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OSI Reference Model

Open Systems Interconnection (OSI) Reference Model

• A conceptual framework showing us how data moves throughout a network and how networks operate.

• It is only a reference model (a very good teaching model), so don't get wrapped up in the details. It is not implemented in the real world. TCP/IP model is!

OSI Reference Model Stack

• The OSI model breaks down the complex task of computer-to-computer communication into 7 layers.

  • User Layers (Host Layers)

    Handled by the host computer and performs application-specific functions, such as data formatting, encryption, and connection management.

  • Lower Layers (Media Layers)

    Provide network-specific functions, such as routing, addressing, and flow control.

   

• The lower three layers are implemented on all network nodes.

  The higher four layers typically run only on end-hosts and NOT on the intermediate switches and routers.

• In networking, a protocol data unit (PDU) is the basic unit of exchange between entities that communicate using a specified networking protocol.

Description of Layers

• Application Layer

  Standardize common type of exchanges

  High-level protocols such as for resource sharing or remote file access, e.g., HTTP

• Presentation Layer

  Concerned about the format of data exchanged between peers

  Translates data between a networking service and an application; including character encoding, data compression and encryption/decryption

• Session Layer

  Provides a name space that is used to tie together the potentially different transport streams that are part of a single application

  Manages communication sessions, i.e., continuous exchange of information in the form of multiple back-and-forth transmissions between two nodes

• Transport Layer

  Implements a process-to-process channel

  Unit of data exchanges in this layer is called a message (or segment (TCP), datagram (UDP))

• Network Layer

  Handles routing among nodes within a packet-switched network

  Unit of data exchange between nodes in this layer is called a packet

• Data Link Layer

  Collects a stream of bits into a larger aggregate called a frame

  Network adapter along with device driver in OS implement the protocol in this layer

  Frames are actually delivered to hosts

• Physical Layer

  Handles the transmission or raw bits over a communication link

OSI Communication