1/18 The OSI Reference Model Surasak Sanguanpong nguan@ku.ac.th http://www.cpe.ku.ac.th/~nguan Last updated: 2 November 2004 Applied Network Research Group Department of Computer Engineering, Kasetsart University 2/18 Outline � Layering Protocol � OSI Reference Model Applied Network Research Group Department of Computer Engineering, Kasetsart University 1
3/18 OSI Model Layer 7 The Open Systems � Application Interconnection (OSI) Layer 6 model was developed by Presentation the ISO as Layer 5 a model for computer � Session communications architecture Layer 4 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 4/18 Relation between layers at boundary 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 2
5/18 Why Seven Layers? Applications Applications Applications Transport Presentation Physical Network Need at least Physical Session and Application layers Data link Transport Physical Network Applications Both end-to-end and hop- Data link by-hop actions are required Data link Physical Transport layer handles end-to-end functions such Physical Separate some as flow control application functions to Datalink concerns all the Network layer handles Presentation and link independent details for Session Layers routing in hop-by hop different link technology fashion Applied Network Research Group Department of Computer Engineering, Kasetsart University 6/18 Encapsulation protocol data unit (PDU) = combination of data from Data the next higher and control information Encapsulation Applications Application PDU AH Data Presentation PDU Presentation PH AH Data Session Session PDU SH PH AH Data Transport PDU TH SH PH AH Data Transport Network PDU Network NH TH SH PH AH Data Data link PDU DH DT Data link NH TH SH PH AH Data Physical DH NH TH SH PH AH Data DT Additional of control information to data is referred to encapsulation Applied Network Research Group Department of Computer Engineering, Kasetsart University 3
7/18 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 Data link DH NH TH SH PH AH Data DT Physical Applied Network Research Group Department of Computer Engineering, Kasetsart University 8/18 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 Data link Data link Physical protocol Physical Physical Physical Media Applied Network Research Group Department of Computer Engineering, Kasetsart University 4
9/18 Layering Classification Application Programs Layer 7 Application User-oriented Layer 6 functions Presentation Layer 5 Session Layer 4 Segment 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 10/18 Physical Layer It deals with the mechanical and � Layer 7 Application electrical specifications of the interface and transmission Layer 6 Presentation medium. The physical layer is responsible for several jobs: Layer 5 Session � Physical characteristics of Layer 4 interface and media Transport � Representation of bits Layer 3 Network � Transmission rate Layer 2 � Synchronization Data Link � Topology Layer 1 Physical � Transmission mode Applied Network Research Group Department of Computer Engineering, Kasetsart University 5
11/18 Datalink Layer Layer 7 The data link layer provides reliable � Application transit of data across a physical Layer 6 link. This layer is concerned with: Presentation � manages physical addressing Layer 5 schemes such as MAC addresses Session for Ethernet networks Layer 4 Transport � checks for physical transmission errors and packages bits into Layer 3 Network frames Layer 2 � physical link management Data Link (Medium access control) Layer 1 Physical Applied Network Research Group Department of Computer Engineering, Kasetsart University 12/18 Network Layer Layer 7 The network layer is responsible � Application for source to destination delivery of a packet possibly across Layer 6 Presentation multiple paths. The network layer is concerned with the following Layer 5 Session primary functions: Layer 4 � maintains logical addresses such Transport as IP addresses Layer 3 Network � Routing (directing packet from one network to another) Layer 2 Data Link Layer 1 Physical Applied Network Research Group Department of Computer Engineering, Kasetsart University 6
13/18 Transport Layer Layer 7 The transport layer is responsible for � Application source-to-destination (end-to-end) Layer 6 delivery). This layer is concerned with: Presentation Connection control � Layer 5 Session Error control (ensures complete data � transfer by: Layer 4 Transport � detect errors and lost data Layer 3 Network � Recover lost data Layer 2 � Manage retransmission of data Data Link Flow control (process of adjusting the flow � Layer 1 of data between sender and receiver to Physical ensure that the receiver can handle all of the incoming data) Applied Network Research Group Department of Computer Engineering, Kasetsart University 14/18 Session Layer Layer 7 The session layer provides the � Application mechanism for managing the dialogue between end-user Layer 6 Presentation application processes such as: Layer 5 � Dialogue control (either duplex Session or half-duplex operation) Layer 4 Transport � Synchronization : allow process to establishes checkpoints Layer 3 Network (synchronization points) Layer 2 Data Link Layer 1 Physical Applied Network Research Group Department of Computer Engineering, Kasetsart University 7
15/18 Presentation Layer Layer 7 The presentation layer handles the � Application conversion of data between a Layer 6 Standards-based or platform Presentation independent formats to a format Layer 5 understood by the local machine. Session Specific responsibilities of the presentation layer are as followings: Layer 4 Transport � Data translations Layer 3 Network � Data encryption Layer 2 � Data compression Data Link Layer 1 Physical Applied Network Research Group Department of Computer Engineering, Kasetsart University 16/18 Application Layer Layer 7 It interfaces directly to and � Application performs common application Layer 6 services for the application Presentation processes. Examples of applications that utilize the Layer 5 Session network are: Layer 4 � Remote login Transport � File Transfer Layer 3 Network � Electronics mail Layer 2 Data Link Layer 1 Physical Applied Network Research Group Department of Computer Engineering, Kasetsart University 8
17/18 Example of representation Application Application Presentation Presentation Session Session Transport Transport Network Network Network Data Link Data Link Data Link Data Link Physical Physical Physical Physical Applied Network Research Group Department of Computer Engineering, Kasetsart University 18/18 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 Data Link Network Access Network Access Physical Physical Physical Physical Applied Network Research Group Department of Computer Engineering, Kasetsart University 9
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