The OSI model ( minus the physical medium ) is shown in Fig. 1. 1. This model is based on a proposal developed by the International Standards Organization ( ISO ) as a first step toward international standardization 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.
Figure 1. 1. The ISO - OSI reference model
The OSI model has seven layers. The principles that were applied to arrive at the seven layers can be briefly summarized as follows :
1. A layer should be created where a different 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
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.
Below we will discuss each layer of the model in turn, starting at the bottom layer. Note that the OSI model itself is not a network architecture because it does not specify the exact services and protocols to be used in eachlayer. It just tells what each layer should do. However, ISO has also produced standards for all the layers,although these are not part of the reference model itself. Each one has been published as a separate international standard.
1. 2. The Physical Layer
The physical layer is concerned with transmitting raw bits over a communication channel. The design issueshave 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, howmany nanoseconds a bit lasts, whether transmission may proceed simultaneously in both directions, how theinitial connection is established and how it is torn down when both sides are finished, and how many pins thenetwork 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.
In this subchapter we will look at the lowest layer depicted in the hierarchy. It defines the mechanical, electrical, and timing interfaces to the network. We will begin with a theoretical analysis of data transmission, only to discover that exists some limits on what can be sent over a channel.
Information can be transmitted on wires by varying some physical property such as voltage or current. By representing the value of this voltage or current as a single-valued function of time, f (t), we can model the behavior of the signal and analyze it mathematically.
To see what all this has to do with data communication, let us consider a specific example: the transmission of the ASCII character ''b'' encoded in an 8 - bit byte. The bit pattern that is to be transmitted is 01100010. The left-hand part of Fig. 1. 2. (a) shows the voltage output by the transmitting computer. The Fourier analysis of this signal yields the coefficients :
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