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                                                           Intermediate                          Receiver
                                                            Switching
                                                              office
                                                      Toll            Toll
                                                     office          office








                        Sender                    Dedicated
                                                    Path



                                      Figure 6.3 Example of a telephone circuit in a telephone network
            As telephone circuits were susceptible to single point failure, during the 1960s, Paul Baran, along with Donald Davies
            and Len Kleinrock, came forward with the idea of fault resistant network based on digital packet switching. Unlike circuit
            switching, which requires a dedicated circuit for the entire session (duration of communication), packet switching
            works on the principle of sharing the links in the network. Messages to be transmitted are divided into small chunks
            called packets. A packet carries the following information: destination address, data, and a sequence number for
            arranging the packets in the correct sequence at the receiver end. The different packets in a session may take different
            paths from a sender to the receiver.



                  ARPANET: First Public Packet-switched Computer network.

            Advanced Research Project Agency (ARPA), set up by US Department of Defence (DoD), realised the need to connect
            geographically separated research computers to form a network. Consequently,  Advanced Research Projects Agency
            Network (ARPANET) evolved with the networking of computers of four host institutes (UCLA, Stanford, UCSB and the
            University of Utah) and later grew to 100’s of hosts. In Figure 6.4, we show ARPANET introduced in the late 1960s.
                                       ARPANET GEOGRAPHIC MAP, OCTOBER 1980




                                                                                                  MIT44
                                                                                         LINCOLN    MIT6
                           LBL
                   MOFFETT                                                                            CCA
                                                                                            AFGL       RCC5
                  AMES 15  AMES 16  LLL                                                  NYU           RCC49
                              SR12                                               CORADCOM         DEC      RCC71
                        SRI51  XEROX    UTAH                  GWC       ANL                           BBN40
                                                                                      RADC
                        STANFORD  TYMSHARE                                          CMU                BBN63
                                                                                                       BBN 72
                        SUMEX    NPS                                           WPAFB              HARVARD
                                                   DOCB                    DTI        DARCOM   ABERDEEN
                 HAWAII  ACCAT NOSC  UCLA                          STLA         ANDRW
                           AFSD  CIT                                  SCOTT       NRL      NSA  NBS
                                                                                  DCEC
                            ISI27  USC          AFWL                              SDAC              NORSAR
                             RAND
                            ISI52                                                  MITRE
                                     YUMA                                             ARPA    PENTAGON
                            ISI22                       WSMR   COLLINS   GUNTER       BRAGG
                                                                               EGLIN  ROBINS
                              SATELLITE CIRCUIT                                                     LONDON
                              IMP                           TEXAS
                              TIP
                              PLURIBUS IMP
                              PLURIBUS TIP
                              C30
                            (NOTE: THIS MAP DOES NOT SHOW ARPA'S EXPERIMENTAL SATELLITE CONNECTIONS)
                            NAMES SHOWN ARE IMP NAMES, NOT (NECESSARILY) HOST NAMES
                                                        Figure 6.4 ARPANET
                                                                                             Computer Networks   223
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