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OSI Reference Model

  • ISO- International Organizations for Standard
  • OSI- Opens System Interconnections
  • Stats developing in late 1970s
  • Approved by 1984
  • The term “Open” in Open System Interconnections denotes “to communicate with any 2 systems”
  • There are 7 layers in OSI Reference model
  • It is also called OSI layered architecture /OSI Protocol architecture
  • The process of breaking up the functions or tasks of networking into layers reduces complexity.
  • Each layer provides a service to the layer above it in the protocol specification.
  • Each layer communicates with the same layer’s software or hardware on other computers.
  • The lower 4 layers are concerned with the flow of data from end to end through the network
  • The upper Three layers of the OSI model are orientated more toward services to the applications

Physical Layer

  • The physical layer coordinates the functions required to carry 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 procedures and functions that physical devices and interfaces have to perform for transmission to occur.
  • The following figure shows the position of the physical layer with respect to the transmission medium and the data link layer.

Data Link Layer

  • The data link layer transforms the physical layer, a raw transmission facility, to a reliable link.
  • It makes the physical layer appear error-free to the upper layer (network layer).
  • The figure shows the relationship of the data link layer to the network and physical layers.

Network Layer

Other responsibilities of the network layer include the following:

  • 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 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 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. One of the functions of the network layer is to provide this mechanism.

Session Layer

  • The services provided by the first three layers (physical, data link, and network) are not sufficient for some processes.
  • The session layer is the network dialog controller. It establishes, maintains, and synchronizes the interaction among communicating systems.

Presentation Layer

Specific responsibilities of the presentation layer include the following:

  • Translation. The processes (running programs) in two systems are usually exchanging information in the form of character strings, numbers, and so on. The information must be changed to bit streams before being transmitted. 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.
  • Encryption. To carry sensitive information, a system must be able to ensure privacy. Encryption means that the sender transforms the original information to another form and 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 contained in the information. Data compression becomes particularly 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 such as electronic mail, remote file access and transfer, shared database management, and other types of distributed information services.
  • The figure shows the relationship of the application layer to the user and the presentation layer.

Specific services provided by the application layer include the following:

  • Network virtual terminal. A network virtual terminal is a software version of a physical terminal, and it allows a user to log on to a remote host. To do so, the application creates a software emulation of a terminal at the remote host. The user’s computer talks to the software terminal which, in turn, talks to the host, and vice versa. The remote host believes it is communicating with one of its own terminals and allows the user to log on.
  • File transfer, access, and management. This application allows a user to access files in a remote host (to make changes or read data), to retrieve files from a remote computer for use in the local computer, and to manage or control files in a remote computer locally.
  • Mail services. This application provides the basis for e-mail forwarding and storage.
  • Directory services. This application provides distributed database sources and access for global information about various objects and services.
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