OSI MODEL

 

OSI Model

 

INTRODUCTION

•        The OSI reference model was introduced by the International Organization for Standardization (ISO) in 1984, in order to provide a reference model to make sure products of different vendors would interoperate in networks.

•        OSI stands for Open System Interconnection, is a reference model that describes how information from a software application in one computer moves through a physical medium to the software application in another computer.

•        It is layered approach i.e. OSI consists of seven layers.

•        Each layer performs a particular network function & have its own function.

•        Every layers adds up a header to the data after performing function.

•        Header added by the layer can only be verified and removed by the peer layer.

 

CHARACTERISTICS

 

- The OSI model is divided into two group layers : The Upper layers and the lower layers.
- The upper layers – Application layers are more often layers that interact with the end software on the computer. While the data transport layers deals with raw network data including the physical layer. The application layer directly interacts with the end user.
- The physical layer and the data link layer are implementations of both hardware and software combinations. The lowest layer, the physical layer directly interacts with the network medium which is a network cable or a wireless medium

DATA ENCAPSULATION & DECASPSULATION

All communications on a network originate at a source, and are sent to a destination. The information sent on a network is referred to as data or data packets.

If one computer (host A) wants to send data to another computer (host B), the data must first be packaged through a process called encapsulation.

Encapsulation wraps data with the necessary protocol information before network transit.

Therefore, as the data packet moves down through the layers of the OSI model, it receives headers, trailers, and other information.

 

PROTOCOL DATA UNITS (PDU)

"Protocol Data Unit." A PDU is a specific block of information transferred over a network. It is often used in reference to the OSI model, since it describes the different types of data that are transferred from each layer

 

 

 

 

 

PROTOCOLS

 

OSI protocols are a family of standards for information exchange. These were developed and designed by the International Organization of Standardization (ISO).

Each layer of the ISO model has its own protocols and functions. The OSI protocol stack was later adapted into the TCP/IP stack

Function of each layer :-

1. Application layer (7^th Layer)

 

•       The application layer is responsible for making the network request (sending computer) or servicing the request (receiving computer). For example, when a user submits a request from a web browser, the web browser is responsible for the submission of the request and is running at this layer. When that web request reaches the web server, the web server, running at the application layer, accepts that request.

 

Features of application layer:

•       It is a top most layer or it is seventh layer of OSI model.

•       It gives user interface to access network resources.

•       Identifies process by port number.

•       PDU is raw data.

•       Application layer gives network services.

Network services

•       Web service – HTTP, HTTPS

•       E-mail service – SMTP, POP3, IMAP4

•       File service – FTP, TFTP, SFTP

•       DHCP service

•       DNS service

•       Remote service – RDS, Telnet, SSH

 

2. Presentation layer (6^th Layer)

 

•       It is 6^th layer of OSI model. It is responsible for ensuring that the communications that are passing through the network are in proper formatting for the end user.

•       After the request is made, the application layer passes the data down to the presentation layer, where it is to be formatted so that the data (or request) can be interpreted by the receiving system. When the presentation layer receives data from the application layer to be sent over the network, it makes sure that the data is in the proper format if it is not, the presentation layer converts the data. On the receiving system, when the presentation layer receives network data from the session layer, it makes sure that the data is in the proper format and once again converts it if it is.

 

Benefits of presentation layer

•       Standard data format.

•       Data encryption and decryption.

•       Data compression and decompression.

•       Encoding and decoding.

 

 

3. Session layer (5^th Layer)

 

•       It establish, manage, and terminate communications between two computers.

•       When a session is established, three distinct phases are involved :-

         I. Establishment phase - The requestor initiates the service and the rules for communication between the two                                                     systems.

                                                 - These rules could include such things as who transmits and when, as well as how                                                     much data can be sent at a time. Both systems must agree on the rules.

         II. Data transfer phase - Both sides know how to talk to each other, the most efficient methods to use, and                                                how to detect errors, all because of the rules defined in the first phase.

         III. Termination phase – It occurs when the session is complete and communication ends in an orderly                                                          fashion.

