Voice and Data Communication

Data Communication: Early computers were being used as stand-alone systems in organization fulfilling their own requirements with the widespread use of computers there was a realization that it would be advantageous in many situations to use computers from remote points.

Types of communication with and among computers: Users would prefer to access to a server from their place of work or heir homes without having to go to the computer center. Such access can be provided by connecting the user’s PCs with communication lines to the server. As a user enters a program and data manually the speed of communication to the server is slow.

Characteristics of communication channels: In order to enable computers to communicate we need communication channels connecting them. In this and subsequent sections, we will examine the types of communication channels their characteristics and methods used to transmit data using them. signals transmitted may be classified as either analog or digital. An analog channel transmits continuously varying signals such as sine waves. A digital channel on the other hand transmits binary digits represented by voltage 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 computer channels is shared in such a way as to maximize the utilization 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 packaged in a multiwire cable at a telephone exchange.

Frequency Division Multiplexing: Frequency division multiplexing is the technique is used to divide the bandwidth available in a physical medium into a number of smaller independent logical channels with each channel having a small bandwidth.

Time Division Multiplexing: The TDM is another popular method of utilizing the capacity of a physical channel effectively. Each user of the channel os allowed 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 Media: Bonded media Unshielded 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 Media: wireless media are called unbounded as wires which restrict the movement of devices that communicate with one another are not used. The signals are radiated on the atmosphere and picked up by the device to which they are sent. Communication satellites are now becoming very popular for data communication between computers. communication satellites are now launched either by rockets or by space shuttles and parked in geostationary orbit at 3600 km above the equator. The speed of satellites in this orbit equals the speed of rotation of the earth and thus the satellite is stationary relative earth.