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

The OSI Model

• 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.

• The OSI model is a layered framework for the design of network systems that allows for communication across all types of computer systems.

• The OSI model is built of seven ordered layers:

1. (layer 1) physical layer

2. (layer 2) data link

3. (layer 3) network layer

4. (layer 4) transport layer

5. (layer 5) session layer

6. (layer 6) presentation layer

7. (layer 7) application layer

THE SEVEN OSI REFERENCE MODEL

LAYERS

Reference ModelsO

S

I

Physical Layer

• 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.

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

Physical Layer• The physical layer deals with the physical characteristics of the

transmission medium.

• It defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between end systems.

• Converts bits into electronic signals for outgoing messages

• Converts electronic signals into bits for incoming messages

• This layer manages the interface between the the computer and the network medium (coax, twisted pair, etc.)

Physical layerThe 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.

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

• Physical topology

• Transmission Mode

Data Link Layer

• The data link layer transforms the physical layer, a raw transmission facility, to a reliable link and is responsible for node-to-node delivery. It makes the physical layer appear error free to the upper layer (network layer).

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

Data Link Layer

• Handles special data frames (packets) between the Network layer and the Physical layer

• At the receiving end, this layer packages raw data from the physical layer into data frames for delivery to the Network layer

• At the sending end this layer handles conversion of data into raw formats that can be handled by the Physical Layer

• It provides reliable transit of data across a physical link by using the Media Access Control (MAC) addresses

Node-to-node delivery

Functions of the data link layer:

• 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.

• 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 Layer

•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 LayerFunctions:•Logical addressing.•Routing

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

Source-to-destination delivery

• 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.

Transport Layer

• 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.

Transport layer

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

Reliable process-to-process delivery of a message

Functions of the transport layer

• 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.

Functions of the transport layer• 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.

• The session layer is the network dialog controller. It was designed to establish, maintain, and synchronize the interaction between communicating devices.

• The presentation layer was designed to handle the syntax and semantics of the information exchanged between the two systems. It was designed for data translation, encryption, decryption, and compression.

• The application layer enables the user to access the network. It provides user interfaces and support for services such electronic email, remote file access, WWW, and so on.

LAYER 5: SESSION LAYER

• The session layer defines how to start, control and end conversations (called sessions) between applications.

• This includes the control and management of multiple bi-directional messages using dialogue control.

• It also synchronizes dialogue between two hosts' presentation layers and manages their data exchange.

• Examples :- SQL, ASP(AppleTalk Session Protocol).

Enables two networked resources to hold ongoing communications (called to exchange data for the duration of the session responsible for initiating, maintaining and terminating sessions a session) across a network

Responsible for security and access control to session information (via session participant identification)

Responsible for synchronization services

(checkpoints)

The Session Layer

• Responsibilities:– establishes, manages, and terminates sessions

between applications.– service location lookup

LAYER 6: PRESENTATION

• The presentation layer ensures that the information that the application layer of one system sends out is readable by the application layer of another system.

• Concerned with Semantics and syntax of the information

• Provides encryption and compression of data.• Examples :- JPEG, MPEG, ASCII, EBCDIC, HTML.

Manages data-format information for networked communications (the network’s translator)

For outgoing messages, it converts data into a generic format for network transmission; for incoming messages, it converts data from the generic network format to a format that the receiving application can understand. If necessary, the presentation layer translates between multiple data formats by using a If necessary, the presentation layer translates between multiple data formats by using a

common format. common format.

This layer is also responsible for certain protocol conversions, data encryption/decryption, or data compression/decompression.

The Presentation Layer

• Responsibilities:– data encryption– data compression– data conversion

The top layer of the OSI model

Provides a set of interfaces for sending and receiving applications to gain access to and use network services, such as: networked file transfer, message handling and database query processing

LAYER 7: APPLICATIONLAYER 7: APPLICATION

• The application layer is the OSI layer that is closest to the user.

• It provides network access to the user.

• It differs from the other layers in that it does not provide services to any other OSI layer, but rather, only to applications outside the OSI model.

• Examples of such applications are spreadsheet programs, word processing programs, and bank terminal programs.

Application layer

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

Headers

Process

Transport

Network

Data Link

Process

Transport

Network

Data Link

DATA

DATA

DATA

DATA

H

H

H

H

HH

What are the headers?

Physical: no header - just a bunch of bits.

Data Link: – address of the receiving endpoints– address of the sending endpoint– length of the data– checksum.

Layering & Headers • Each layer needs to add some control information

to the data in order to do it’s job. • This information is typically appended to the data

before being given to the lower layer.• Once the lower layers deliver the the data and

control information - the peer layer uses the control information.

Summary

• Data-Link: communication between machines on the same network.

• Network: communication between machines on possibly different networks.

• Transport: communication between processes (running on machines on possibly different networks).

Summary of duties

Protocol Hierarchies

Services to Protocols Relationship

• The relationship between a service and a protocol.

Reference Models

The OSI reference model.