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OSI Model: Layers, Protocols, and Functions

Learn about the OSI model, its layers, protocols, and their functions in computer networks. Understand how data is constructed and reduced throughout the communication process.

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OSI Model: Layers, Protocols, and Functions

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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

  16. Layer 5 Layer 5 Layer 4 Layer 4 Layer 3 Layer 3 Layer 2 Layer 2 Physical Physical ProtocolsConstruction versus Reduction DATA Construction H5 DATA H4 DATA UNIT H3 DATA UNIT H2 DATA UNIT T2 BITS Reduction H – header (pakethuvud): control data added at the front end of the data unit T – trailer (svans): control data added at the back end of the data unit Trailers are usually added only at layer 2 Computer Networks

  17. Illustration of the Construction and Reduction Process • Observe how headers and trailer are added at the sender and removed at the receiver Animation of Figure 2.4 in the book Computer Networks

  18. Figure 2.4An exchange using the Internet model Computer Networks

  19. Figure 2.3Peer-to-peer processes Computer Networks

  20. Layer 5 Layer 5 Layer 4 Layer 4 Layer 3 Layer 3 Layer 2 Layer 2 Layer 1 Layer 1 An Example of Five Layers Network Machine 2 Machine 1 Layer 5 protocol Layer 4/5 interface Layer 4 protocol Layer 3/4 interface Layer 3 protocol Layer 2/3 interface Layer 2 protocol Layer 1/2 interface Layer 1 protocol Physical medium The path through which the actual transmission take place Computer Networks

  21. The Concept of Layers • Layer n on one machine communicates with layer non the other machine via layer n protocol. • The communication is virtual • Peers are entities comprising the corresponding layers on different machines. • There is an interface between each pair of adjacent layers for communication with the layer above and the layer below. Computer Networks

  22. Peer-to-peer Processes • The processes on the two machines that communicate at a given layer are called peer-to-peer processes • At the physical layer communication is direct • At the upper layers the communication has to go down through the layers on the sender machine, than to be transmited through the physical layer and than to go back up to the same layer at the receiving machine Computer Networks

  23. Messages and Protocol Stacks • On the sender machine, each layer: • Accepts an outgoing message from the layer above • Adds a header and does other processing • Passes resulting message to next lower layer • On the receiver, each layer: • Receives an incoming message from the layer below • Removes the header for that layer and performs other processing • Passes the resulting message to the next higher layer Computer Networks

  24. Illustration of the Source-to-end Delivery at the Network Layer • Observe how data are sent from node to node to reach the final destination. Animation of Figure 2.11 in the book Computer Networks

  25. Figure 2.5Physical layer Computer Networks

  26. Note: The physical layer is responsible for transmitting individual bits from one node to the next. Computer Networks

  27. Figure 2.6Data link layer Computer Networks

  28. Note: The data link layer is responsible for transmitting frames from one node to the next. Computer Networks

  29. Figure 2.7Node-to-node delivery Computer Networks

  30. Example 1 In Figure 2.8 a node with physical address 10 sends a frame to a node with physical address 87. The two nodes are connected by a link. At the data link level this frame contains physical addresses in the header. These are the only addresses needed. The rest of the header contains other information needed at this level. The trailer usually contains extra bits needed for error detection Computer Networks

  31. Figure 2.8Example 1 Computer Networks

  32. Figure 2.9Network layer Computer Networks

  33. Note: The network layer is responsible for the delivery of packets from the original source to the final destination. Computer Networks

  34. Figure 2.10Source-to-destination delivery Computer Networks

  35. Example 2 In Figure 2.11 we want to send data from a node with network address A and physical address 10, located on one LAN, to a node with a network address P and physical address 95, located on another LAN. Because the two devices are located on different networks, we cannot use physical addresses only; the physical addresses only have local jurisdiction. What we need here are universal addresses that can pass through the LAN boundaries. The network (logical) addresses have this characteristic. Computer Networks

  36. Figure 2.11Example 2 Computer Networks

  37. Figure 2.12Transport layer Computer Networks

  38. Note: The transport layer is responsible for delivery of a message from one process to another. Computer Networks

  39. Figure 2.12Reliable process-to-process delivery of a message Computer Networks

  40. Example 3 Figure 2.14 shows an example of transport layer communication. Data coming from the upper layers have port addresses j and k (j is the address of the sending process, and k is the address of the receiving process). Since the data size is larger than the network layer can handle, the data are split into two packets, each packet retaining the port addresses (j and k). Then in the network layer, network addresses (A and P) are added to each packet. Computer Networks

  41. Figure 2.14Example 3 Computer Networks

  42. Figure 2.15Application layer Computer Networks

  43. Note: The application layer is responsible for providing services to the user. Computer Networks

  44. Figure 2.16Summary of duties Computer Networks

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