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

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Applied Network Research Group Department of Computer Engineering, Kasetsart University 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

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OSI 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

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Relation between layers at boundary

• 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 N-1

Service #1 Service #2 Service #3 Service #M

Layer Boundary

= Service Access Point

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Why Seven Layers?

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

Applications Physical Applications Data link Physical Applications Transport Network Data link Physical

Need at least Physical and Application layers Datalink concerns all the link independent details for different link technology

Applications Presentation Network Data link Physical Session Transport

Separate some application functions to Presentation and Session Layers

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Encapsulation

Data Data AH

Applications Presentation Session Transport Network Data link Physical

Data AH PH Data AH PH SH Data AH PH SH TH Data AH PH SH TH NH Data AH PH SH TH NH DH DT

Application PDU Presentation PDU Session PDU Transport PDU Network PDU Data link PDU

protocol data unit (PDU) = combination of data from the next higher and control information Additional of control information to data is referred to encapsulation

Encapsulation

Data AH PH SH TH NH DH DT

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Decapsulation

Data Data AH

Applications Presentation Session Transport Network Data link Physical

Data AH PH Data AH PH SH Data AH PH SH TH Data AH PH SH TH NH Data AH PH SH TH NH DH DT

In each layer, corresponding header/trailer has been removed

Data AH PH SH TH NH DH DT

Decapsulation

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Virtual Communications

Session protocol Network Access Protocol Application protocol

Applications Presentation Network Data link Physical Session Transport Applications Presentation Network Data link Physical Session Transport

Physical Media

Presentation protocol Transport protocol Datalink protocol Physical protocol

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Layering Classification

Layer 1 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

Application Programs

Segment

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Physical Layer

Layer 1 Physical Layer 2 Data Link Layer 3 Network Layer 4 Transport Layer 5 Session Layer 6 Presentation Layer 7 Application

• It deals with the mechanical and

electrical specifications of the interface and transmission

• medium. The physical layer is

responsible for several jobs:

Physical characteristics of

interface and media

Representation of bits Transmission rate Synchronization Topology Transmission mode

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Datalink Layer

Layer 1 Physical Layer 2 Data Link Layer 3 Network Layer 4 Transport Layer 5 Session Layer 6 Presentation Layer 7 Application

• The data link layer provides reliable

transit of data across a physical

• link. This layer is concerned with:

manages physical addressing

schemes such as MAC addresses for Ethernet networks

checks for physical transmission

errors and packages bits into frames

physical link management

(Medium access control)

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Network Layer

Layer 1 Physical Layer 2 Data Link Layer 3 Network Layer 4 Transport Layer 5 Session Layer 6 Presentation Layer 7 Application

• The network layer is responsible

for source to destination delivery

• f a packet possibly across

multiple paths. The network layer is concerned with the following primary functions:

maintains logical addresses such

as IP addresses

Routing (directing packet from

• ne network to another)

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Transport Layer

Layer 1 Physical Layer 2 Data Link Layer 3 Network Layer 4 Transport Layer 5 Session Layer 6 Presentation Layer 7 Application

• The transport layer is responsible for

source-to-destination (end-to-end) delivery). This layer is concerned with:

• Connection control
• Error control (ensures complete data

transfer by:

detect errors and lost data Recover lost data Manage retransmission of data

• Flow control (process of adjusting the flow
• f data between sender and receiver to

ensure that the receiver can handle all of the incoming data)

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Session Layer

Layer 1 Physical Layer 2 Data Link Layer 3 Network Layer 4 Transport Layer 5 Session Layer 6 Presentation Layer 7 Application

• The session layer provides the

mechanism for managing the dialogue between end-user application processes such as:

Dialogue control (either duplex

• r half-duplex operation)

Synchronization : allow process

to establishes checkpoints (synchronization points)

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Presentation Layer

Layer 1 Physical Layer 2 Data Link Layer 3 Network Layer 4 Transport Layer 5 Session Layer 6 Presentation Layer 7 Application

• The presentation layer handles the

conversion of data between a Standards-based or platform independent formats to a format understood by the local machine. Specific responsibilities of the presentation layer are as followings:

Data translations Data encryption Data compression Applied Network Research Group Department of Computer Engineering, Kasetsart University 16/18

Application Layer

Layer 1 Physical Layer 2 Data Link Layer 3 Network Layer 4 Transport Layer 5 Session Layer 6 Presentation Layer 7 Application

• It interfaces directly to and

performs common application services for the application

• processes. Examples of

applications that utilize the network are:

Remote login File Transfer Electronics mail

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Example of representation

Application Presentation Session Transport Network Data Link Physical

Network Data Link Physical Data Link Physical

Application Presentation Session Transport Network Data Link Physical Applied Network Research Group Department of Computer Engineering, Kasetsart University 18/18

OSI and TCP/IP

TCP/IP OSI

Application Application Presentation Presentation Session Session Transport Transport Network Network Data Link Data Link Physical Physical Application Application Transport Transport Internet Internet Network Access Network Access Physical Physical