Write about Data Transmission Media
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Data Transmission Media
The most basic hardware required for communication is the media through which data is transferred. There are several types of media, and the choice of the right media depends on many factors such as cost of transmission media, efficiency of data transmission and the transfer rate. We will describe some of these transmission media.
- Twisted Pair Cable
The most common form of wiring in data communication application is the twisted pair cable. It consists of a pair of insulated conductors (copper wires) that are twisted together. It is also called unshielded twisted-pair (UTP) cable because other than the plastic insulation around the two individual bunches of copper wires, nothing else shields it from outside interference.The advantage of a twisted pair cable is to provide better immunity from spurious noise signals. Twisted pair cable is used for communication upto a distance from 1 km and can achieve transfer rates of 1-2 megabytes per second. But, as the speed increased the maximum transmission distance is reduced, and may require repeaters. Twisted pair cable is widely used in telephone networks and are increasingly being used for data.
- Coaxial Cable
Coaxial cables are groups of specially wrapped and insulated wire lines capable of transmitting data at high rates. They consist of a central copper wire surrounded by a PVC insulation over which there is a sleeve of copper mesh. Te copper mesh sleeve is shielded again by an outer shield of thick PVC material. Larger the cable diameter, lower is the transmission loss, and higher transfer speeds can be achieved. A coaxial cable can be used over a distance of about 1 Km. and can achieve transfer rate of upto 100 megabytes per second.
A coaxial cable is of two types-a 75 ohm cable which is used by the cable TV operators and the 50 ohm cable which is used in high speed broadband network and are low loss cables. They are also used by telephone companies to transmit dat. Telephone companies often package several coaxial cables into a very large cable, which can handle over 40,000 telephone calls simultaneously.
- Optical Fibre Cables
An Optical Fibre Cable carries signals in the form of fluctuating light in a glass or plastic fibre. An optical fibre cable consists of a glass or plastic core surrounded by a closing made of similar material but with a lower refractive index. The core transmits the light while the change in refractive index between the core and the cladding causes total internal reflection, thus minimizing the loss of light from fibre. A light waves gave a much wider band width then the electric signal and are immune from elettromagnetic interferences, this leads to high data transfer rates of about 1000 megabytes per second and can be used for long and medium distance transmission links.
- Radio, Microwave and Satellite Channels
Radio, microwave and satellite channels are use electromagnetic propagation in open space. The advantage of these channels lie in their capability to cover large geographical areas and being inexpensive then the wired installation. The demarcation between radio, microwave and satellite channels lies in the frequencies in which they operate. Frequencies below 100 MHz are radio frequencies and higher are the microwave frequencies. Radio frequency transmission may be below 30 MH and thus the techniques of transmission are different. Owing to the characteristics of the ionosphere, frequencies below 30 MHz are reflected back towards the surface of the earth. Above 30 MHz propagation is one line-of-sight paths. Antennas are placed in between the line of sight paths to increase the distance. Radio frequencies are prone to attenuations and, thus, they require repeats along the path to enhance the signal. Radio frequencies can achieve data transfer rates of 100 Kbps to 400 Kbps.
Microwave systems use very high frequency radio signals to transmit data through space. However, at microwave frequencies, electromagnetic waves cannot bend or pass obstacles like tall buildings or hills. Hence, transmitter and receiver of a microwave system , mounted on very high towers, should be in line of sight. This may not be possible for very long distance transmission. Moreover, signals become weaker after travelling a certain distance and require power amplification. To overcome problems of line of sight and power amplification of weak signals, microwave systems use repeaters at intervals of about 25 km to 30 kms in between transmitting and receiving stations. Microwave systems have speed of about 16 giga bps. They can support about 250,000 voice channels simultaneously.
Main problem with microwave communication is that curvature of earth, mountains and other structures often block the line of sight. This problem is overcome by using satellites. Communication satellites are microwave relay stations placed in outer space. Each satellite can receive and retransmit signals to slightly less than half the earth’s surface. Therefore,at least three satellites are needed in geosynchronous orbit to provide data transmission service worldwide. The INSAT series of Indian satellites are positioned in outer space in a manner to be accessible from any place in India. In satellite communication microwave signal at 6 GHz or 14 GHz is transmitted from a transmitter on earth to a satellite positioned in space. By the time signal reaches the satellite, it becomes weak due to 36,000 kms travel. A transponder, mounted on the satellite, amplifies the weak signal and transmits it back to earth at a frequency of $ GHz or 11 GHz. A transponder can support typically 1200 voice channels each of 4800 bps, or 400 digital channels each of 64 Kbps (Kilo bits per second). A satellite has many transponders. Therefore, a single satellite has enormous data communication capability.
Use of 4 GHz to 6 GHz band of frequencies for transmission and retransmission of microwave signals in a satellite communication system is called C-band transmission, and use of 11 GHz to 14 GHz band of frequencies is called Ku-band transmission.
To enable a large number of users to benefit from satellite communication, it was realized that size and cost of receive-transmit earth stations has to be considerably reduced, so that one can easily install a private earth station. To meet this requirement, smaller dish antennas, having lower power transmitters and receivers, were built for installation at users’ sites. These dish antenna systems are popularly known as VSATs (Very Small Aperture Terminals ).
- Digital and Analog Transmission
Analog transmission has dominated communication for the last few decades. Analaog signals carry data as continuous wave form. In analog transmission, normally continuously varying voltage is used for such transmission. Telephone line is a good example of analog transmission media. Analog transmission is generally used for the transmission of voice signals or TV signals. Amplitude of an analog signal is measured n volts and its frequency (f) in hertz (Hz). When analog signals are transmitted over long distances, they become weak and distorted as they travel. Hence, amplifiers are used at periodic intervals along analog communication lines between modems to amplify weak analog signals.In terma of voltage, voice can be represented as follows:
Digital Transmission
Digital signal is a sequence of voltage pulses represented in binary form.When digital signals are transmitted over long distances, repeaters are used at periodic intervals along digital communication lines to strengthen weak digital signals.
Computer generated data is digital, whereas telephone lines used for data communication in computer networks usually carry analog signals. Digital transmission of digital data is preferred to analog transmission of digital data due to its lower cost, high transmission speed and lower error rate.
