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    Voice and Data Communication

    Data Communication:Early computers were being used as stand alone systems in organisation fullfilling their own requirementswith widespread use of computers there was a relization that it would be advantageous in many situations to use computers from remote points.
    Types of communicatin with and among computers:Users would prefer to access to aserver from their place of work or heir homes without having to go to the computer centre.Such access can be provbided by connecting the users PC's with communication lines to the server. As a user enters a program and data manually the speed of communication to the server is slow.
    Characterstics of communication channels: In order to enable computer to communiucate we need communication channels connecting them.In this and subsequent sections we will examine the types of commuinication channelstheir charaterstics and methods used to transmit data using them.signals transmitted may be classified as either analog or digital.An analog channel transmits continously varying signals such asine waves.A digital channel on the other hand transmits binary digits represented by voltagge pulses.
    Allocation of Channel:A communication channel is an expensive resource. In computer communication except for short communication lines a channel is shared by many computersA channel is shared in such a way as to maximize the utilizatiion of channel capacity.
    Space Division Multiplexing:Space division multiplexing is creating a communication channel by grouping together a number of individual communication lines.e.g number of subscriber lines packeged in a multiwire cable at a telephone exchange.
    Frequency Division MultiplexingFrequecy division multiplexing is the technique is used to divide the bandwidth available in aphysical madium into number of smaller independent logical channels with each channel having a small bandwidth.
    Time Division MultiplexingThe TDM is another popular method of utilizing the capacity of a physical channel effectively.Each user of the channel os alloted small time intervals during which he may transmit a message.Thus total time available in the channel;is divided and each user is allocated time slices.
    Physical Communication MediaBonded media Unsheilded twisted pair of wires Known as UTP is the main medium used in local telephone communication and short distance less than 1 km digital data transmission.The medium is inexpensive and easy to install and use. Most modern buildings are prewired with UTP cables.
    Unbounded Mediawireless media are called unbounded as wires which restrict the movement of devices which communicate with one another are not used.The signals are radiated on the aatmosphere and picked up by the device to which they are sent.Communication satellites are now becoming very popular fro data communication between computers.communication satellites are now lauched either by rockets or by space shuttles and parked in geostationary orbit at 3600 km above the eqator.The speed of satellites in this orbit equals the speed of rotation of the earth and thus the satallite is stationary relative earth.

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