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Chapter 2

Chapter 2. Network Models. Figure 2.1 Sending a letter. Layered Protocols. Communication tasks are divided into series of layers or levels Each layer is responsible for particular task and act on them by using one or more protocols Each layer is built upon one bellow it

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Chapter 2

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  1. Chapter 2 NetworkModels Computer Networks

  2. Figure 2.1Sending a letter Computer Networks

  3. Layered Protocols • Communication tasks are divided into series of layers or levels • Each layer is responsible for particular task and act on them by using one or more protocols • Each layer is built upon one bellow it • The number and name of the layers differ from network to network Computer Networks

  4. Figure 2.17The OSI seven layer model Computer Networks

  5. Application Application R Presentation Presentation Session Session Transport Transport Network Network Network Data Link Data Link Data Link Physical Physical Physical The Layers of OSI Model End System Intermediate System Computer Networks

  6. To allow access to network resources Application To translate, encrypt and compress data Presentation To establish, manage and terminate sessions Session To provide reliable end-to-end message delivery and error recovery Transport To move packets from source to destination; to provide internetworking Network Data Link To organize bits into frames, to provide node-to-node delivery Physical To transmit bits over a medium; to provide mechanical and electrical specifications Summary of OSI Layer Functions Computer Networks

  7. Open System Interconnection (OSI) • Developed by International Standard Organization (ISO) as a first step towards international standardization • De jure protocol • Deals with interconnecting systems that are open for communication with other systems • Open protocol suite • Good as theoretical model, but not widely implemented in practice Computer Networks

  8. The OSI layers • Session layer • Provides the control structure for communication between applications (dialog control) • Establishes, manages and terminate connections (sessions) between cooperating applications • Presentation layer • Provides independence to the application processes from differences in data representation • Application layer • Provides access to the OSI environment for users and provides distributed information services Computer Networks

  9. The OSI layers • Physical layer • Transmission of unstructured bit stream • Deals with the mechanical, electrical, functional and procedural characteristics to access the physical medium • Data link layer • Provides reliable transfer across the physical link between two ends connected via single link • Sends blocks of data (frames) with the necessary synchronization, error control and flow control • Can add header and trailer Computer Networks

  10. The OSI layers • Network layer • Provides upper layers with independence from the data transmission and switching technologies accross internetwork • Responsible for source-to-destination delivery, addressing and routing in the internetwork • Transport layer • Provides transparent transport of data between end points that might not be connected via single link • Provides source-to-destinationconnection, error recovery and flow control Computer Networks

  11. Protocol Suites • Open System Interconnection (OSI) • Today used mostly as a reference model • Prevously used in X.25 based protocols • Internet (TCP/IP) • Most popular suite today • Xerox Networking Sysytems (XNS) • System Network Architecture (SNA – IBM) • Digital Network Architecture (DNA – DEC) • NetBIOS (Software interface) • AppleTalk Computer Networks

  12. The TCP/IP five layer model Computer Networks

  13. TCP/IP-modellen Exempel: SMTP, HTTP TCP, UDP IP Ethernet Computer Networks

  14. TCP/IP Protocol Suite • De facto (and after that de jure) standards • Open (All modification and newly proposed protocols are published in a form of RFC (Request for Comments) • RFC as well as drafts are published on the Internet • can be found on many URL (one is www.rfc-editor.org) • RFC becomes a standard when it is: • Stable and well understood • Technically competent • Implemented on multiple independent places Computer Networks

  15. The TCP/IP Protocol Suite (Cont.) • Allows computers of many sizes, vendors and operating systems to communicate with each other • History: • Developed as de facto standard before OSI • 1960’s: started as goverment financed research project • 1990’s: most widely used form of networking • Forms the basis for the Internet (capital “I”) (a WANthat spans the globe) Computer Networks

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