The OSI Model

The OSI Model

Advantages of Reference Models

  1. It divides the network communication process into smaller and simpler components, thus aiding component development, design, and troubleshooting.
  2. It allows multiple-vendor development through standardization of network components.
  3. It encourages industry standardization by defining what functions occur at each layer of the model.
  4. It allows various types of network hardware and software to communicate.
  5. It prevents changes in one layer from affecting other layers, so it does not hamper development.

The OSI Reference Model

The OSI has seven different layers, divided into two groups. The top three layers define how the applications within the end stations will communicate with each other and with users. The bottom four layers define how data is transmitted end to end. 

  1. Upper Layer
    APPLICATION LAYER Provides a user interface
    PRESENTATION LAYER Presents data, Handles processing such as encryption
    SESSION LAYER Keeps different applications, Data separate
  2. Lower Layer
    TRANSPORT LAYER Provides reliable or unreliable delivery, Performs error correction before retransmit
    NETWORK LAYER Provides logical addressing, Which routers use for path determination
    DATA LINK LAYER Combines packets into bytes and bytes into frames, Provides access to media using MAC address, Performs error detection not correction
    PHYSICAL LAYER Moves bits between devices, Specifies voltage, wire speed and pin-out of cables

The following network devices operate at all seven layers of the OSI model:

  • 1. Network management stations (NMSs)
    2. Web and application servers
    3. Gateways (not default gateways)
    4. Network hosts


The OSI reference model has seven layers:

  • The Application Layer

    The Application layer of the OSI model marks the spot where users actually communicate to the computer.

    The Application layer is also responsible for identifying and establishing the availability of the intended communication partner and determining whether sufficient resources for the intended communication exist. It’s important to remember that the Application layer is acting as an interface between the actual application programs.

    Example, FTP and TFTP.
  • The Presentation Layer

    The Presentation layer gets its name from its purpose: It presents data to the Application layer and is responsible for data translation and code formatting.
  • The Session Layer

    The Session layer is responsible for setting up, managing, and then tearing down sessions between Presentation layer entities. It coordinates communication between systems and serves to organize their communication
  • The Transport Layer

    The Transport layer segments and reassembles data into a data stream. They provide end-to-end data transport services and can establish a logical connection between the sending host and destination host on an internetwork. Transport layer using two types of protocol TCP and UDP.

    Features of TCP:
    1. Flow Control

      Flow control prevents a sending host on one side of the connection from overflowing the buffers in the receiving host.

      • The segments delivered are acknowledged back to the sender upon their reception.
      • Any segments not acknowledged are retransmitted.
      • Segments are sequenced back into their proper order upon arrival at their destination.
      A manageable data flow is maintained in order to avoid congestion, overloading, and Data loss.
    2. Connection-Oriented Communication

      In reliable transport operation, a device that wants to transmit sets up a connection-oriented communication with a remote device by creating a session. Which is called a call setup or a three- way handshake.
      • • The first “connection agreement” segment is a request for synchronization.
        • The second and third segments acknowledge the request and establish connectionparameters
        • The final segment is also an acknowledgment. It notifies the destination host that the connection agreement has been accepted and that the actual connection has been established.
        A service is considered connection-oriented if it has the following characteristics:

        • A virtual circuit is set up (e.g., a three-way handshake).
        • It uses sequencing.
        • It uses acknowledgments.
        • It uses flow control. The types of flow control are buffering, windowing, and congestion avoidance.
    3. Windowing

      The quantity of data segments (measured in bytes) that the transmitting machine is allowed to send without receiving an acknowledgment for them is called a windowing.
    4. Acknowledgments

      Reliable data delivery ensures the integrity of a stream of data sent from one machine to the other through a fully functional data link. It guarantees that the data won’t be duplicated or lost. This is achieved through something called positive acknowledgment with retransmission.
  • The Network Layer

    The Network layer (also called layer 3) manages device addressing, tracks the location of devices on the network, and determines the best way to move data. Two types of packets are used at the Network layer: data and route updates.
    1. Data packets

      Used to transport user data through the internetwork. Protocols used to support data traffic are called routed protocols; examples of routed protocols are IP and IPv6.
    2. Route update packets

      Used to update neighboring routers about the networks connected to all routers within the internetwork. Protocols that send route update packets are called routing protocols; examples of some common ones are RIP, RIPv2, EIGRP, and OSPF.
    3. Metric

      The distance to the remote network. Different routing protocols use different ways of computing this distance. 
  • The Data Link Layer

    The Data Link layer provides the physical transmission of the data and handles error notification, this means that the Data Link layer will ensure that messages are delivered to the proper device on a LAN using hardware addresses. The IEEE Ethernet Data Link layer has two sub layers:
    1. Media Access Control (MAC) 802.3

      Defines how packets are placed on the media. Contention media access is “first come/first served”
    2. Logical Link Control (LLC) 802.2

      Responsible for identifying Network layer protocols and then encapsulating them.
    Switches and Bridges at the Data Link Layer

    Layer 2 switching is considered hardware-based bridging because it uses specialized hardware called an application-specific integrated circuit (ASIC). ASICs can run up to gigabit speeds with very low latency rates. Latency is the time measured from when a frame enters a port to the time it exits a port.
  • The Physical Layer

    Physical layer does two things: It sends bits and receives bits. The Physical layer specifies the electrical, mechanical, procedural, and functional requirements for activating, maintaining, and deactivating a physical link between end systems. This layer is also where you identify the interface between the data terminal equipment (DTE) and the data communication equipment (DCE)

    DCE (data circuit-terminating equipment.) The DCE is usually located at the service provider, while the DTE is the attached device.

    The services available to the DTE are most often accessed via a modem or channel service unit/data service unit (CSU/DSU)

    Hubs at the Physical Layer

    A hub is really a multiple-port repeater. A repeater receives a digital signal and re-amplifies or Regenerates that signal and then forwards the digital signal out all active ports without looking at any data.

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