The OSI Reference model

Open system Interconnection Model

• The OSI Model is a standard developed on the

area of the computer networking

• It enables devices from different vendors to

work together( Interoperability)

• It enables us to visualize the process involved

in data communication

Introduction

• The OSI Reference model divides the process of data communication into 7 layers

• Each layers perform a specific task

• In each layer there is a set of protocols involved, or devices

• The seven layers1. Physical layer

2. Data link layer

3. Network layer

4. Transport layer

5. Session layer

6. Presentation layer

7. Application layer

Application layer

• It is the layer through which users or

application programs access to the network

• It provides the interface for application

programs to access the network

• Includes programs and protocols like

– FTP: File transfer protocol

– SMTP: Simple mail transfer protocol

Presentation Layer

• Handles formatting of data to common form

• In the sending computer, it translates data from

the format sent down from the application layer

into a commonly recognized, intermediary format

• At the receiving computer, this layer translates

the intermediary format into a format that can be

useful to that computer's application layer

• It handles encryption and decryption,

compression and decompression

Session layer

• open, use, and close a connection called a

session.

• Performs dialog between computers

Transport layer

• Prepare the data for transportation

• Divide message into smaller part that can be transmitted at a time. This process is called segmentation

• The transport layer provides flow control and error handling, and participates in solving problems concerned with the transmission and reception of packets

• Data at this layer is called segment

Network layer

• Responsible for computer’s logical addressing

• It translate logical address to physical address

• Determines the route through which the data

is transmitted

• Devices at this layer can process data using

the logical address(like IP address)

– Router

• Data at this layer is called packet

Data Link Layer

• Adds physical address on the data

• Converts data to a form that can be transmitted into the network media

• It controls the electrical impulses that enter and leave the network cable.

• Devices on this layer can process data by using the physical address(MAC Address)

– NIC, Switch, Bridge

• Data at this layer is called frame

Physical layer

• It is the layer on which the actual data is transmitted

• Defines the medium through which the bits are transmitted

• The physical layer is responsible for transmitting bits (zeros and ones) from one computer to another

• Devices at this layer do not process data but simply receive and pass data in a form of bits

– Cables, connectors, repeaters, hubs

Network Protocol

• Protocols are rules and procedures for

communicating

• Two computers can communicate with each

other if

– They use the same protocol or

– They use compatible protocols

Protocol stack(Suit):

• Set of protocols that work together

• Each layer of the stack specifies a different

protocol for handling a function or subsystem of

the communication process

• Protocol stack defines the process of networking

into layers. And in each layers there is a specific

protocol to perform a specific task on that layer

• Protocols exist at each layer of these stacks,

performing the tasks specified by that layer

Protocol stack examples

• The ISO/OSI protocol suite.

• TCP/IP

• IPX/SPX

TCP/IP

• Transmission control protocol/Internet

protocol

• TCP/IP was developed by the United States

Department of Defense as a set of wide area

network (WAN) protocols

• It is currently the standard protocol in the

Internet

TCP/IP and OSI

• The TCP/IP protocol does not exactly match the OSI reference model

• It divides the process of networking into four layers

– Network interface layer

– Internet layer

– Transport layer

– Application layer

• Each of these layers corresponds to one or more layers of the OSI reference model.

Network Interface Layer

• corresponding to the physical and data-link

layers of the OSI reference model,

Internet Layer

• corresponding to the network layer of the OSI reference model

• Protocols– Internet Protocol(IP): is a protocol that performs

addressing and route selection

– Address resolution protocol(ARP) : The ARP determines hardware address (MAC addresses) that correspond to an IP address

– Reverse Address Resolution Protocol (RARP): changes a MAC address to an IP address

– ICMP(Internet control management protocol : protocols to send and receive status reports about information being transmitted eg: pinging

Transport layer

• corresponding to the transport layer of the OSI reference model

• The transport layer provides acknowledgment of receipt, flow control, and sequencing of packets.

• Transport layer uses either UDP or TCP– TCP- is connection oriented. i.e. each segment is

acknowledged by the receiver (data communication on the internet)

– UDP(user data gram protocol) –connectionless. In this protocol the sender doesn’t need acknowledgement to send the next data(e.g: TV broadcast on the Internet)

TCP VS UDP

TCP

• TCP is connection-oriented protocol. When a file or message send it will get delivered unless connections fails. If connection lost, the server will request the lost part. There is no corruption while transferring a message.

• Reliable

• slower

UDP

• UDP is connectionless

protocol. When you a send

a data or message, you

don't know if it'll get there,

it could get lost on the way.

There may be corruption

while transferring a

message.

• Unreliable

• fast

TCP vs. UDP

Application layer

• Corresponding to the session, presentation,

and application layers of the OSI reference

model

Protocol Data Units(PDUs)

• PDUs is the data with control information at each

layer of the OSI reference model

• The control information is attached to the header

or the trailer

• Each layer protocol data unit is defined as follows

– Transport layer PDUs: Segment

– Network layer PDUs: Packet

– Data link layer PDUs: Frame

– Physical layer PDUs:bit

Data Encapsulation

• Data Encapsulation is a process of taking one Protocol Data Unit (PDU) and enveloping it within a set of protocol header and trailer. So that it can be handled by the next lower layer– It is performed by the sending computer

– Data->segment->packet->frame->bits

• De-encapsulation is the process of taking one PDU and removing the header and trailer of the PDU. So that it can be handled by the next upper layer.– It is the reverse of encapsulation

– it is performed in the receiving computer

– Bits->frame->packet->segment->data