Reference ModelsOverviewSlide 3The OSI Reference ModelPowerPoint PresentationThe OSI Reference Model - PrinciplesSlide 7OSI – The Physical LayerOSI – The Data Link LayerSlide 10OSI – The Network LayerSlide 12OSI – The Transport LayerSlide 14Slide 15OSI – The Session LayerSlide 17OSI – The Presentation LayerSlide 19OSI – The Application LayerSlide 21ISO/OSI Reference ModelData Transmission in the OSI ModelSlide 24Slide 25The TCP/IP Reference ModelSlide 27TCP/IP – The Internet LayerSlide 29Slide 30TCP/IP – The Transport LayerSlide 32TCP/IP – The Application LayerSlide 34A comparison of OSI and TCP/IPSlide 36Slide 37A hybrid modelReference ModelsReference Models22OverviewOverviewThe OSI Reference ModelThe OSI Reference ModelThe TCP/IP Reference ModelThe TCP/IP Reference ModelA comparison of the OSI and TCP/IP A comparison of the OSI and TCP/IP Reference ModelsReference ModelsA hybrid modelA hybrid model33Reference ModelsReference ModelsNow that we have discussed layered Now that we have discussed layered networks in abstract, it is time to look at networks in abstract, it is time to look at some examples. Next, we will discuss two some examples. Next, we will discuss two important network architectures – the OSI important network architectures – the OSI reference model and the TCP/IP reference reference model and the TCP/IP reference model.model.44The OSI Reference ModelThe OSI Reference ModelThe OSI models is shown on Fig. 1. (minus the physical The OSI models is shown on Fig. 1. (minus the physical medium). medium). This model is based on a proposal developed by the This model is based on a proposal developed by the International Standards Organization (ISO) as a first International Standards Organization (ISO) as a first step toward international standardization of the protocols step toward international standardization of the protocols used in the various layers. used in the various layers. The model is called the The model is called the ISO OSI (Open Systems ISO OSI (Open Systems Interconnection)Interconnection) Reference Model because it deals with Reference Model because it deals with connecting open systems – that is, systems that are connecting open systems – that is, systems that are open for communication with other systems. We will open for communication with other systems. We will usually just call it the OSI model for short. usually just call it the OSI model for short.55Fig. 1. The OSI reference model66The OSI Reference Model - PrinciplesThe OSI Reference Model - PrinciplesThe OSI model has seven layers.The OSI model has seven layers.The principles that were applied to arrive at the The principles that were applied to arrive at the seven layers are as follows:seven layers are as follows:A layer should be created where a different level of A layer should be created where a different level of abstraction is needed.abstraction is needed.Each layer should perform a well defined function.Each layer should perform a well defined function.The function of each layer should be chosen with an The function of each layer should be chosen with an eye toward defining internationally standardized eye toward defining internationally standardized protocols.protocols.The layer boundaries should be chosen to minimize The layer boundaries should be chosen to minimize the information flow across the interfaces.the information flow across the interfaces.The number of layers should be large enough that The number of layers should be large enough that distinct functions need not be thrown together in the distinct functions need not be thrown together in the same layer out of necessity, and small enough that same layer out of necessity, and small enough that the architecture dose not become unwieldy.the architecture dose not become unwieldy.77The OSI Reference ModelThe OSI Reference ModelNote that the OSI model itself is not a network Note that the OSI model itself is not a network architecture, because it does not specify the architecture, because it does not specify the exact services and protocols to be used in each exact services and protocols to be used in each layer. It just tells what each layer should do. layer. It just tells what each layer should do. However, ISO has also produced standards for However, ISO has also produced standards for all the layers, although these are not part of the all the layers, although these are not part of the reference model itself. Each one has been reference model itself. Each one has been published as a separate international standard. published as a separate international standard. Next, we will discuss each layer of the model in Next, we will discuss each layer of the model in turn, starting at the bottom layer.turn, starting at the bottom layer.88OSI – The Physical LayerOSI – The Physical LayerThe The physical layerphysical layer is concerned with transmitting raw bits is concerned with transmitting raw bits over a communication channel. The design issues have over a communication channel. The design issues have to with making sure that when one side send a 1 bit, it is to with making sure that when one side send a 1 bit, it is received by the other side as a 1 bit, not as a 0 bit.received by the other side as a 1 bit, not as a 0 bit.Typical questions here are:Typical questions here are:how many volts should be used to represent a 1 and how may how many volts should be used to represent a 1 and how may for a 0for a 0how many microseconds a bit lasts how many microseconds a bit lasts whether transmission may proceed simultaneously in both whether transmission may proceed simultaneously in both directionsdirectionshow the initial connection is established and how it is torn down how the initial connection is established and how it is torn down when both sides are finishedwhen both sides are finishedAnd how many pins the network connector has and what each And how many pins the network connector has and what each pin is used for.pin is used for.The design issues here largely deal with The design issues here largely deal with mechanical, mechanical, electricalelectrical, and procedural interfaces, and the , and procedural interfaces, and the physical physical transmission mediumtransmission medium, which lies below the physical , which lies below the physical layer.layer.99OSI – The Data Link LayerOSI – The Data Link LayerThe main task of the The main task of the data link