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•  Message: Information being communicated.
        •  Channel: The medium through which data is transmitted.

                                                          DATA

                                                  Communication Channel

                                         Node 1           DATA             Node 2
                                               Figure 6.1 Communication System

        Figure 6.2 shows a collection of interconnected nodes that communicate through some channel that forms a computer
        network.  Each participating node in a network has a unique Media Access Control address (MAC address). For example,
        if you are using WINDOWS® you can find your computer’s MAC address (physical address) by executing the following
        command from the command prompt:

        ipconfig /all



                 What is the MAC address of your computer?



                                                Computer System 1

                                                                         Mobile






                                                                          Printer

                                   Computer
                                   System 2



                                                    Computer                  Computer
                                                    System 3                  System 4
                                                Figure 6.2 Computer Network

        One or more nodes in a computer network may provide resources and services to other nodes. An application program
        that provides a service in a computer network is called a server. For example, an FTP server installed on a computer
        enables the transfer of files to various nodes in the network. In informal use, the node that hosts a service is sometimes
        called a sever, even though it does not confirm to the technical meaning of the term.

        6.2 Evolution of Networking

        In 1876, Bell developed a communication paradigm based on telephone lines. Initially, each telephone was connected
        by a dedicated wire to another phone it wished to communicate with. However, the network soon became a mess
        of cables. In 1877, his Bell Telephone Company established the Public Switched Telephone Network (PSTN), allowing
        several home phones to connect to a dedicated office that would establish a dedicated connection between the
        interested parties. This form of end to end communication remained the primary mode of communication for the
        next hundred years.  Establishing a dedicated connection (also called path or circuit) between the two end-users often
        requires passing through several intermediate switches en route. So, setting up a telephone circuit is called circuit
        switching. Figure 6.3 shows a telephone network.


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