OSI Reference Model

Q. What do you understand by the term OSI Model? Explain the functions and protocols and services of the each layer?


Answer: - The OSI Reference Model is explained as follows:

The OSI model (excluding the physical medium) is shown in Fig 4. This model is based on a proposal obtained by the International Standards Organization (ISO) as a first step toward international standardization of the protocols used in the number of layers (Day and Zimmermann, 1983). It was revised again in 1995(Day, 1995). The model is known as the ISO-OSI (Open Systems Interconnection) Reference Model because it deals with  linking open  systems-that is, systems which are open for communication with the other systems. The OSI model comprises of seven layers. The principles which were applied to arrive at the seven layers can be briefly summarized as written below:

1. A layer must be created where a different abstraction is required.

2. Each layer must execute a well-defined function.

3. The function of each layer must be chosen with an eye toward defining internationally standardized protocols.

4. The layer boundaries must be chosen to decrease the information flow across the interfaces.

5. The number of layers must be large enough that separate functions need not be thrown together in the same layer out of necessity and small enough that the architecture does not turn out to be unwieldy.

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The OSI Reference Model


1. The Physical Layer:

The physical layer is linked with transmitting raw bits over a communication channel. The

design issues have to do with making certain that when one side sends a 1 bit, it is obtained by the other side as the 1 bit, not as a 0 bit.

2. The Data Link Layer:

The major task of the data link layer is to transform a raw transmission facility into a line that seems free of undetected transmission errors to the network layer. It accomplishes this task by having the dispatcher break up the input data into the distinct data frames (typically a few hundred or a few thousand bytes) and transmits the frames in sequence. If the service is dependable, the receiver confirms correct receipt of each frame by sending back the acknowledgement frame. Another problem that arises in the data link layer (and most of the higher layers as well) is how to maintain a fast transmitter from drowning a slow receiver in data. Some traffic regulation mechanism is often required to let the transmitter know that how much buffer space the receiver has at a moment. Often, this flow regulation and the error handling are integrated.

3. The Network Layer:

The network layer controls the procedure of the subnet. A key design issue is determining how the packets are routed from starting place to destination. Routes can be based on static tables which are ''wired into'' the network and rarely altered. These can also be determined at the beginning of each conversation, for example, a terminal session (e.g., a login to a remote machine). Finally, they can be very dynamic, being determined anew for each packet, to reflect the present network load. If too many packets are there in the subnet at the same time, they will get in one or the way, forming bottlenecks. The control of such overcrowding also belongs to the network layer. More commonly, the quality of service provided (delay, jitter, transit time, etc.) is also the network layer issue. When a packet has to be sent from one network to another to get to its destination, number of trouble can occur. The addressing made in use by the second network may be dissimilar from the first one. The second may not accept the packet at all because it is too large in size. The protocols might be different, and so on. It is dependent on the network layer to overcome all the above problems to allow heterogeneous networks to be interlinked. In broadcast networks, the routing problem is easy, so the network layer is frequently thin or even nonexistent.

4. The Transport Layer:

The basic function of the transport layer is to accept data from above, split it up into minor units if need be, pass these to the network layer, and make sure that the pieces all arrive accurately at the other end. Furthermore, all this must be done effectively and in a way that isolates the upper layers from the inevitable alterations in the hardware technology.  The transport layer also determines what type of service to grant the session layer, and, eventually to the users of the network. The most admired type of transport connection is an error-free point-to-point channel that delivers messages or bytes in the order in which they were sent. Here the other possible kinds of transport service are the transporting of isolated messages, with no assured about the order of delivery, and the broadcasting of messages to the number of multiple destinations. The type of service is determined when the association is established. The transport layer is in end-to- end layer, all the way from the source to the destination. In other words, a program on the source machine carries on a conversation with the equivalent program on the destination machine, using the message headers and control messages. In the lower layers, the protocols are between every machine and its instant neighbours, and not between the ultimate source and destination machines, which may be unconnected by many routers.

5. The Session Layer:

The  session  layer  permits the  users  on  different  machines  to  establish  sessions in  between  them.

Sessions offer number of services, synchronization (check pointing long transmissions to allow them to continue from where they were after a crash) including dialog control (keeping track of whose turn is to transmit), and token management (preventing two parties from attempting the same critical operation at the same instance),

6. The Presentation Layer:

The presentation layer is concerned with semantics and syntax of the information transmitted.

In order to make it achievable for computers with different data representations to communicate, the data structures to be exchanged can be described in an abstract way, along with a standard encoding to be used ''on the wire.'' The presentation layer manages the abstract data structures and allows the higher-level data structures (e.g., banking records), to be described and exchanged.

7. The Application Layer:

The application layer contains a number of of protocols that are generally needed by users. One extensively-used application protocol is HTTP (Hypertext Transfer Protocol), which is the base for the World Wide Web. When a browser requires a Web page, it sends the name of the page it wants to the server using HTTP. The server then sends a page back. The other application protocols which are used for file transfer, electronic mail, and network news.

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