NETWORK MODELS

T.Najah Al_Subaie

Kingdom of Saudi Arabia

Prince Norah bint Abdul Rahman University

College of Computer Since and Information System

NET331

Chapter 2

LAYERED TASKS OSI Model TCP/IP Model

Layered Tasks

We use the concept of layers in our daily life. As an example, let us consider two friends who communicate through postal mail.

The process of sending a letter to a friend would be complex if there were no services available from the post office.

Tasks Involved in Sending a Letter

The OSI Model

Established in 1947, the International Standards Organization

(ISO) is a multinational body dedicated to worldwide agreement on international standards.

An ISO standard that covers all aspects of network communications is the Open Systems Interconnection (OSI) model.

It was first introduced in the late 1970s.

Note

ISO is the organization. OSI is the model.

Seven layers of the OSI model

The OSI Model

ISO model is not a protocol. It is a model for understanding and designing a network architecture.

It shows how to facilitate communication between different system without requiring changes to the logic of the underlying hardware and software.

The OSI Model

It is a layered framework for the design of network systems that allows communication between all types of computer systems.

Consists of seven separated but related layers.

Each layer defines a part of the process of moving information across a network

Layered Architecture

Each layer groups the networking functions that had related uses.

Each layer uses the services provided by the layer just below it.

Between machines, the corresponding layers are communicated using a set of protocols. Peer to Peer processes.

The interaction between layers in the OSI model

The philosopher-translator-secretary architectu

Interfaces Between Layers

Each interface defines the information and services a layer must provide for the layer above it.

Interfaces allows the specific implementation of the layer’s functionality to be changed or replaced without requiring changes to the surrounding layers.

An exchange using the OSI model

Organization of the Layer

At the sender: each layer adds a header or possibly a

trailer. At the receiver:

each layer removes its corresponding header or trailer and performs the appropriate actions.

When the message reaches layer 7 (application layer) , the message is in a form appropriate to the applications.

Encapsulation

The data portion of a packet at level N-1 carries the whole packet ( data and header/ trailer) from level N.

Layers in the OSI Model.

We briefly describe the functions of each layer in the OSI model. Physical Layer Data Link Layer Network Layer Transport Layer Session Layer Presentation Layer Application Layer

Physical Layer

The physical layer is responsible for movements of individual bits from one hop (node) to the next.

Physical Layer

It is also concerned with: Physical characteristics of interfaces and

medium. Representation of bits. Data rate. Synchronization of bits. Line configuration. Physical topology. Transmission mode.

Data Link Layer

The data link layer (DLL) is responsible for moving frames from one hop (node) to the next.

Data Link Layer

Data Link Layer

It is also concerned with: Framing.

Data units in DLL. Physical addressing

Adding header to define header/receiver of the frame. Flow control

To avoid overwhelming the receiver. Error control

A set of mechanisms to keep a reliable transmission. Access control

To control the traffic in the link.

Hop-to-hop delivery by the DLL

Network Layer

The network layer is responsible for the delivery of individual packets from the source host to the destination host.

Network Layer

Source-to-Destination Delivery

Transport Layer

The transport layer is responsible for the delivery of a message from one process to another.

Transport Layer

Transport Layer

It is also concerned with: Service point addressing. Segmentation and reassembly. Connection control. Flow control. Error control.

Process to Process Delivery by the Transport Layer

Session Layer

The session layer is responsible for dialog control and synchronization.

Its functionalities: Dialog control. Synchronization.

Session Layer

Presentation Layer

The presentation layer is responsible for translation, compression, and encryption.

Presentation Layer

Application Layer

The application layer is responsible for providing services to the user.

Application Layer

Summary of Layers

TCP/IP Reference Model

the TCP/IP protocol suite is made of five layers: physical, data link, network, transport, application.

TCP/IP and OSI model