OSI MODEL

 What is the OSI model?


The Open Systems Interconnection (OSI) model describes seven layers that computer systems use to communicate over a network. The first standard model for network communications was adopted by all major computer and telecommunication companies in the early 1970s late. In plain English, the OSI provides a standard for different computer systems to be able to communicate with each other.

The OSI model can be seen as a universal language for computer networking. It’s based on the concept of splitting up a communication system into seven abstract layers, each one stacked upon the last.

The OSI Model has 7 defined layers of networking. these layers are interdependent. What this means is that each layer within the OSI model is served by the layer below it and each layer in this model serves the layer above it.







Why does the OSI model matter?


The OSI model is still very useful for troubleshooting network problems, even though the modern Internet does not explicitly implement it (it instead follows the simpler Internet protocol suite). The OSI model will help break down a problem and isolate the root of the problem, whether it's one person who can't get their laptop online or a website that's down for thousands of users.

PHYSICAL LAYER :



  • The lowest or first layer of the OSI Model.
  • The physical layer defines the relationship between a device and a transmission medium, such as a copper or optical cable.
  • This includes the layout of pins, voltages, cable specifications, hubs, repeaters, network adapters, host bus adapters (HBA used in storage area networks) and more.
  • Major function :
Line Configuration: It defines the way how two or more devices can be connected physically.

Data Transmission: It defines the transmission mode whether it is a simplex, half-duplex or full-duplex mode between the two devices on the network

Topology: It defines the way how network devices are arranged

Signals: It determines the type of signal used for transmitting the information.

 

 

DATALINK LAYER :



  • A second layer of the OSI Model.
  • The data link layer establishes and terminates a connection between two physically connected nodes on a network. It breaks up packets into frames and sends them from source to destination.
  • This layer is composed of two parts :
  1.  Logical Link Control (LLC) - Identifies network protocols, performs error checking and synchronizes frames
  2. Media Access Control (MAC) - Uses MAC addresses to connect devices and define permissions to transmit and receive data.
  • Major Function :
·        Framing: Data-link layer takes packets from Network Layer and encapsulates them into Frames. Then, it sends each frame bit-by-bit on the hardware. At the receiver’ end, the data link layer picks up signals from hardware and assembles them into frames.

 

·        Addressing: Data-link layer provides layer-2 hardware addressing mechanism. The hardware address is assumed to be unique on the link. It is encoded into hardware at the time of manufacturing.

 

·        Synchronization: When data frames are sent on the link, both machines must be synchronized to transfer to take place.

 

·        Error Control: Sometimes signals may have encountered a problem in transition and the bits are flipped. These errors are detected and attempted to recover actual data bits. It also provides an error reporting mechanism to the sender.

 

·        Flow Control: Stations on the same link may have different speed or capacity. The Data-link layer ensures flow control that enables both machines to exchange data at the same speed.

 

·        Multi-Access: When a host on the shared link tries to transfer the data, it has a high probability of collision. The data-link layer provides mechanisms such as CSMA/CD to equip the capability to access shared media among multiple Systems.





Network Layer

  • Responsible for the delivery of the packet from the original source to the final destination.
  • Its duties including logical addressing and routing
  • The third layer in the OSI model
  • Setting up the routes for data packets to take, checking to see if a server in another network is up running and receiving IP packets from another network happen at the network layer.
  • The network layer and the Internet layer are basically the same things, but they come from different models of how the internets works.
  • Protocols that used at the network layers is IP, IPsec, ICMP, IGMP  and  GRE.

 








Transport Layer

  • Responsible for delivery of a message from one process to another.
  • Its duties including port addressing, segmentation and connection, flow and error control
  • Fourth layer in the OSI model.
  • The transport layer collects message segments from ap. plications and transmits them into the network layer.
  •  Also responsible for the management of error correction, providing quality and reliability to the end-user.
  • Enables the host to send and receive error-corrected data, packets or messages over a network, and the network component that allows multiplexing.

Session Layer


The session layer is responsible for maintains, establishes and synchronizes the interaction between communicating systems.
  • Dialogue control
 allows two systems to enter into a dialogue (communication between two processes.)
  • Synchronization
 allows a process to add checkpoints into a stream of data


Presentation Layer






The presentation layer is responsible for the syntax and semantics of the information exchanged between two systems
  •  Translation
 the running programs must be changed to bitstreams before being transmitted.
  • Encryption / Decryption
 to carry sensitive information, a system must be able to ensure privacy.
  • Compression
 reduces the number of bits contained in the information.





Application Layer

  • The application layer is used by end-user software such as web browsers and email clients.
  • It provides protocols that allow the software to send and receive information and present meaningful data to users.
  • A few examples of application layer protocols are the Hypertext Transfer Protocol (HTTP), File Transfer Protocol (FTP), Post Office Protocol (POP), Simple Mail Transfer Protocol (SMTP), and Domain Name System (DNS)
  • In our cases, OUM’s student is trying to access the OnlineLearning system located in the OUM network facilities. The application layer will establish a connection with IT Center where the OnlineLearning System server is located. It will allow access to network resources in the server. As I told you, the application layer will give supports network access and provide services for user applications.

How the data travels from the student's computer to reach the OnlineLearning System which is in the OUM IT Center?

There are 4 steps of the data travel which includes all of the OSI Model.

 1) By OSI Model 7, 6 & 5     
    - The student visits the OnlineLearning website that must be approved by the OUM IT              Center.
    - As the web browser is secured, the web browser initiates a request and instructs the              networking stack to transmit it to the OnlineLearning web server.


2) By OSI Model 4 & 3
    - If required, the TCP stack divides the request into packets and transmits them over the          data channel.
    - It regulates the rate at which packets are sent and resends packets as needed. This is the       operating system in.

3) By OSI Model 2
    - The data link encrypts each packet in a network frame and transmits it via the physical link       to the computer's next switch or router.
    - Students connect to their home network via modem, whilst OUM connects to the college          network.

4) By OSI Model 1
    - The wire and electrical signals that actually convey the data are referred to as the physical       link.