osi network reference model

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Computer Networks

OSI-ISO Model

Chandrakant Mallick

Network Models2

The design of a computer network that includes the hardware, software, access methods and protocols used.

Network models define strategies for connecting host computers and other communicating equipment.

Defines necessary elements for data communication between devices.

Network Models Contd..3

A communication model, therefore, defines a standard for the communicating hosts.

Network model provides only a conceptual framework for communications between computers

A programmer formats data in a manner defined by the communication architecture and passes it on to the communication software.

Separating communication functions adds flexibility, for example, we do not need to modify the entire host software to include more communication devices.

It typically has a layered structure

Examples of Networks Architecture

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Example of a Home Networks 5

Layered Architecture – Scenario 16

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Layered Architecture – Scenario 2

(The philosopher-translator-secretary architecture)

Benefits of Layered Architecture8

Layer architecture simplifies the network design.It is easy to debug network applications in a

layered architecture network.The network management is easier due to the

layered architecture.Network layers follow a set of rules, called

protocol.The protocol defines the format of the data being

exchanged, and the control and timing for the handshake between layers.

Open Systems Interconnection (OSI) Model9

International standard organization (ISO) established a committee in 1977 to develop an architecture for computer communication.

Open Systems Interconnection (OSI) reference model is the result of this effort.

In 1984, the Open Systems Interconnection (OSI) reference model was approved as an international standard for communications architecture.

Term “open” denotes the ability to connect any two systems which conform to the reference model and associated standards.

Open Systems Interconnection (OSI) Model contd.. 10

An ISO (International Standard Organization) that covers all aspects of network communications is the Open System Interconnection (OSI) model.

An open system is a model that allows any two different systems to communicate regardless of their underlying architecture (hardware or software).

The OSI model is not a protocol; it is model for understanding and designing a network architecture that is flexible, robust and interoperable.

Open Systems Interconnection (OSI) Model contd.. 11

The OSI model is now considered the primary Architectural model for inter-computer communications.

The OSI model describes how information or data makes its way from application programmes (such as spreadsheets) through a network medium (such as wire) to another application programme located on another network.

The OSI reference model divides the problem of moving information between computers over a network medium into SEVEN smaller and more manageable problems .

This separation into smaller more manageable functions is known as layering.

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OSI: A Layered Network Model

Analogies

Peculiar

Seriously

Take

Not

Do

Please

All

People

Seem

To

Need

Data

Processing

Seven layers of the OSI model

The interaction between layers in the OSI model

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A Layered Architecture of OSI Model

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The process of breaking up the functions or tasks of networking into layers reduces complexity.

Each layer provides a service to the layer above it in the protocol specification.

Each layer communicates with the same layer’s software or hardware on other computers.

The lower 4 layers (transport, network, data link and physical —Layers 4, 3, 2, and 1) are concerned with the flow of data from end to end through the network.

The upper four layers of the OSI model (application, presentation and session—Layers 7, 6 and 5) are orientated more toward services to the applications.

Data is Encapsulated with the necessary protocol information as it moves down the layers before network transit.

Peer-to-Peer Process15

Within a single machine, each layer calls upon services of the layer just below it.

Layer 3, for example, uses the services provided by layer 2 and provides services for layer 4.

Between machines, layer x on one machine communicates with layer x on another machine, by using a protocol (this is Peer-to-Peer Process).

Communication between machines is therefore a peer-to-peer process using protocols appropriate to a given layer.

An data exchange using the OSI model

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Interfaces between LayersThere is an interface between each pair of adjacent

layers.This interface defines what information and services

a layer must provide for the layer above it.

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Physical Layer18

The physical layer coordinates the functions required to transmit a bit stream over a physical medium.

It also defines the procedures and functions that physical devices and interfaces have to perform for transmission occur.

Physical Layer

19The physical layer is responsible for transmitting individual bits from one node to the next.

Physical Layer20

The physical layer is concerned with the following: Physical characteristics of interfaces and

media: The physical layer defines the characteristics of the interface between devices and the transmission media, including its type.

Representation of the bits: the physical layer data consist of a stream of bits without any interpretation. To be transmitted, bits must be encoded into signals –electrical or optical-. The physical layer defines the type of encoding.

Data rate: The physical layer defines the transmission rate, the number of bits sent each second.

Physical Layer21

Line configuration: the physical layer is concerned with the connection of devices to the medium.

Physical topology: The Physical layer is responsible for defining the physical layout called topology of the underlying network such as star,ring,bus,mesh and tree topology.

