The OSI model is based on a proposal developed by the International Standards Organization (ISO) as a first step toward international standardisation of the protocols used in the various layers (Day and Zimmermann, 1983). It was revised in 1995 (Day, 1995). The model is called the ISO OSI (Open Systems Interconnection) Reference Model because it deals with connecting open systems—that is, systems that are open for communication with other systems. We will just call it the OSI model for short.
The OSI model has seven layers. The principles that were applied to arrive at the seven layers can be briefly summarised as follows:
1. A layer should be created where a different abstraction is needed.
2. Each layer should perform a well-defined function.
3. Function of each layer should be chosen with an eye toward defining internationally standardised protocols.
4. The layer boundaries should be chosen to minimise 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.
1.The Physical Layer
The physical layer is concerned with transmitting raw bits over a communication channel. The design issues have to do with making sure that when one side sends a 1 bit, it is received by the other side as a 1 bit, not as a0 bit.
Typical questions here are how many volts should be used to represent a 1 and how many for a 0, how many nanoseconds a bit lasts, whether transmission may proceed simultaneously in both directions, how the initial connection is established and how it is torn down when both sides are finished, and how many pins the network connector has and what each pin is used for.
The design issues here largely deal with mechanical, electrical, and timing interfaces, and the physical transmission medium, which lies below the physical layer.
2.The Data Link Layer
The main task of the data link layer is to transform a raw transmission facility into a line that appears free of undetected transmission errors to the network layer. It accomplishes this task by having the sender break up the input data into data frames and transmit the frame sequentially.
If the service is reliable, the receiver confirms correct receipt of each frame by sending back an acknowledgement frame.
Another issue that arises in the data link layer is how to keep a fast transmitter from drowning a slow receiver in data. Some traffic regulation mechanism is often needed to let the transmitter know how much buffer space the receiver has at the moment.
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, the medium access control sublayer, deals with this problem.
3.The Network Layer
The network layer controls the operation of the subnet. A key design issue is determining how packets are routed from source to destination. Routes can be based on static tables that are ”wired into” the network and rarely changed. They can also be determined at the start of each conversation, for example, a terminal session(e.g., a login to a remote machine). Finally, they can be highly dynamic, being determined anew for each packet, to reflect the current network load.
If too many packets are present in the subnet at the same time, they will get in one another’s way, forming bottlenecks. The control of such congestion also belongs to the network layer. When a packet has to travel from one network to another to get to its destination, many problems can arise.
The addressing used by the second network may be different from the first one. The second one may not accept the packet at all because it is too large. The protocols may differ, and so on. It is up to the network layer to overcome all these problems to allow heterogeneous networks to be interconnected. In broadcast networks, the routing problem is simple, so the network layer is often thin or even non-existent.
4.The Transport Layer
The basic function of the transport layer is to accept data from above, split it up into smaller units if need be, pass these to the network layer, and ensure that the pieces all arrive correctly at the other end. Furthermore, all this must be done efficiently and in a way that isolates the upper layers from the inevitable changes in the hardware technology.
The transport layer also determines what type of service to provide to the session layer, and, ultimately, to the users of the network. The most popular 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. However, other possible kinds of transportservice are the transporting of isolated messages, with no guarantee about the order of delivery, and the broadcasting of messages to multiple destinations.
The transport layer is a true 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 a similar program on the destination machine, using the message headers and control messages. In the lower layers, the protocols are between each machine and its immediate neighbours, and not between the ultimate source and destination machines, which may be separated by many routers.
5.The Session Layer
The session layer allows users on different machines to establish sessions between them. Sessions offer various services, including dialog control (keeping track of whose turn it is to transmit), token management(preventing two parties from attempting the same critical operation at the same time), and synchronisation(checkpointing long transmissions to allow them to continue from where they were after a crash).
6.The Presentation Layer
Unlike lower layers, which are mostly concerned with moving bits around, the presentation layer is concerned with the syntax and semantics of the information transmitted. In order to make it possible for computers with different data 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 allows higher-level data structures (e.g., banking records), to be defined and exchanged.
7.The Application Layer
The application layer contains a variety of protocols that are commonly needed by users. One widely-used application protocol is HTTP (HyperText Transfer Protocol), which is the basis for the World Wide Web. When a browser wants a Web page, it sends the name of the page it wants to the server using HTTP. The server then sends the page back. Other application protocols are used for file transfer, electronic mail, and network news.
anand.johari1994