QuickNote - The OSI Reference Model

  1. The OSI is a logical model, not a physical one.

  2. It also provides a framework for creating and implementing :

    • networking standards

    • devices

    • internetworking schemes

  3. The OSI has seven different layers, divided into two groups.

    • The upper layers

      • Application (Layer 7)

        • PDU : Data

        • Functions : Provides a user interface

        • Note : The Application layer works as the interface between actual application programs. This means end-user programs like Microsoft Word don’t reside at the Application layer.

      • Presentation (Layer 6)

        • PDU : Data

        • Functions :

          • Data encryption

          • Data compression

          • Translation services

      • Session (Layer 5)

        • PDU : Data

        • Functions : Session establishment, maintenance and termination

        • Command : netstat -a -o

          • -o switch displays the process identifier (PID)

          • -a switch displays active TCP connections, TCP connections with the listening state, as well as UDP ports that are being listened to

        • Communication modes:

          • Simplex

          • Half-duplex

          • Full-duplex

    • The lower layers

      • Transport (Layer 4)

        • PDU : Segment

        • Functions :

          • Segmentation

          • Flow control

            • Buffering

              • storing data in memory until it gets processed

            • Windowing

              • The number of data segments allowed per transmission

            • Congestion Prevention

              • When the local buffer is full, the receiver sends request to the sender to stop transmitting until it is ready to receive data again (called source quench)

          • Error Handling

          • Segment sequencing

          • Service addressing

      • Network (Layer 3)

        • PDU : Packet

        • Device : Router, MLS (multi-layer switch)

        • Functions :

          • Logical Addressing (based on IP)

          • Routing

        • Routers' functions :

          • Breaks down broadcast and collision domains

          • Uses logical addresses and access lists

          • Can provide layer-2 bridging

          • Routes between LAN and VLANs

      • Data Link (Layer 2)

        • PDU : Frame

        • Device : Switch, Bridge

      • Physical (Layer 1)

        • PDU : Bit

        • Device : Hub, NIC

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