Open Systems Interconnection(OSI) Reference Model

Most systems do not use the specific OSI protocols which were developed based on the OSI model.  However, the concepts involved are useful in understanding data communications, and the layers are approximated by popular protocols such as TCP/IP, SPX/IPX, and ethernet.

OSI Reference Model
7 Application Layer 6 Presentation Layer 5 Session Layer 4 Transport Layer 3 Network Layer 2 Data Link Layer 1 Physical Layer

Layers

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  Application Layer



• The application layer is the actual user program. (Many programs also include the presentation layer. in some cases, a program may include functions down through the data link layer.)


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  Presentation Layer



• The presentation layer is responsible for formatting data for network transport, and converting it back to system specific format on receipt.


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  Session Layer



• The session layer is responsible for establishing the communications link between applications. Security and other administrative activities are handled at the session level.


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  Transport Layer



• The transport layer is responsible for establishing the communications link between systems. The TCP and SPX protocols function at the transport layer level.


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  Network Layer



• The network layer is responsible for connecting the transport layer to the data link layer; The IP and IPX protocols function at the network layer level.


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  Data Link Layer



• The data link layer specifies the formatting of data for transport by the physical layer. The data link layer typically detects errors occurring in the physical layer. Ethernet protocols function at the data link layer level. On PCs, the data link layer is typically handled by firmware on a network interface card (NIC.)


• 
  

  Physical Layer



• the physical layer is usually represented by cabling, such as UTP, fiber optic cable, or coaxial cable. Space itself is the physical layer used by radio or infrared links, however. The physical layer includes the physical, electrical, and functional aspects of the connection.