Services offered :

•       Dialog Control -      This layer allows two systems to start communication with each other in half-duplex or full-                               duplex.

•       Dialog Separation –   Keep different application data separate from each other.

•       Synchronization -    This layer permitted a process to add checkpoints which are referred to as synchronization                                  points  into the stream of data

 

 

4. Transport Layer (4^th Layer)

- Responsible to deliver entire data from end to end
- It receives data from the Session Layer and convert into segment.
- Each segment is made up of Source and Destination Port number, Protocol/Services and Sequence number
- Transport layer has two well-known protocols— TCP and UDP.

5. Network Layer (3^rd Layer)

•      The main aim of this layer is to deliver packets from source to destination across multiple links (networks).

•      It also divides the outgoing messages into packets and to assemble incoming packets into messages for higher levels.

 

FUNCTIONS OF NETWORK LAYER

•       

•      Internetworking :

•      It is the main responsibility. It provides a logical connection between      different devices.

                      

•           Addressing :    

•       A Network layer adds the source and destination address to the header of                     the frame. Addressing is used to identify the device on the internet.

•      Routing : Routing is the major component of the network layer, and it determines the                          best optimal path out of the multiple paths from source to the destination.

 

 

•      Packetizing :

 

 

•      A Network Layer receives the packets from the upper layer and converts                                    them into packets. This process is known as Packetizing. It is achieved by                           internet protocol (IP)

 

 

6. Datalink Layer (2^nd Layer)

 

 

 

•       The three main functions of the data link layer are to deal with transmission errors, regulate the flow of data, and provide a well-defined interface to the network layer.

•       When it transfers the data, it uses timers and sequence numbers to check for errors to ensure that all data is successfully received. 

•       The Data link layer adds a header to the frame that contains a destination address. The frame is transmitted to the destination address mentioned in the header.

•       Datalink layer contains two sub-layers:

 

I.              Logical Link Control Layer

 

 

•       It is responsible for transferring the packets to the Network layer of the receiver that is receiving.

•       It identifies the address of the network layer protocol from the header.

•       It also provides flow control.

 

II. Media Access Control Layer

 

•       A Media access control layer is a link between the Logical Link Control layer and the network's physical layer.

It is used for transferring the packets over the network.

 

 

Function of datalink layer

 

 

 

- Node to Node OR Hope to Hope delivery

- Flow Control : - It is the technique through which the constant data rate is maintained on both the                            sides so that no data get corrupted

                           - Method – 1. Stop & Wait        2. GBN              3. Selective Repeat

- Error Control : - Check data is not corrupted.

                            - Method – 1. CRC (DL)  2. Checksum (TL)

- Access Control : - When two or more devices are connected to the same communication channel,                                       then the  data link layer protocols are used to determine which device has                                             control over the link at a given t    - Method – 1. CSMA/CD        2. Aloha (Pure/Slotted) 3. Token (Bus/Ring)

- Physical Addressing : It adds a header to the frame that contains a destination address

- Framing : - Translates the physical's raw bit stream into packets known as Frames. 

                   - adds the header and trailer to the frame.

 

 

7. Physical Layer (1^st Layer)

•      The physical layer deals with bit-level transmission between different devices and supports electrical or mechanical interfaces connecting to the physical medium for synchronized communication.

•      When receiving data, this layer will get the signal received and convert it into 0s and 1s and send them to the Data Link layer, which will put the frame back together.

•      For transmission, it defines all the procedures and functions that physical devices and interfaces have to perform.

 

FUNCTIONS OF PHYSICAL LAYER

•      Defining bits: Determines how bits are converted from 0s and 1s to a signal.

•      Data rate: Determines how fast the data flows, in bits per second.

•      Synchronization: Ensures that sending and receiving devices are synchronized.

•      Transmission mode: Determines the direction of transmissions and whether those are                                                          simplex, half-duplex or full-duplex

•      Interface: Determines how devices are connected to a transmission medium such as Ethernet or                radio waves.