OSI model

The word OSI model stands for the "Open System Interconnection" model.  It has been created by ISO – ‘The International Organization of Standardization, in the year 1984.

The purpose of designing this model is to prescribe the process or the steps to be used to transfer data over a transmission medium from one networked device to another.

Therefore, the ISO OSI model is basically a conceptual model which enables the communication between different systems without requiring a change to the logic of the underlining hardware and software.

It consists of seven separate but related layers, each defining a part of the process of moving information across a network.

Note: OSI model is a conceptual model. In the real industry, only TCP/IP model works not the OSI.

The architecture of the OSI Model: 

OSI Model

• OSI consists of seven layers as shown above. (KEYWORD: PDNTSPA).
• OSI model is divided mainly divided into 2 layers: upper and lower layers.
• The upper layers contain the four top layers of the OSI model (Application, presentation, session, and transport). These layers mostly deal with application-related issues.
• Lower layers contain physical, data link, and network layer.
• In this architecture, each layer serves the layer above it and, in turn, is served by the layer below it. So, in a given message between users, there will be a flow of data down through the layers in the source computer, across the network, and then up through the layers in the receiving computer.

Adjacent Layer Interaction: On a single computer, one lower layer provides a service to the layer just above. The software or hardware that implements the higher layer request that the next layer performs the needed function. (Example: Data from the application layer to the presentation Layer)

Same-Layer Interaction: The two computers use a protocol to communicate with the same layer on another computer. The protocol defines a header that communicates with each computer wants to do.

While understanding this model we also must learn 2 basic terms:

• Encapsulation: Adding header and trailer from data.

•  Decapsulation: Removing header and trailer from data

(ADD Encapsulation Diagram)

• When the formatted data unit passes through a physical layer it is changed into an electromagnetic signal and transported along a physical line.

The function of each layer of the OSI model:

1. Physical layer:

• Coordinate the function required to carry a bit stream over a physical medium.
• Deals with the mechanical and electrical specifications of the interface and transmission medium.
• Configuration of the physical device.
• Define transmission medium: the characteristics of the interface between the device and medium.
• Define the type of encoding: Streams of its are encoded into signals(electrical and optical).
• Transmission mode: Ex. Simplex or Duplex 
• Line configuration: Connection of the device to media (Ex. Point to point configuration, multipoint configuration)
• Physical topology: How the device is connected to make a network (Ex. Star, Ring, mesh, etc.

    2. Data link layer:

    

Sub Layers

• The data here is called frames (Framing).
• Assign physical address:
• If frames are to be distributed to different systems on the same network, this layer adds a header to the frame to define the sender and/or receiver of a frame.
• If the frame is intended for a system outside the sender network, the receiver address is the address of the device that connects the network to the next one.
• Flow control:
• The rate at which the data is absorbed by the receiver < (less than) the rate at which data are produced in the sender, this leads to overwhelming the receiver. Hence, to avoid this situation this layer imposes a flow control mechanism.
• Error control: This is normally achieved by adding a trailer to the end of the frame.
• Make error-free data from the physical layer to the network layer.
• Transforms a raw transmission facility to a reliable link.
• Protocol: Switch, Bridge, PPP, Ethernet.

      3. Network layer:

• The data here is called Packets.
• Responsible for source-to-destination delivery of packets.
• Logical addressing(works with an IP address).
• Routing:
• When independent network or links are connected to create internetwork or a large network, the connecting device route or switch the packets to their final destinations, and this mechanism is provided by the network layer.
• Protocols: IP, ICMP, IGMP, IPSec

     4. Transport Layer(heart of OSI model): 

• The data here is called “Segments”.
• Provide end-to-end connection between two devices. 
• It breaks data into segments. 
• It takes care of the end-to-end delivery of messages. Provide acknowledgment, also retransmit data if the error is found.
• Add source and destination port numbers in its header. Flow & error control take place here.
• Port Number: The destination port number is defined. The source port number can be random. For example, if you are surfing any website you use HTTPS/HTTP (80,443) is defined. But you don’t know which port you will use to surf.
• Services: Connection Less and Connection-Oriented.
• Protocols: TCP/UDP

      5.  Session Layer:

• Start, terminate, and authenticate the communication between two devices. 
• It synchronizes data with checkpoints. In any case of disconnection, these checkpoints are helpful. 
• Start communication with half duplex or full duplex. 

      6. Presentation Layer:

• Encryption/decryption of data takes place here. 
• Translate the incoming data. So the application layer can understand or compress the data before delivering it to the next layer.
• Protocols: JPEG, PNG, IMAP, SSH, SSL

      7. Application Layer:

• This is the only layer that directly interacts with the user, It can be any Application, Website, or SSH session. Web browsers and email clients rely on this layer.
• It does not define the application itself, but the protocol that is used by applications.
• Protocols: HTTP, SMTP, POP3