OSI Model Overview

OSI Model

• The Open Systems Interconnection (OSI) model is a framework for developing protocol standards.
• It describes communication between heterogeneous computers.
• The OSI model was proposed to reduce design complexity and make it possible to interconnect different kinds of network systems.

Principles of the OSI Model

• Each layer should perform a well-defined function.
• A layer should be created where a different level of abstraction is needed.
• The function of each layer should be chosen within internationally standardized protocols.
• The layer boundaries should be chosen to minimize the information flow across the interfaces.
• Each layer only uses the functions of the layer below, and only exports functionality to the layer above.

OSI Model Architecture

• The OSI model has seven layers, organized from the physical layer to the application layer.
• The physical layer deals with the physical transmission of data.
• The data link layer is responsible for error detection and correction, as well as framing data.
• The network layer establishes the route and handles packet segmentation for data transmission.
• The transport layer manages end-to-end communication, regulates the data flow, and ensures reliable data delivery.
• The session layer establishes, maintains, and terminates user sessions between machines, managing dialogue control and data synchronization.
• The presentation layer handles data representation, ensuring that data is in a format that can be understood by both sender and receiver.
• The application layer provides services for users, such as file transfer, email, and remote login.

Data Link Layer

• The Logical Link Control (LLC) sublayer manages communication between devices over a single link.
• The Medium Access Control (MAC) sublayer uniquely identifies devices on a network.

Network Layer

• The network layer is responsible for routing data packets across the network.
• Static and dynamic routing is used for network control.
• Packet segmentation helps manage data transmission over heterogeneous networks.

Transport Layer

• The transport layer splits data into smaller units (packets) and passes them to the Network Layer.
• It ensures that all data pieces are delivered correctly at the receiving end.
• The transport layer performs error detection and recovery.

Session Layer

• The session layer manages user sessions between machines.
• It synchronizes data flow using checkpoints.
• Data retransmission only occurs after a crash and for data after the last checkpoint.

Presentation Layer

• The presentation layer handles data representation, such as data compression and decompression, as well as cryptography and encryption.

Application Layer

• The application layer interacts with users and applications, providing services like file transfer, email, and remote login.
• It maps the virtual world to the real world, allowing communication between different file systems and naming conventions.

OSI Model Communication

• Each layer handles a specific aspect of communication.
• Each layer encapsulates the data from the layer above into a protocol data unit (PDU).
• Data is passed down the layers, with each layer adding its own header and trailer information to the PDU.

Data Transmission

• Data transmission occurs through the different layers of the model.
• Each layer performs its own specific function on the data before passing it down to the next layer.
• Data is transmitted as a stream of bits through the physical layer.
• Header information from each layer is added to ensure proper routing and delivery of data.