research Group Department of Computer Engineering, Kasetsart University

5/9 Service Access Point (SAP) Each layer has its own � Service #M Service #1 Service #2 Service #3 functionality Layer N When a layer needs � functionality of another Layer Boundary layer, it must issue a Layer N-1 request. At each layer boundary, � there is at least one point of services called SAP = Service Access Point Applied Network Research Group Department of Computer Engineering, Kasetsart University 6/9 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 Applied Network Research Group Department of Computer Engineering, Kasetsart University

7/9 Protocol Data Unit protocol data unit (PDU) = combination of data from Data the next higher and control information Applications Encapsulation Application PDU AH Data Presentation Presentation PDU PH AH Data Session PDU Session SH PH AH Data Transport Transport PDU TH SH PH AH Data Network Network PDU NH TH SH PH AH Data Datalink Datalink PDU DH NH TH SH PH AH Data DT Physical DH DT NH TH SH PH AH Data Additional of control information to data is referred to encapsulation Applied Network Research Group Department of Computer Engineering, Kasetsart University 8/9 Decapsulation Data Decapsulation Applications AH Data Presentation PH AH Data SH PH AH Data Session In each layer, corresponding TH SH PH AH Data Transport header/trailer NH TH SH PH AH Data Network has been removed DH NH TH SH PH AH Data DT Datalink DH NH TH SH PH AH Data DT Physical Applied Network Research Group Department of Computer Engineering, Kasetsart University

9/9 OSI and TCP/IP OSI TCP/IP Application Application Application Application Presentation Presentation Session Session Transport Transport Transport Transport Network Network Internet Internet Data Link Network Access Data Link Network Access Physical Physical Physical Physical Applied Network Research Group Department of Computer Engineering, Kasetsart University