iso osi and tcp-ip reference models
DESCRIPTION
computer networksTRANSCRIPT
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Reference models
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Objectives
• Understand the architecture of Internet with an analogy.
• ISO/OSI Reference model
• TCP/IP Reference model
• Compare ISO/OSI and TCP/IP
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Analogy: Airline System
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Horizontal Layering
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Protocol Layers• To reduce the design complexity, most
networks are organized as a stack of layers or levels, each one built upon the one below it.
• The purpose of each layer is to offer certain services to the higher layers, shielding those layers from the details of how the offered services are actually implemented.
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The OSI Reference Model
• 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.
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The OSI Reference Model
Application LayerPresentation Layer
Session LayerTransport LayerNetwork LayerData link layerPhysical Layer
• The seven layers of ISO/OSI model
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The OSI Reference Model…
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• The physical layer is concerned with transmitting raw bits over a communication channel.
• The design issues here largely deal with – mechanical, electrical, and timing interfaces, and – the physical transmission medium, which lies
below the physical layer
The Physical Layer
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• 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.
• The sender break the input data up into data frames and transmit the frames sequentially.
The Data Link Layer
DataFrame1
Frame2
Data Link Layer
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• The network layer is concerned with controlling the operation of the subnet.
• A key design issue is determining how packets are routed from source to destination.
The Network Layer
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• The basic function of the transport layer is to accept data from above, split it up into smaller units if need be,
• Pass the DU to the network layer, and ensure that the pieces all arrive correctly at the other end.
The Transport Layer
DU1
DU2
Data Transport Layer
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• The session layer allows users on different machines to establish sessions between them.
• Dialog control – (keepingtrack of whose turn it is to transmit)
• Token management – (preventing two parties from attempting the same
critical operation at the same time) and • Synchronization – (checkpointing long transmissions to allow they to
continue from where they were after a crash).
The Session Layer
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• Concerned with the syntax and semantics of the information transmitted.
The Presentation Layer
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• The application layer contains a variety of protocols that are commonly needed.
• One widely-used appplication 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.
The Application Layer
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The TCP/IP Reference Model
• ISO OSI Model TCP/IP Model
Application LayerPresentation Layer
Session LayerTransport LayerNetwork LayerData link layerPhysical Layer
Application Layer
Transport LayerInternet Layer
Link LayerPhysical Layer
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• Permit hosts to inject packets into any network and have them travel independently to the destination.
• The internet layer defines an official packet format and protocol called IP (Internet Protocol).
• The job of the internet layer is to deliver IP packets where they are supposed to go.
The Internet Layer
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• The first protocol defined is TCP(Transmission Control Protocol)
• TCP is a reliable connection-oriented protocol that allows a byte stream originating on one machine to be delivered without error on any other machine in the internet.
• It fragments the incoming byte stream into discrete messages and passes each one onto the internet layer.
The Transport Layer
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• The second protocol in this layer, UDP (User Datagram Protocol)
• UDP is an unreliable, connectionless protocol for applications that do not want TCP’s sequencing or flow control and wish to provide their own.
• It is also widely used for one-shot, client-server type request-reply queries and applications in which prompt delivery is more important than accurate delivery, such as transmitting speech or video.
The Transport Layer…
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• It contains all the higher-level protocols.• The early ones included virtual terminal
(TELNET), file transfer (FTP), and electronic mail (SMTP)
• The TELNET allows a user on one machine to log into a distant machine and work there.
• The FTP provides a way to move data efficiently from one machine to another.
• SMTP was originally just a kind of file transfer, but later a specialized protocol was developed for it.
The Application Layer
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• Domain Name Service (DNS) for mapping host names onto their network addresses
• HTTP, the protocol used for fetching pages on the World Wide Web
The Application Layer…
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• Below the internet layer is a great void.• The host has to connect to the network using
some protocol so it can send IP packets over it.
• This protocol is not defined and varies from host to host and network to network.
The Host-to-Network Layer
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OSI Vs TCP/IP
•Did not originally clearly distinguish between service, interface, and protocol• The protocols came first, and the model was really just a description of the existing protocols.•Four layers•connectionless in the network layer but supports both modes in the transport layer
• Three concepts are central: Services, Interfaces, Protocols
• The OSI reference model was devised before the protocols were invented
• Seven layers• Connectionless and
connection-oriented communication in the network layer, but only connection-oriented communication in the transport layer