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OSI Model

OSI Model: –

An ISO standard that covers all aspect of network communications in the Open System Interconnection (OSI Model). An open system is a model that allows only two different systems to communicate regardless of their underlying architecture. Open system interconnection model is not a protocol it is a model for understanding & designing a network architecture that is flexible and interpretable.

Open system interconnection model is a layered frame work for the design of network systems that allows for communication across all types of computer systems. It consists of seven separate layers but related layers, each of which defines a segment of the process of moving information across a network.


  • Layered Architecture: –

The open system interconnection model is built of seven ordered layers: Physical, Data Link, Network, Transport, Session, Presentation and Application. The following figure shows the layers involved when a message is sent from device A to device B.

osi model

As the message travels from A to B, it may pass through many intermediate nodes. These nodes usually involve only first three layers of the open system interconnection model.

  • Peer-to-Peer Process: –

Within a single machine, each layer calls upon the service of the layer just below it. E.g. layer 3 uses the services provided by layer 2 and provides service for layer 4. Between machines, layer x on one machine communicates with layer x on another machine. This communication is governed by protocols. The processes on each machine that communicate at a given layer are called peer-to-peer processes.

Each layer in the sending machine adds its own information to the message. It receives from the layer just above it and passes the whole package to the layer just below it. This information is added in the form of headers or trailers. Headers are added to the message at layers 6, 5, 4, 3 & 2. A trailer is added at layer 2.


  • Interface Between Layers: –

The passing of the data and information down through layers of sending machine & block up through the layers of receiving machine is made possible by an interface between each pair of adjacent layers. Each interface defines what information & services a layer must provide for the layer above it well defined interface & layer functions provide modulating to a network.


  • Organization of Layers: –

The seven layer campus area network is through of as belonging to three subgroups.

  • Layer 1, 2 and 3 – Physical, Data Link and Network are the network support layers they deal with the physical aspects of moving data from one device to another.
  • Layer 5, 6 and 7 – Session, Presentation and Application campus area network is through of as the user support layers. They allow interpretability among unrealities software system.
  • Layer 4 – Transport layer ensure end-to-end variable data transmission while layer 2 ensure reliable transmission on s single link.


Function of the Layers: –


Physical Layer: –

  • The physical layer coordinates the functions required to transmit a bit stream over a physical medium. It deals with the mechanical and electrical specifications of the interface and transmission medium. It also defines the procedure and functions that the physical devices and interfaces have to perform for transmission to occur.The physical layer is concern with the following.


  • Physical Characteristic of Interfaces & Media: –

The physical layer defines the interface between the devices & the transmission medium.


  • Representation of Bits: –

The physical data layer consists of a stream of bits without any interpretation to be transmitted they must be encoded into signals. The physical layer defines the type of encoding.


  • Data Rate: –

The transmission rate is also defined by the physical layer.


  • Synchronization of Bits: –

The sender & receiver clocks must be synchronized at the bit level.


  • Line Configuration: –

The physical layer is concerned with the connection of devices to the medium. In a point-to-point configuration, two devices are connected together through a dedicated link. In a multipoint configuration, a link is shared between several devices.


  • Physical Topology: –

The physical topology defines how devices are connected to make a network.


  • Transmission Mode: –

The physical layer also defines the direction of transmission between two devices: Simplex, half duplex and full duplex.


Data Link Layer: –

  • The data link layer transforms the physical layer, a row transmission facility, to a variable link and is responsible for node-to-node delivery. It makes physical layer appear error free to the upper layer specific responsibilities of the data link layer include following.


  • Framing: –

The data link layer divides the stream 0 bits received from the network layer into manageable data units called frames.


  • Physical Addressing: –

If frames are to be distributed to different systems on the network; the DLL adds a header to the frame to define the physical address of the sender that is source address and receiver that is destination address of the frame.


  • Flow Control: –

The rate at which the data are assorted by the receiver is less than the rate produced in the sender; the DLL imposes a flow control mechanism to prevent over the receiver.


  • Error Control: –

The DLL adds reliability to the physical layer by adding mechanism to detect & transmit damage or lost frames. It also uses a mechanism to prevent duplication of frames. Error control is normally achieved through a traitor added to the end of the frame.


  • Access Control: –

When two or more devices are connected to the same link, data link protocols are necessary to determine which device has control over the link at any given time.


