OPEN SYSTEM FOR INTERCONNECTION (OSI MODEL)

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  1. Physical Layer – It is responsible for the movement of individual bits from one node to another. It consists of stream of bits that is sequence of 0’s and 1’s. The transmission rate- the number of bits sent each second is also defined by the physical layer. Syncronization of bits is done at this layer. It defines the physical topology of a network which means it defines how devices are connected to make a network. It also defines the direction of transmission between two devices- simplex, half duplex or full duplex.
  2. Data link layer – This layer is responsible for moving frames one node to another. The data link layer divides the stream of bits manageable data units called frames. It is responsible for flow control, error control and access control. It makes the physical layer appear error free to the upper layer (network layer).
  3. Network layer – This layer is responsible for the delivery of individual packets from the source host to the destination post. The data link layer over sees the delivery of frames between two systems on the same network link, the network layer ensures that each packet gets from its point of origin to its final destination. Routing – when independent network or links are connected to create internetworks (network of networks) the connecting devices called routers or switches route or switch the packets to their final destination. One of the functions of network layer is to provide this mechanism.
  4. Transport Layer – The transport layer is responsible for process to process delivery of the entire message. A process is an application programme running on a host. The network layer oversees source to destination delivery of individual packets it does not recognize any relationship between those packets. It is responsible for segmentation and reassembly of the packets. Like data link layer the transport layer is also impossible for flow control and error control. However flow control at this layer is performed end to end rather than across a single link.
  5. Session layer – The session layer is network dialog controller. It establishes, maintains and synchronizes the intuation among communicating systems. It allows a process to add checkpoints or synchronization point to a stream of data.
  6. Application layer – It enables the user whether human or software, to access the network. It provides user interface and support for services such as electronic mail, remote file access and transfer, shared data base management and other types of distributed information services.
  7. Presentation layer – The presentation layer is concerned with the syntax and simentics of the information exchanges between two systems.

Specific responsibilities of presentation layer

  1. Translation – because different computers use different encoding systems the presentation layer is responsible for interoperability between these different encoding methods. The presentation layer at the sender changes the information from its sender dependent format into a common format. The presentation layer at the receiving machine changes the common format into its receiver dependent format.
  2. Encryption – to carry sensitive information on system must be able to ensure privacy. It means that the sender transforms the original information to another form and sends the resulting message out over the network. Decryption on the other hand reverses the original process to transform the message back to its form.
  3. Compression – The data compression reduces the number of bits contained in the information. Data compression becomes particularly important in the transmission of multimedia such as audio, video and graphics.
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