The Open Systems Interconnection model (OSI model) (ISO/IEC 7498-1) is a product of the Open Systems Interconnection effort at the International Organization for Standardization. It is a prescription of characterizing and standardizing the functions of a communications system in terms of abstraction layers. Similar communication functions are grouped into logical layers. A layer serves the layer above it and is served by the layer below it.
- Layer 1 is the physical layer, which consists of the basic networking transmission technologies of a network.
- Layer 2 is the data link layer, which is the protocol layer that transfers data between adjacent network nodes.
- Layer 3 is the network layer responsible for packet forwarding including routing through intermediate routers, whereas the data link layer is responsible for media access control, flow control and error checking.
- Layer 4 is the transport layer, which provides end-to-end or host-to-host communication services for applications within a layered architecture.
- Layer 5 is the session layer, which provides the mechanism for opening, closing and managing a session between end-user application processes. This layer is not used in modern communication networks.
- Layer 6 is the presentation layer, which serves as the data translator for the network. It is sometimes called the syntax layer. This layer is not used in modern communication networks.
- Layer 7 is the application layer. In the Internet model, the application layer is an abstraction layer reserved for communications protocols and methods designed for process-to-process communications across an Internet Protocol (IP) computer network. Application layer protocols use the underlying transport layer protocols (Layer 4) to establish process-to-process connections via ports.
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