OSI Model

OSI Model

The 7 Layers of the OSI Model

The Open System Interconnection (OSI) model defines a networking framework to implement protocols in seven layers. Control is passed from one layer to the next, starting at the application layer in one station, and proceeding to the bottom layer, over the channel to the next station and back up the hierarchy. The seven layers are:

Application Layer (7th Layer)

Everything at this layer is application-specific. This layer supports application and end-user processes. This layer provides application services for file transfers, e-mail, and other network software services. Telnet and FTP are applications that exist entirely in the application level. Tiered application architectures are part of this layer.

Presentation Layer (6th Layer)

It provides independence from differences in data representation (e.g., encryption) by translating from application to network format, and vice versa. This layer works to transform data into the form that the application layer can accept. This layer formats and encrypts data to be sent across a network, providing freedom from compatibility problems. It is sometimes called the syntax layer.

Session Layer (5th Layer)

It sets up, coordinates, and terminates conversations, exchanges, and dialogues between the applications at each end. It deals with session and connection coordination.

Transport Layer (4th Layer)

This layer provides transparent transfer of data between end systems, or hosts, and is responsible for end-to-end error recovery and flow control. It ensures complete data transfer.

Network Layer (3rd Layer)

This layer provides switching and routing technologies, creating logical paths called virtual circuits, for transmitting data from node to node. Routing, forwarding, addressing, internetworking, error handling, congestion control and packet sequencing are functions of this layer.

Data Link Layer (2nd Layer)

At this layer, data packets are encoded and decoded into bits. It furnishes transmission protocol knowledge and management and handles errors in the physical layer, flow control and frame synchronisation. The data link layer is divided into two sub layers: The Media Access Control (MAC) layer and the Logical Link Control (LLC) layer. The MAC sub layer controls how a computer on the network gains access to the data and permission to transmit it. The LLC layer controls frame synchronisation, flow control and error checking.

Physical Layer (1st Layer)

This layer conveys the bit stream — electrical impulse, light or radio signal — through the network at the electrical and mechanical level. It provides the hardware means of sending and receiving data on a carrier, including defining cables, cards and physical aspects. Fast Ethernet, RS232, and ATM are protocols with physical-layer components.

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