Unit - 4
COMPUTER NETWORK & INTERNET
Introduction
• Data communication technologies deal with means and methods of data transfer from one location to another.
• A computer network is a network of computers. It connects multiple computers in a manner to enable meaningful transmission and exchange of data. Sharing of information, resources (both hardware and software), and processing load among the connected computers is the main objective of a computer network.
COMPUTER NETWORK
Communication is the process of transferring a message from one point to another. The three basic elements of any communication system are:
1. Sender 2. Medium 3. Receiver
Basic elements of a Communication System
• For example, when you speak to your friend on telephone, you are the sender, telephone line is medium, and your friend is the receiver. This is an example of voice communication.
• Similarly in data communication, sender and receiver are normally machines (such as computers, peripheral devices like printers, plotters, disk etc.) and transmission medium is a wired/wireless data communication channel (such as telephone lines, optical fiber, satellite links etc.)
• Electronic systems that transfer data from one point to another are called data communication systems.
Simplex
• In this type of transmission mode, data can be sent only in one direction i.e. communication is unidirectional. It is suitable for connecting send – only (such as keyboard) or receive – only (such as printer) devices.
• At first thought , this mode may appear adequate for applications in which flow of information is unidirectional.
• However, almost all data processing applications require bidirectional communication because even when flow of data is unidirectional, an application requires a return path to send acknowledgement, control or error signals.
• Without this capability, a sender might send data and never know that the receiver never received it (due to some problem)
Half - Duplex
• Half-duplex data transmission means that data can be transmitted in both directions on a signal carrier, but not at the same time.
Half - Duplex
• For example, on a local area network using a technology that has half-duplex transmission, one workstation can send data on the line and then immediately receive data on the line from the same direction in which data was just transmitted.
• Hence half-duplex transmission implies a bidirectional line (one that can carry data in both directions) but data can be sent in only one direction at a time.
• Example of half duplex is a walkie- talkie in which message is sent one at a time but messages are sent in both the directions.
Full - Duplex
• A half duplex system needs to switch direction each time the direction of data transfer reverses. This requires a special switching circuit and a delay of about 150 milliseconds.
• Many times, applications require bi-directional data transfer simultaneously.
• In full duplex system we can send data in both the directions as it is bidirectional at the same time in other words, data can be sent in both directions simultaneously.
• Example of Full Duplex is a Telephone Network in which there is communication between two persons by a telephone line, using which both can talk and listen at the same time.
Full - Duplex
Twisted – Pair Wire
• Consists of two bunches of thin copper wires, each bunch enclosed separately in a plastic insulation, then twisted around each other to reduce interference by adjacent wires.
It is also called unshielded twisted pair cable because other than the plastic insulation around the two individual bunches of copper wires, nothing else shields it from outside interference.
Twisted – Pair Wire
• Commonly used in local telephone communication and short distances ( up to about 1 km) digital data transmission.
• Normally for distance up to 100 meters, they provide data transmission speed of up to 9600 bps, and for longer distances (local telephone lines) they provide speed of the order of 1200 bps.
• UTP cables are inexpensive, and easy to install and use.
• Their use is limited because they easily pick up noise signals when line length extends beyond from 100 meters.
Coaxial cables consists of a central copper wire surrounded by a PVC insulation over which there is a sleeve of copper mesh. The copper mesh sleeve is shielded again by an outer shield of thick PVC material. Signal is transmitted by inner copper wire, and is electrically shielded by the outer copper mesh sleeve.
Coaxial Cable - Features
• Offer much higher bandwidths than UTP cables, and can transmit digital signals at rates up to 10 mega bps.
• They are used extensively in long distance telephone lines for both voice and data transmission.
• Telephone companies often packages several coaxial cables into a very large cable, which can handle over 40,000 telephone calls simultaneously.
• Furthermore, coaxial cables have much higher noise immunity, and can offer cleaner data transmission without distortion or loss of signals.
Microwave System
• These systems use very high frequency radio signals to transmit data through space (wireless communication).
• However, at microwave frequencies, electromagnetic waves cannot bend or pass obstacles like tall building or hills. Hence, transmitter and receiver of a microwave system, mounted on very high towers, should be in line of sight. But this may not be possible for very long distance transmission. Moreover, signals become weaker after travelling a certain distance and require power amplification.
Microwave System
• To overcome problems of line of sight and power amplification of weak signals, microwave systems use repeaters at intervals of about 25 to 30 kms in between transmitting and receiving stations.
• First Repeater in line of sight of transmitting station and its last repeater in line of sight of receiving station.
• Each of these stations receives, amplifies and retransmits data signals.
• Have speed of about 16 Giga bps (1 Giga = 109). They can support about 2,50,000 voice channels simultaneously. As they support wireless communication, they do not require laying of cables and associated cost of land digging.
Satellite Communication
• A space shuttle launches a satellite and places it in outer space precisely 36,000 kms with an orbit speed that matches earth’s rotation speed exactly.
• In satellite communication, microwave signal at 6GHz or 14GHz is transmitted from a transmitter on earth . By the time this signal reaches the satellite, it becomes weaker due to 36,000 kms travel.
• Transponders are there to amplify and transmit it back to earth at a frequency of 4GHz to 11GHz.
Advantages
• Data transmission costs are independent of distance between two points as long as these two points are within the satellite’s area coverage.
• Satellite with Many transponders has enormous data communication capability.
• Error detection is easy as a transmitting station can receive back its own transmission .
• As they support wireless transmission, they do not require laying of cables and associated costs of land digging.
Disadvantages
• Initial cost of placing a satellite into orbit is vey high.
• Microwave signals have to travel a long distance that results into a propagation delay of 270 msec between sender and receiver.
• While retransmitting, satellite broadcast it to all stations within its coverage so system needs special security to prevent unauthorized access of information.
• Atmospheric disturbance, like thunder and lightning affects K band communication system so in these times C – band is used
Optical Fibers
• Optical fibers are hair – thin threads of glass used as data transmission medium.
• They transmit light signals instead of electrical signals.
• As light travels much faster than electricity, optical fibers can transmit data at much higher speed with no significant loss of intensity over long distances.
At sender side: It uses a Light Emitting diode (LED) or a laser diode to convert electrical to light waves
At Receiver Side: It uses photoelectric diode.
Advantages
• Larger Bandwidth
• Low loss
• Immunity to electromagnetic interference
• Small size and lightweight
• Security
• Analog and digital data transmission
Disadvantages
• Aligning and joining two fiber optic cables is not so simple as for twisted copper wire pairs or coaxial cables. It requires special equipment to do so.
