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OSI Model

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The Open Systems Interconnection (OSI) reference model provides a set of international rules and standards to allow any system obeying these protocols to communicate with other systems that use them. These protocols are arranged in seven separate but interconnected layers, each dealing with a different aspect of the communication. Among other things, this allows hardware, like routers and firewalls, to focus on the particular aspect of communication that applies to them, and ignore other parts.

The seven OSI layers are as follows:

1. Physical layer: This layer deals with the physical connection between two points. This is the lowest layer, and its major role is communicating raw bit streams. This layer is also responsible for activating, maintaining, and deacti- vating these bit-stream communications.
2. Data-link layer: This layer deals with actually transferring data between two points. The physical layer takes care of sending the raw bits, but this layer provides high-level functions, such as error correction and flow control. This layer also provides procedures for activating, maintaining, and deactivating data-link connections.
3. Network layer: This layer works as a middle ground, and its key role is to pass information between lower and higher layers. It provides addressing and routing.
4. Transport layer: This layer provides transparent transfer of data between systems. By providing a means to reliably communicate data, this layer allows the higher layers to worry about other things besides reliable or cost-effective means of data transmission.
5. Session layer: This layer is responsible for establishing and then maintaining connections between network applications.
6. Presentation layer: This layer is responsible for presenting the data to applications in a syntax or language they understand. This allows for things like encryption and data compression.
7. Application layer: This layer is concerned with keeping track of the requirements of the application.

When data is communicated through these protocols, it's sent in small pieces called packets. Each packet contains implementations of these protocols in layers. Starting from the application layer, the packet wraps the presentation layer around that data, which wraps the session layer around that, which wraps the transport layer, and so forth. This process is called encapsulation. Each wrapped layer contains a header and a body: The header contains the protocol information needed for that layer, while the body contains the data for that layer. The body of one layer contains the entire package of previously encapsulated layers, like the skin of an onion or the functional contexts found on a program stack.

When two applications existing on two different private networks communicate across the Internet, the data packets are encapsulated down to the physical layer where they are passed to a router. Because the router isn't concerned with what's actually in the packets, it only needs to implement protocols up to the network layer. The router sends the packets out to the Internet, where they reach the other network's router. This router then encapsulates this packet with the lower-layer protocol headers needed for the packet to reach its final destination. This process is shown in the following illustration.

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