note2 - Computer Networks Network architecture Saad...

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Unformatted text preview: Computer Networks Network architecture Saad Mneimneh Computer Science Hunter College of CUNY New York- Networks are like onions- They stink?- Yes, no, they have layers Shrek and Donkey 1 Introduction (ISO OSI, IETF, and Shrek stan- dard) When designing complex systems, such as a network, a common engineering approach is to use the concepts of modules and modularity. In this approach, the design of the system evolves by breaking the big task into smaller tasks. Each module is responsible for a specific task and provides services to the other modules to accomplish their tasks. We can interact with a module as a black box that provides certain functionality without knowing the details of how it works. We only need to know how to interface with the module. Someone can remove the module and update it with a newer one, and we would still be able to continue our work in the same way. Moreover, modularity is important to simplify tasks (divide and conquer). For instance, in a network, reliability of message delivery and routing of messages can be treated separately by different modules. Changing one would not impact the other. If a better routing procedure is employed, it only affects the module responsible for it. Certainly, we do not desire a change in routing to affect our ability to reliably deliver messages. Modules often interact in a hierarchy. A network is designed as a hierarchical or layered architecture in which every module or layer provides services to the upper layer. Users, sitting at the top layer of the network, communicate as if there is a virtual link between them, and need not be aware of the details of the network. The following figure illustrates the standardized 7 layers of a network. A description of each one follows. 1 physical DLC DLC network data link control network transport session presentation application DLC DLC network physical physical physical physical external site subnet node subnet node external site in out in out physical data link control network transport session presentation application physical link virtual bit pipe virtual link for reliable packets virtual link for end to end packets virtual link for end to end messages virtual session virtual network service user user Figure 1: Network architecture: the 7 layers of each node in the network 1.1 The top layers 1.1.1 Application layer This is the application that is used to access the network. Each application performs something specific to the user needs, e.g. browsing the web, transferring files, sending email, etc... 1.1.2 Presentation layer The main functions of the presentation layer are data formats, data encryption/decryption, data compression/decompression, etc......
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note2 - Computer Networks Network architecture Saad...

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