02Chapter02 - CHAPTER 2 Data Communication Models Todays...

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15 CHAPTER 2 Data Communication Models Today’s networks and internetworks need to connect end systems (for example, com- puter stations) and intermediate systems (for example, routers) manufactured by a vari- ety of vendors. The complexity increases when we realize that internetworking is not only the connecting of end systems or intermediate systems, but also the need to force different application programs to communicate with one another. In other words, real communication is between one application program on one end system and another application program on another end system. This means the need for harmony: the application programs should understand each other, the end systems should coordinate, the intermediate system should facilitate the routing of information, the transmission media should be able to transmit the infor- mation with the rate and accuracy needed by end systems, and so on. The task is very complex. The complexity is due to the fact that some part of the systems should handle a subset of the task and others another subset. An end system cannot be responsible for carrying information from one place to another; this should be done by a transmission medium such as a cable or the air. Switching and routing of data are not functions of an end system; these tasks should be done by some intermedi- ate systems. 2.1 LAYERED ARCHITECTURE In a layered architecture approach, the complex task of communication between two applications is broken into smaller subtasks and each subtask is assigned to a layer . Different protocols use different numbers of layers with the duty of each layer being different from protocol to protocol. Peer-to-Peer Communication One of the ideas of layered architecture is to create peer-to-peer communication pro- tocols. The idea is that each given layer in one system logically communicates with its corresponding layer at the other system. In other words, layer N at the source logically
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16 CHAPTER 2 DATA COMMUNICATION MODELS (not necessarily physically) communicates with layer N at the destination and all inter- mediate nodes (if they have a layer N ). Figure 2.1 shows the idea. Protocol Data Unit (PDU) In a layered architecture, the communication between two corresponding layers requires a unit of data (packet) called a protocol data unit (PDU). The PDU used at layer 1 is called 1 -PDU, the protocol used at layer 2 is called 2 -PDU, and, in general, the PDU is used at layer N is called N -PDU. Encapsulation/Decapsulation Although logical communication occurs between two corresponding layers, the actual communication happens through the layers. At the source, the data flows downward through the layers. At the destination, the data flows upward through the layers. In the intermediate systems, the data flows first upward and then downward. Through this movement, headers or trailers are added to or removed from the PDU delivered by upper or lower layers. This process is called encapsulation/decapsulation because it
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This note was uploaded on 01/07/2012 for the course CIS 067A taught by Professor Behrouzforouzan during the Fall '11 term at DeAnza College.

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02Chapter02 - CHAPTER 2 Data Communication Models Todays...

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