Types of Communication Media/Channel/ Transmission Media:
There are several types of communications media/Channel/transmission media through which data can be transmitted from one point to another are as follows:
a. Wire Pairs/Twisted Wires
b. Coaxial Cable
c. Microwave System
d. Communications Satellite
e. Optical Fiber
a. Wire Pairs/Twisted Wires:
The most popular form of the communication is twisted-pair cable, two insulated copper wires twisted in a spiral. twisted-pair cable makes up the bulk of the world's public telephone communication system. That's also the reason twisted-pair is the most commonly used communication medium; it's already in place and it's relatively simple for you organization to set up a network using twisted-pair cable. Although it's the most commonly used medium, twisted-pair cable is also the slowest and least reliable of the guided communication media.
A single twisted-pair cable is capable of providing a capacity of up to 16 mbps for short distance. More realistically for network communication though, twisted-pair cable can handle up to 64 kilobits per second (kbps). Distance, noise on the line, and interference trend to limit its reliability.
||10 to 100Mbps
||32 to 2 seconds
||2 hours, 38 minutes
|ISDN Phone Line
||41 minutes, 40 seconds
|DSL phone Line
||8.192 Mbps download
|T-1 phone Line
||3 minutes, 27 seconds
||10 Mbps download
||300 Bps - 10 Mbps
||56Kbps - 200 Mbps
||256 Kbps - 100 Mbps
||256 Kbps - 100 Mbps
||500Kbps - 10 Gbps
For human speech communication, this is not much of a problem; if there's a noise on the line, you can still understand what your friend is saying. Information communication across a network, however, requires much greater accuracy. For example, a noise that changes the credit card number from 5244 0811 2643 741 to 5244 0810 2643 741 is more than an irritation; in business it means retransmitting the information or applying a charge to the wrong person's credit card.
Wire pairs are inexpensive medium of data transmission. They are easy to install and use, however, their use is limited because they easily pickup noise signals which results in high error rates when the line length extends beyond 100 meters.
b. Coaxial Cable:
A faster and more reliable alternative to twisted-pair cable is coaxial cable. Coaxial cables are group of specially wrapped and insulated wire lines that are able to transmit data at high rates. they consist of a central copper wire surrounded by a PVC insulated over which a sleeve of copper mesh is placed. An other shield of thick PVC material again shields the metal sleeve. The signal is transmitted by the inner copper wire and is electrically shielded by the outer metal sleeve.
Coaxial cables offer much higher bandwidths and are capable of transmitting digital signals at very high rates of 500mege bits per seconds, or about 15,000 voice calls, simultaneously. Because of its particular construction, coaxial cables have much higher noise immunity and offer a cleaner and crisper data transmission without distortion or loss signals up to 300-600 meters
c. Microwave System:
An other popular transmission media is microwave which is unguided communication media. This is a popular way, of transmitting data since it does not incur the expenses of laying cable. Microwave systems use very high frequency radio signals, to transmit data through space. Microwave are called "line-of-sight" media because at microwave frequencies the electromagnetic wave cannot bent or pass obstacles like hill. Hence, the transmitter and receiver of microwave system, which are mounted on very high towers, should be in line of sight.
This may not possible for very long distance transmission. Moreover, the signals become weaker after traveling a certain distance and require power amplification. Microwave systems permit data transmission rates of about 16 Giga baud.
Furthermore it can carry thousand of voice channels of same time. the link can support 250,000 voice channels. However, the capital investment needed to install microwave links is very high and hence they are mostly used to link big metropolitan cities with heavy telephone traffic between them.
d. Communications Satellite:
The main problem with microwave communication is that the curvature of the earth, mountains and other structures often block the line of sight. Due to this reason, several repeater stations are normally required for long distance transmission between two points. Using satellites, which are relatively newer, and more promising data transmission media overcome this problem.
A communication satellite is basically a microwave rely station placed in outer space. These satellite are launched either by rockets or by space shuttles above the equator with an orbit speed that exactly matches the earth's rotation speed. It is stationary relative to earth and always stays over the same point on the ground . This allows a ground station to aim its antenna at fixed point in the sky. Dozen of satellites are now in the orbit to handle international and domestic data, voice, and video communication needs. The main advantage of satellite communication is that it is a single microwave relay station visible from any point of a very-very large area.
However, a major drawback of satellite communication has been the high cost of placing the satellite into its orbit. Moreover, a signal sent to a satellite is broadcast to all receivers within the satellite range (security problems). Hence, necessary security measures are to be taken to prevent unauthorized tempering of information. data transmitting speed to satellite is 6GHz and data-receiving speed is 4 GHz, which is another drawback of satellite transmission.
e. Optical Fiber:
A potential cost effective, alternative to satellite communication is the use of optical fiber based upon laser technology. In fiber optical, semiconductor lasers transmit information in form of light along hair-thin glass (optical) fiber at the seed of light (186,000 miles/sec), with no significant loss of intensity over very long distance.
Characteristics of Optical Fibers:
The out standing characteristics of optical fibers are:
- They provide high quality transmission of signals at very high speeds.
- Fiber optical transmission are not affected by electromagnetic interface. Hence noise and distortion are also reduced.
- Optical fiber are most commonly used for point-to-point communication link because it is extremely difficult and expensive to tap a fiber optic cable at various points. The feature also provides security against unauthorized tampering of information.
Submarine Fiber Optical Network:
Submarine Fiber Optical cables are used for increased network reliability secure and faster data exchange all over the word. The largest submarine cable laid to data is SEA-ME-WE-III (South East Asia-Middle east- Western Europe). It is a high cable joining thirty three (33) countries led by France telecom and Singapore Telecom. It is providing high capacity digital communication links of 20 Gigabit capacities from South East Asia to Middle East and Western Europe via the Indian Ocean, the Red Sea and the Mediterranean Sea for 403333 MIUs KM .