note15 - CSCI 120 Introduction to Computation Computer...

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Unformatted text preview: CSCI 120 Introduction to Computation Computer Networks (draft) Saad Mneimneh Visiting Professor Hunter College of CUNY 1 Introduction Primitive forms of data networks have a long history. After all, it is all about communication. Early societies used smoke signal to communicate information. In the 19 th century, telegraphy was used as means of communicating messages. Messages were manually encoded into strings of symbols and then manually transmitted and received. When necessary, the messages were manually relayed at intermediate points. Example of such manual encoding and transmission is the Morse code, created by Samuel Morse in the 1830s. Morse code used standardized sequences of short and long marks or pulses (commonly known as dots and dashes) to encode letters, numerals, and punctuation marks. For instance the Morse code for the letter V is ... 1 . Reflecting on the knowledge that we have accumulated in this course so far, this is equivalent to bits! A dot corresponds to a 0 and a dash corresponds to a 1. Therefore, we can say that telegraphy used binary symbols . In the 1050s and 1960s, we have seen that central computers were connected to terminals by links to form time-sharing real-time processing systems (see Lecture 11). This constitute the first form of modern computer networks that we are familiar with today. terminal central processor printer Figure 1: Central computer network 1 The 4 opening notes of Beethovens fifth symphony, named Victory, represent the Morse code for the letter V: three short notes followed by a long note. This is just a coincidence. Beethoven died in 1827 before Morse invented his code. Later on, multiplexers were used to collect all the traffic from a set of pe- ripherals in the same geographic area and send it on a single link to the central processor. To free the processor from the burden of handling communication, a special processor, called front end, was also developed to control communication. terminal central processor printer front end Figure 2: Multiplexers and front end added In these networks, and in contrast to telegraphy, communication is auto- mated. However, while such a system can be referred to as a computer network, it is simpler to view as one computer with remote peripherals. Real networks emerged in the 1970s when ARPANET and SYMNET were introduced. These were the first large scale general purpose computer networks connecting geo- graphically distributed computer systems, users, peripherals, etc... CPU subnet terminal personal computer CPU Figure 3: Computer network Therefore, instead of having a computer as the center of the network, the subnet (communication part of the network) becomes central. Inside the subnet we have a set of nodes (these are usually computers in their own right), various pairs of which are connected by communication links. Outside the subnet, various computers, databases, terminals, and other devices, are connected via the subnet. A message originates at an external device (the sender), passes through the subnet from one node to another on the links, and goes out to the...
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This note was uploaded on 03/27/2010 for the course CSCI 120 taught by Professor Saadmneimneh during the Spring '09 term at CUNY Hunter.

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note15 - CSCI 120 Introduction to Computation Computer...

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