OSI Layers

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What are OSI Layers?

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 OSI model takes the task of internetworking and divides that up into what is referred to as a vertical stack that consists of the following layers.

1. Physical (Layer 1) – This layer conveys the bit stream, electrical impulse, light, or radio signal through the network at the electrical and mechanical level. Fast Ethernet, RS232, and ATM are protocols with physical layer components.

2. Data Link (Layer 2) – At this layer, data packets are encoded and decoded into bits. The data link layer is divided into two sub layers: The Media Access Control (MAC) layer and the Logical Link Control (LLC) layer.

3. Network (Layer 3) – This layer provides switching and routing technologies, creating logical paths, known as virtual circuits, for transmitting data from node to node.

4. Transport (Layer 4) – 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.

5. Session (Layer 5) – This layer establishes, manages and terminates connections between applications.

6. Presentation (Layer 6) – This layer provides independence from differences in data representation (e.g., encryption) by translating from application to network format, and vice versa.

7. Application (Layer 7) – 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.

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