The OSI model (Open Systems Interconnection model) is a framework that helps to understand how data is transmitted over a network. It divides the process of transmitting data into seven layers, each with its own specific functions. The OSI model is a logical model, not a physical one, and it is used as a reference model to describe the protocols used in networking.

The seven layers of the OSI model are:

  1. Physical Layer: This layer deals with the physical aspects of transmitting data, such as the electrical and mechanical connections between devices. It defines the types of cables and connectors used, as well as the signals that are sent over the network.
  2. Data Link Layer: This layer is responsible for creating a reliable link between two devices on the same network segment. It deals with issues such as error detection and correction, and it also provides a means for devices to access the network.
  3. Network Layer: This layer is responsible for routing packets of data between different devices on the network. It provides a logical addressing scheme, such as IP addresses, which allows devices to find each other on the network.
  4. Transport Layer: This layer is responsible for ensuring that data is delivered reliably and in the correct order. It provides services such as flow control, error checking, and retransmission of lost packets.
  5. Session Layer: This layer establishes, maintains, and terminates connections between applications on different devices. It also synchronizes communication between the applications.
  6. Presentation Layer: This layer is responsible for translating data between the format used by the application and the format used on the network. It also performs data compression and encryption.
  7. Application Layer: This layer provides a means for applications to access the network. It defines the protocols and interfaces that applications use to communicate over the network.

Each layer of the OSI model communicates with the layer directly above and below it, and only with those layers. The data flows from the top layer (Application) to the bottom layer (Physical), and then back up again as it travels from the source to the destination. Each layer adds its own header or trailer to the data as it passes through, which contains control information for that layer.