The ISO-OSI Reference model with diagram

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The OSI Reference Model:

                                    The OSI model is based on a proposal develop by ISO as a first step toward international standardization of the protocols used in the various layers. The model is called ISO OSI (Open Systems Interconnection) Reference Model.Open system is a system open for communication with other systems.

The OSI model has 7 layers . The principles that were applied to arrive at the seven layers are as follows:

1. A layer should be created where a different level of abstraction is needed.
2. Each layer should perform a well defined function.
3. The function of each layer should be chosen with an eye toward defining internationally standardized protocols.
4. The layer boundaries should be chosen to minimize the information flow across the interfaces.
5. The number of layers should be large enough that distinct functions need not be thrown together in the same layer out of necessity, and small enough that the architecture does not become unwieldy.

 

Fig. . The OSI reference model.
The OSI model is not a network architecture – it does not specify the exact services and protocols. It just tells what each layer should do. However, ISO has also produced standards for all the layers as a separate international standards.

1.The Physical Layer

                     The main task of the physical layer is to transmit raw bits over a communication channel.
Typical questions here are:

  • how many volts should be used to represent 1 and 0, 
  • how many microseconds a bit lasts, 
  • whether the transmission may proceed simultaneously in both directions, 
  • how the initial connection is established and how it is turn down, 
  • how many pins the network connector has and what each pin is used for. 
  • The design issues deal with mechanical, electrical, and procedural interfaces, and the physical transmission medium, which lies below the physical layer.
  • The user of the physical layer may be sure that the given stream of bits was encoded and transmitted.He cannot be sure that the data came to the destination without error. This issue is solved in higher layers.

2. The Data Link Layer

             The main task of the data link layer is to take a raw transmission facility and transform it into a line that appears free of undetected transmission errors to the network layer. To accomplish this, the sender breaks the input data into data frames (typically a few hundred or a few thousand bytes), transmits the frames sequentially, and processes the acknowledgment frames sent back by the receiver.

The issues that the layer has to solve:

  • to create and to recognize frame boundaries – typically by attaching special bit patterns to the beginning and end of the frame, 
  • to solve the problem caused by damaged, lost or duplicate frames (the data link layer may offer several different service classes to the network layer, each with different quality and price), 
  • to keep a fast transmitter from drowning a slow receiver in data, 
  • if the line is bi-directional, the acknowledgment frames compete for the use of the line with data frames. 
  • Broadcast networks have an additional issue in the data link layer: how to control access to the shared channel. A special sublayer of the data link layer (medium access sublayer) deals with the problem.

  • The user of the data link layer may be sure that his data were delivered without errors to the neighbor node. However, the layer is able to deliver the data just to the neighbor node.

3. The Network Layer

                      The main task of the network layer is to determine how data can be delivered from source to destination. That is, the network layer is concerned with controlling the operation of the subnet.

The issues that the layer has to solve:

  • to implement the routing mechanism, 
  • to control congestions, 
  • to do accounting, 
  • to allow interconnection of heterogeneous networks. 
  • In broadcast networks, the routing problem is simple, so the network layer is often thin or even nonexistent.
  • The user of the network layer may be sure that his packet was delivered to the given destination. However, the delivery of the packets needs not to be in the order in which they were transmitted.

4. The Transport Layer

                                The basic function of the transport layer is to accept data from the session layer, split it up into smaller units if need be, pass them to the network layer, and ensure that the pieces all arrive correctly at the other end. All this must be done in a way that isolates the upper layers from the inevitable changes in the hardware technology.

The issues that the transport layer has to solve: 

  • to realize a transport connection by several network connections if the session layer requires a high throughput or multiplex several transport connections onto the same network connection if network connections are expensive, 
  • to provide different type of services for the session layer, 
  • to implement a kind of flow control. 
  • The transport layer is a true end-to-end layer, from source to destination. In other words, a program on the source machine carries on a conversation with a similar program on the destination machine. In lower layers, the protocols are between each machine and its immediate neighbors.
  • The user of the transport layer may be sure that his message will be delivered to the destination regardless of the state of the network. He need not worry about the technical features of the network.

5. The Session Layer

                           The session layer allows users on different machines to establish sessions between them. A session allows ordinary data transport, as does the transport layer, but it also provides enhanced services useful in some applications.

Some of these services are: 

  • Dialog control – session can allow traffic to go in both directions at the same time, or in only one direction at a time. 
  • If traffic can go only in one way at a time, the session layer can help to keep track of whose turn it is. 
  • Token management – for some protocols it is essential that both sides do not attempt the same operation at the same time. 
  • The session layer provides tokens that can be exchanged. Only the side holding the token may perform the critical action. 
  • Synchronization – by inserting checkpoints into the data stream the layer eliminates problems with potential crashes at long operations. After a crash, only the data transferred after the last checkpoint have to be repeated.
  • The user of the session layer is in similar position as the user of the transport layer but having larger possibilities.

6. The Presentation Layer

                                  The presentation layer perform certain functions that are requested sufficiently often to warrant finding a general solution for them, rather than letting each user solve the problem. This layer is, unlike all the lower layers, concerned with the syntax and semantics of the information transmitted.
                                  A typical example of a presentation service is encoding data in a standard agreed upon way. Different computers may use different ways of internal coding of characters or numbers. In order to make it possible for computers with different representations to communicate, the data structures to be exchanged can be defined in an abstract way, along with a standard encoding to be used “on the wire”. The presentation layer manages these abstract data structures and converts from the representation used inside the computer to the network standard representation and back.

7. The Application Layer                             

                         The application layer contains a variety of protocols that are commonly needed.For example, there are hundreds of incompatible terminal types in the world. If they have to be used for a work with a full screen editor, many problems arise from their incompatibility. One way to solve this problem is to define network virtual terminal and write editor for this terminal. To handle each terminal type, a piece of software must be written to map the functions of the network virtual terminal onto the real terminal. All the virtual terminal software is in the application layer.

                               Another application layer function is file transfer. It must handle different incompatibilities between file systems on different computers. Further facilities of the application layer are electronic mail, remote job entry, directory lookup ant others.

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