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End office
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

