EXPLAIN OSI MODEL.(physical layer,network layer,data link layer)


Introduction of OSI Model with all layers:

          Established in 1947, the international standards organization (ISO) is a multination body dedicated to worldwide  agreement on international standards. An ISO standard that covers all aspects of network communications is the open systems interconnection model. it was first introduced  in the late 1970s.

An open system is a set of protocols that allows any two different systems to communicate regardless of their underlying  architecture. The purpose of the OSI model is to show how to facilitate  communication  between different systems without requiring changes to the logic of the underlying hardware and software. The OSI model is not a protocol; it is a model for understanding and designing a network architecture that is flexible, robust, and interoperable.

    7.  Application

    6. Presentation

    5.  Session

    4.  Transport

    3.  Network

    2.  Data Link

  1.  Physical

àNetwork processes to applications

àData representation

àInterhost communication

àEnd –to-end communication

àAddress and best path

àAccess to media

àBinary transmission

         The OSI model is a layered framework for the design of network systems that allows communication between all types of computer systems. It consists of seven separate but related layers, each of which defines a part of the process of moving information across a network (see figure). An understanding of the fundamentals of the OSI model provides a solid basis for exploring data communication.


OSI layered architecture



   The OSI model is based on the international standard organization (ISO) as towards international standardization of the protocols used in the different layers. The model is called ISO-OSI. The OSI model  is composed of seven ordered layers such as physical (layer1), data link (layer2), network (layer3), transport (layer4), session (layer5), presentation (layer6), application (layer7).figure shows the layers involved when a message is sent from device A to device B. As the message travels from A to B, it may pass through many intermediate nodes. These intermediate nodes usually involve only the first three layer of the OSI model.

             We will discuss each layers of the model one by one, starting at the bottom layers. The OSI model itself is not network architecture because it does not specify the exact services and protocols to be used in each layer. It just tells what each layer each layer should do. Each one has been published as a separate international standard.


The physical layer:


  • The lowest, bottom layer – responsible for the physical        connection between devices. The NIC converts the data (bits) in to transmission signals. Transmission may be analogue or digital. It’s responsible for the rate of transmission and also includes all components such as the types of connector (RJ-45, token ring). Devices at this level include NICs, repeaters, hubs.
  • The physical layer concerns itself with the transmission of bits. It also manages the network card’s hardware interface to the network. The hardware interface involves the type of cabling (coax, twisted pair, etc.), frequency of operation (1 mbps, 10 mbps, etc.), voltage levels, cable terminations, topography (star, bus, ring, etc.), etc.
  • Physical characteristics of interfaces and medium. The physical layer defines the characteristics of the interface between the devices and transmission medium. It also defines the type of transmission medium.
  • Representation of bit : the physical layer data consists of a stream of bits (sequence of Os and I s ) with no interpretation. To be transmitted, bits must be encoded into signals-electrical or optical. The physical layer of defines the types of encoding (how Os and I s are changed to signals).
  • Data rate: the transmission rate-the number of bits sent each seconds-it also defined by the physical layer. In other words, the physical layer defines the duration of a bit, which is how long it lasts.
  • Synchronization of bits: the sender and receiver not only must be use the same bit rate but also must be synchronized at the bit level. In other words, the sender and the receiver clocks must be synchronized.
  • Transmission mode : the physical layer also defines the direction of transmission between two devices: simplex, half-duplex, or full-duplex. In simplex mode, only one devices can send; the other can only receiver. The simplex mode is a one –way communication. In the half-duplex  mode, two devices can send and receive, but not at the same time. In a full-duplex (or simply duplex) mode, two devices can send and receive at the same time.



The data link layer:


  • The data link layer is a firmware layer of the network interface card. The data link layer puts the datagram’s into packets (frames of bits: 1s & 0s) for transmission, and assembles receiver packets into datagram’s. the data link layer works at the bit level, and adds start /stop flags and bit error checking (CRC or parity) to the packet frame. Error checking is at the bit level only: packets with errors are discarded and a request for re-transmission is sent out. The data link layer is primarily concerned with bit sequence.
  • Framing: the data link layer divides the stream of 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 data link layer adds a header to the frame to define the sender and/or receiver of the frame. If the frame is intended for a system outside the sender’s network, the receiver address is the address of the device that connects the network to the next one.
  • Error control: the data link layer adds reliability to the physical layer by adding mechanisms to detects and retransmit damaged or lost frames. It also uses a mechanism to recognize duplicate frames. error control is normally achieved through a trailer added to the end of the frame.


The network layer:


  • The network layer is responsible for the source-to-destination delivery of a packet, possibly across multiple networks (links) . whereas the data link layer oversees the delivery of the packet between two systems on the same network (links), the network layer ensures that each packet gets from its point of origin to its final destination.
  • The network layer is concerned with the path through the network. It is responsible for routing switching, and controlling  the flow of information between hosts. The network layer converts the segments into smaller datagram’s than the network can handle: network hardware source and destination addresses are also added. The network layer does not guarantee  that the datagram will reach its destination.
  • Logical addressing: the physical addressing implemented by the data link layer 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 header to the packets coming from the upper layer that, among other things, includes the logical addresses of the sender and receiver.
  • Routing: when independent networks or links are connected to create internetworks (network of networks) or a large network, the connecting devices (called routers or switches) route or switch the packets to their final destination. Once of the functions of the network layer is to provide this mechanism.  


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by sharmin   haveliwala  in Networking  on 9/19/2015 5:41:18 AM  

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