Transmission Mode: The physical layer is also responsible for defining the transmission mode such as simplex, half-duplex, full-duplex mode.

Data Link Layer22

The data link layer transforms the raw data streams of the physical layer to data frames to be transmitted over a reliable link and is responsible for error free node-to-node delivery of data.

It makes the physical layer appear error free to the upper layer (network layer).

Data Link Layer

23The data link layer is responsible for transmitting frames from one node to the next.

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Node-to-node delivery

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Framing. The data link layer divides the stream of bits received from the network layer into data units called frames.

Physical addressing. If frames are to be distributed to different systems on the network, the data link layer adds a header to the frame to define the physical address of the sender (source address) and/or receiver (destination address) of the frame.

If the frame is intended for a system outside the sender’s network, the receiver address is the address of the device that connects one network to the next.

Functions of the data link layer:

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Flow Control. If the rate at which the data are absorbed by the receiver is less than the rate produced in the sender, the data link layer imposes a flow control mechanism to prevent overwhelming the receiver.

Error control. The data link layer adds reliability to the physical layer by adding mechanisms to detect and retransmit damaged or lost frames. Error control is normally achieved through a trailer to the end of the frame.

Access Control. When two or more devices are connected to the same link, data link layer protocols are necessary to determine which device has control over the link at any time.


Network Layer28

The Network layer is responsible for the source-to-destination delivery of a packet possible across multiple networks.

If two systems are connected to the same link, there is usually no need for a network layer. However, if the two systems are attached to different networks, there is often a need for the network layer to accomplish source-to-destination delivery.

Network Layer Functions29

The network layer is responsible for the delivery of packets from the original source to the final destination.

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Source-to-destination delivery

Network Layer Functions31

The typical functionalities of network layer are: Logical addressing. The physical addressing

implemented by the data link layer handles the addressing problem locally.

The network layer adds a header to the packet coming from the upper layer, among other things, includes the logical address of the sender and receiver.

Routing. When independent networks or links are connected together to create an internetwork (a network of networks) or a large network, the connecting devices (called routers or gateways) route or switch the packets to their final destination.

It uses the Internet Protocol (IP) for internetworking and routing

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Network Layer FunctionsRouting

Transport Layer33

The transport layer is responsible for process-to-process delivery of the entire message.

The network layer oversees host-to-destination delivery of individual packets, it does not recognize any relationship between those packets.

The transport layer ensures that the whole message arrives intact and in order, overseeing both error control and flow control at the process-to-process level.

34The transport layer is responsible for delivery of a message from one process to another.

Transport Layer

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Reliable process-to-process delivery of a message

Functions of the Transport Layer36

The Specific functions of the transport Layer include:Port addressing: computer often run several

processes (running programs) at the same time. Process-to-process delivery means delivery from a specific process on one computer to a specific process on the other.

The transport layer header include a type of address called port address.

The network layer gets each packet to the correct computer; the transport layer gets the entire message to the correct process on that computer.

Segmentation and Reassembly: a message is divided into transmittable segments, each having a sequence number. These numbers enable the transport layer to reassemble the message correctly upon arrival at the destination.


Connection control: The transport layer can be either connectionless or connection-oriented.

A connectionless transport layer treats each segment as an independent packet and delivers it to the transport layer at the destination machine.

A connection-oriented transport layer makes a connection with the transport layer at the destination machine first before delivering the packets. After all the data are transferred, the connection is terminated.


Flow control: the transport layer performs a flow control end to end. The data link layer performs flow control across a single link.

Error control: the transport layer performs error control end to end. The data link layer performs control across a single link.

Efficiency: It ensures efficient delivery of the data packets over the network to the correct process.

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Functions of the Transport Layer

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The session layer allows two application on separate computers to set up use and terminate a connection called a session.

The session layer is the network dialog controller.

It establishes, maintains, and synchronizes the interaction between communicating devices.

The specific functions of the session layer are:Dialog control: Allows communication either in

half-duplex or full-duplex mode.Synchronization: Adds check points to the

stream of data for acknowledgement and synchronization between sender and the receiver.

The Session Layer

The session layer is responsible for dialog control and synchronization.

The Session Layer Functions41

Presentation Layer42

The presentation layer is concerned with syntax and semantics of the information exchanged between the two systems.

It carries out the functions like Protocol conversion Data translation Encryption and decryption Compression and decompression.

Presentation Layer Functions43

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

Application Layer44

The application layer enables the user to access the network services.

It provides user interfaces and support for services such as Electronic email Remote login using TELNET File transfer through FTP Web services through WWW and so on.

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Application Layer Functions

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

Summary of layers46