OSI MODEL (PART I)

 7 LAYERS OF OSI MODEL

A conceptual framework for describing the operations of a networking system is called the OSI Model (Open Systems Interconnection Model). In order to facilitate interoperability across various goods and software, the OSI model categorizes computing functions into a universal set of guidelines and requirements. Physical, Data Link, Network, Transport, Session, Presentation, and Application are the seven separate abstraction layers into which the communications between computing systems are divided in the OSI reference model.

Figure 1: 7 layers of the OSI Model

1. Physical Layer

• The physical layer is responsible for transmitting individual bits from one node to the next.
• The physical tools used for data transfer, like cables and switches, are included in this layer. This layer is also where the data is transformed into a bit stream, or a series of ones and zeros. In order to differentiate the 1s from the 0s on both devices, the physical layer of both devices must also agree on a signal protocol.

Figure 2: Physical Layer

2. Data Link Layer

• The data link layer is responsible for transmitting frames from one node to the next. Errors that might have happened at the physical layer are likewise fixed at the data link layer.

Data Link Layer is divided into two sublayers:

1. Logical Link Control (LLC)

2. Media Access Control (MAC)

• The MAC sublayer regulates how a networked computer is granted access to the data and authorization to transfer it. Frame synchronization, flow management, and error checking are all within the LLC layer's control.

Figure 3: Data Link Layer

3. Network Layer

• The network layer is responsible for the delivery of packets from the original source to the final destination.
• It also handles packet routing, which is the choice of the shortest route from a variety of options to transmit the packet. The network layer inserts the IP addresses of the sender and receiver in the header.

Figure 4: Network Layer

4. Transport Layer

• The transport layer is responsible for delivery of a message from one process to another.
• The application layer receives services from the transport layer, while the network layer receives services from the transport layer. Segments are the units of data in the transport layer. It is in charge of the full message's delivery from beginning to end. If an error is detected, the transport layer acknowledges the successful data transmission and re-transmits the data.

Figure 5: Transport Layer

5. Session Layer

• The session layer is responsible for maintain, establishes and synchronizes the interaction between communicating systems.
• Dialog control: Enables two systems to begin communicating in half-duplex or full-duplex mode.
• Synchronization: This layer allows a process to insert checkpoints into the data that serve as synchronization points. These synchronization points aid in the detection of errors so that data may be correctly re-synchronized, message ends are not severed prematurely, and data loss is avoided.

Figure 6: Session Layer

6. Presentation Layer

• The presentation layer is responsible for syntax and semantics of the information exchanged between two systems
• Translation: Before being transferred, the running applications must be converted to bit streams.
• Encryption/ Decryption: Data encryption converts data into a different format or code. The ciphertext is the encrypted data, and the plain text is the decoded data. When encrypting and decrypting data, a key value is used.
• Compression: Reduces the number of bits that must be sent across the network.

Figure 7: Presentation Layer

7. Application Layer

• The application layer is responsible for providing services to the user. It handles issues such as network transparency and resource allocation. An application layer is not an application, but it performs the application layer functions. This layer provides the network services to the end-users.

Figure 8: Application Layer