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    1. Mapping The OSI Model To The TCPIP Model…


    2. earn your CCNA, you need to know the OSI model, the TCP/IP model, and how the two


    3. The four layers of the TCP/IP architecture can be compared to certain levels of the OSI


    4. and how these layers map to the OSI model


    5. The Transport layer in the TCP/IP architecture is similar to the Transport layer in the OSI model


    6. Remember that the OSI model uses IP addresses, or “Layer 3


    7. Interface model does the work of both the Data Link and Physical Layers in the OSI


    8. And crank up the paper work to have Det 5 OSI do a Top Secret Plus background check on him


    9. ative, or OSI, an organization that would police the use of the


    10. An Office of Special Investigations, or OSI, team had been flown in to head up the investigation

    11. Finally he was interviewed by the OSI personnel after they’d barged onto the scene and staked their claim as the lead investigative agency


    12. And OSI had made clear it was the lead agency because Daughtrey had been a flyboy


    13. “OSI figure that too?” asked Knox


    14. The OSI team was congregated in a corner going over their notes, Puller could see


    15. One of the OSI team came over to Puller and Knox


    16. “Not necessarily,” said Knox, and the OSI man turned his attention to her


    17. The OSI agent stared at him fixedly and then nodded


    18. “OSI will be on to that angle as well, you can count on that,” said Puller


    19. This part of the book introduces networking concepts and explains both the OSI and TCP/IP models


    20. The Open Systems Interconnection (OSI) reference model defines the functions of each layer and how the layers work together to provide network communications

    21. Chapter 2 covers the OSI reference model in detail


    22. Figure 2-1 The OSI reference model with its seven layers


    23. The OSI stack was originally conceived as the model for the creation of a protocol suite that would conform exactly to the seven layers


    24. Networking professionals, educators, and authors frequently refer to protocols, devices, or applications as operating at a particular layer of the OSI model because using this model breaks a complex process into manageable units that provide a common frame of reference


    25. However, it is important to understand that none of the protocol stacks in common use today conforms exactly to the layers of the OSI model


    26. The primary reason why real protocol stacks differ from the OSI model is that many of the protocols used today (including Ethernet) were conceived before the OSI model documents were published


    27. In fact, the TCP/IP protocols have their own layered model, which is similar to the OSI model in several ways but uses only four layers (see Figure 2-2)


    28. Figure 2-6 Each layer in the OSI model communicates with the layer above and below it


    29. The following sections examine each of the seven layers of the OSI reference model in turn, the functions that are associated with each, and the protocols that are most commonly used at those layers


    30. The physical layer of the OSI model defines the actual medium that carries data from one computer to another

    31. Thus, while the OSI reference model might create a neat division between the physical and data link layers, in the real world, the functionality of the two is more closely intertwined


    32. The OSI document defines five theoretical classes of transport layer protocol, as shown here:


    33. This classification of transport layer services is another place where the theoretical constructs of the OSI model differ substantially from reality


    34. The OSI model document defines two forms of error recovery that can be performed by connection-oriented transport layer protocols


    35. The services provided by the session layer are widely misunderstood, and even at the time of the OSI model’s development, there was some question concerning whether they should be allotted a layer of their own


    36. Most of these services are of interest only to application developers, and some are even duplicated as a result of a compromise that occurred when the two committees creating OSI model standards were combined


    37. This was a much more common occurrence at the time that the OSI model was developed than it is now


    38. All of the discussion in the previous sections about applications utilizing session layer services actually involves the use of the pass-through service at the presentation layer because it is impossible for a process at any layer of the OSI model to communicate directly with any layer other than the one immediately above or beneath it


    39. As you have seen in this chapter, the bottom four layers of the OSI reference model perform functions that are easily differentiated, while the functions of the session, presentation, and application layers tend to bleed together


    40. Many of the application layer protocols listed here contain functions that rightly belong at the presentation or session layers, but it is important not to let the OSI model assert itself too forcibly into your perception of data networking

    41. A bridge is another device used to connect LAN cable segments, but unlike hubs, bridges operate at the data link layer of the OSI model and are selective about the packets that pass through them


    42. In Chapter 2, the protocol operating at the Open Systems Interconnection (OSI) model’s data link layer was compared to a postal system, in which each packet is a piece of mail and the data link layer frame functions as the envelope containing the data generated by the upper layers


    43. In the previous sections, you learned how repeaters, hubs, and bridges can connect network segments at the physical and data link layers of the OSI model, creating a larger LAN with a single collision domain


    44. Whether wired or wireless, network routers work at the network layer of the OSI model


    45. Frame-relay service operates at the data link layer of the OSI reference model and runs at bandwidths from 56 Kbps to 44


    46. The Fibre Channel protocol stack consists of five layers that perform the functions attributed to the physical and data link layers of the Open Systems Interconnection (OSI) reference model


    47. This chapter examines the fundamental Ethernet mechanisms and how they provide a unified interface between the physical layer of the Open Systems Interconnection (OSI) reference model and multiple protocols operating at the network layer


    48. It is up to the protocols operating at higher layers of the OSI model to detect the data loss and to use their own mechanisms to force a retransmission


    49. The frame consists of a header and footer that surround and encapsulate the data generated by the protocols operating at higher layers of the OSI model


    50. Since its inception in the 1970s, the TCP/IP protocol suite has evolved into the industry standard for data transfer protocols at the network and transport layers of the Open Systems Interconnection (OSI) model







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