Network Layer: –

  • The network layer is responsible for the source to destination delivery of a packet possibly across multiple networks. The network layer ensures that each packet gets from its point of origin to its final destination whereas the DLL oversees the delivery of the packets between two systems on the same network.If two systems are connected to the same link there is usually no need for a network layer. However if the two systems are attached to different networks with connecting devices between the network, there is often a need for the network layer to accomplish source to destination delivery.
    • Logical Addressing: –

    The physical addressing implemented by the DLL handles the addressing problem locally. If a packet passes the network boundary we need another addressing system to help distinguish the source and destination systems. The network layer adds a header to the packet coming from the upper layer that among other things includes the logical address of the sender & receiver.


    • Routing: –

    When independent networks and links are connected together to create an internetwork or a large network, the connecting device called routers or gateways route to the packet to their final destination. One of the functions of the network layer is to provide this mechanism.


Transport Layer: –

  • The transport layer is responsible for source to destination delivery or the entire message whereas network layer oversees end-to-end delivery of individual packets. It doesn’t recognize any relationship between those packets. It treats each one independently. The transport layer on the other hand ensure that the whole message arrives infects & in order, overseeing both error controls & flow control at the source to destination level.For added security, the transport layer may create a connection between the two end parts. A connection is a single logical path between the source and destination that is associated with all packets in a message, creating a connecting involves three steps.
  • Connection establishment.
  • Data transfer.
  • Connection release.

Specific responsibilities of the transport layer include the following.


  • Service Point Addressing: –

Computer often runs several programs at the same time, so source to destination delivery means delivery not only from one to other computer but also from a specific process on one to a specific process on the other computer. The transport layer header therefore must include a type of address called service point address or port address.


  • Segmentation & Reassembling: –

A message is divided into transmittable segments each containing a sequence number. These numbers enable the transport layer to reassemble the message correctly upon arriving at the destination and to identify and replace packets that were lost in the transmission.


  • Connection Control: –

A connection less transport layer treats each segment as an independent packet. A connection oriented transport layer makes a connection with the transport layer as the destination machine first before delivering the packets.


  • Flow Control: –

Like the DLL layer & transport layer is responsible for flow control. However flow control at this layer is performed end-to-end rather than across a single link.


  • Error Control: –

Like the DLL & the transport layer is responsible for error control. However, error control at this layer is performed end-to-end rather than across a single link. The sending transport layer makes sure that the entire message arrives at the receiving transport layer without error.


Session Layer: –

  • The services provided by the first three layers are not sufficient for some processes. The session layer is the network dialog controller it establishes, maintain and synchronize the intersection between communication systems.Specific responsibilities of the session layer include following.
    • Dialog Control: –

    The session layer allows two systems to enter into a dialog box. It allows the communication between two processes to take place either in half or full duplex.


    • Synchronization: –

    The session layer allows a process to add checkpoint synchronization points into a stream of bits.


Presentation Layer: –

  • The presentation layer is concerned with the syntax and semantics of the information exchanged between two systems. Specific responsibilities of the presentation layer include the following.
    • Translation: –

    The process in two systems is usually exchanging information in the form of character string, numbers and so on. The information should be change to bit stream before being transmitted. The presentation layer is responsible for interoperability between two different encoding methods.


    • Encryption: –

    To carry sensitive information a system must be able to ensure privacy. Encryption means that the sender transforms the original information to another from & sends the resulting message out over the network. Decryption reverses the original process to transform the message back to its original form.


    • Compression: –

    Data compression reduces the number of bits to be transmitted. Data compression becomes important in the transmission of multimedia such as text, audio and video.


Application Layer: –

  • The application layer enables the user, whether human or software to access the network. It provides user interfaces and support for services.Specific services provided by the application layer include the following.
    • Network Virtual Terminal: –

    A network virtual terminal is a software version of physical terminal and allows the user to logon to a remote host. To do so, the application creates a software emulation of a terminal at the remote host.


    • File Transfer Access & Management (FTAM): –

    This application allows a user to access files in a remote computer, to retrieve files and to manage files in a remote computer.


    • Mail Services: –

    This application provides the basis for e-mail forwarding & storage.


    • Directory Services: –

    This application provides distributed database source and access for global information about various objects & services.


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