Transmission Media





Some medium of communications are as follows:

  • Twisted Pair
  • Co-axial Cables
  • Optical Fibers
  • Microwaves
  • Artificial Satellite
  • Infrared
  • Bluetooth

 

# Twisted Pair cable

 

It is oldest, but very common medium of transmission. It is made from two 1mm thick isolated copper wires twisted to each other like as DNA model. It is used for either analog or digital transmission. It has high error rate beyond 100m. Information is conveyed by the difference in voltage between two wires. If the signal voltage applied to one wire is V1 volts and the signal voltage applied to the other is V2, the signal being transmitted is V1-V2. If voltage on one wire will be V1 + n(noise signal), and the voltage on the other wire will be V2+n. The receiver will take the difference between voltage=v1+v2=v1+n+v2-n=v1-v2. It has two categories: UTP (Unshielded Twisted Pair), and STP (Shielded Twisted Pair).

 

(i) UTP (Unshielded twisted pair): It is a most popular twisted pair. The quality of UTP may vary from telephone-grade wire to extremely high-speed cable. The cable has 4 pair of wires inside the jacket. Each pair is twisted with different number if twists per inch to help eliminate interference from adjacent pairs and other electrical devices. The EIA/ TIA (Electronic Industrial Association / Telecommunication Industry Association) has established standards of UTP and rated five categories of wire:

Category 1: Voice only (Telephone wire)

Category 2: Data to 4 Mbps (Local Talk)

Category 3: Data to 10 Mbps (Ethernet)

Category 4: Data to 20 Mbps (16 mbps Token Ring)

Category 5: Data to 100 Mbps (Fast Ethernet)

 

(ii) STP (Shielded Twisted Pair): A advantage of UTP is that it may be susceptible to radio and electrical frequency interference. STP is suitable for environments with electrical interference; however, the extra shielding can make the cables quite bulky. STP is often used on networks using Token Ring Topology.

 

# Coaxial Cable

 

Coaxial cable is difficult to install. It is highly resistant to signal interference. It also supports greater cable length between networks devices than twisted pair cables.

Comments:

  • Speed: 100 Mega bits/second
  • Larger than cable diameter, lower in the transmission loss.
  • Used in long distance telephone line an as cables for closed circuit TV.
  • Used over distance of 1 KM.
  • It has two type: (a) thin and (b) thick co-axial cables

(i) Thin Coaxial Cable: 10 Base 2 refers to the specifications for thin coaxial cable carrying Ethernet signals. The 2 refers to the 200 meter as length, but in actual condition, the length is only 185 meter observed for efficient communication. This cable is popular in linear bus network.

(ii) Thick Coaxial Cable: 10 Base 5 refers to the specification for thick coaxial cable carrying Ethernet signal. The 5 refers to 500-meter length of cable for efficient transmission of data. Thick coaxial cable has an extra protective plastic cover that helps keep moisture away from the center conductor.

 

# Optical Fiber or Glass Fiber:

Diagram:

Optical Fiber

Optical Fiber


Composition:

(a) Core: It is an innermost part of fiber cable made of glass. Its diameter is 50 micron in the case of multi-mode fiber and 8 to 10 microns in single mode fiber.

(b) Cladding: Glass cladding covers Core. Its refractive index is less than core.

(c)Strengthening Fiber: Kevlar fiber h

(d) Jacket: It is an outermost Teflon or PVC covering of cladding.

 

Comments:

  • Light source: It is at the one end of optical fiber to accept electrical signal and convert it into light pulses. The presence of pulse indicates a 1 bit and absence of pulse indicates a 0 bit.
  • The transmission medium:  It is a hair like thin glass.
  • Detector: It is present to end of fiber cables to covert light pulse into electrical signals.
  • It transmits light pulse with very high speed and each pulse is equivalent to one bit of data. Optical fiber is expensive and hard to install.

The principle behind optical communication is the well-known phenomenon of total internal reflection. In 1970, the corning Glass Company of America developed an optical fiber which has losses of less than 20 dB per KM. The losses arise mainly on account of absorption and scattering of light by imperfections with the glass fiber. These have now been further reduced and today cables are available with loses of less than 1dB per KM. Optical Fiber are thus poised to replace conventional coaxial cables sonner than later.

The first trans-Atlantic fiber optical cable was made operational in 1988. This cable can carry 40,000 simultaneous telephone calls. Optical fiber cable can resist the ill-effect of sea water much better than the metal cables used earlier. However, the optical fibers cables are expensive at the moment and interconnecting them requires skilled operation.

Principle behind Optical Fiber

Principle behind Optical Fiber


 

# Single Mode Cable

 

Single Mode cable is a single stand of glass fiber with a diameter of 8.3 to 10 microns that has one mode of transmission. Single Mode Fiber with a relatively narrow diameter, through which only one mode will propagate typically 1310 or 1550 mm.

Single Mode fiber is a step index fiber with the central core having a diameter of about 3 microns. The cladding is about 60 microns thick. This effectively gives rise to a single transmission bit rate of the order of gigabits per second (1 giga=109).

Carries higher bandwidth than multimode fiber, but requires a light source with a narrow spectral width. Single Mode fiber is also called mono-mode optical fiber, single-mode fiber, single-mode optical waveguide, and uni-mode fiber.

Single-mode fiber gives you a higher transmission rate and up to 50 times more distance than multimode, but it also costs more. Single-mode fiber has a much smaller core than multimode. The small core and single light-wave virtually eliminate any distortion that could result from overlapping light pulses, providing the least signal attenuation and the highest transmission speeds of any fiber cable type.

Single-mode optical fiber is an optical fiber in which only the lowest order bound mode can propagate at the wavelength of interest typically 1300 to 1320nm.

 

Multimode

 

Multimode is a step index fiber with the central core having a diameter between 40 and 50 microns. The outside clad has a low refractive index and a thickness of 10 to 20 microns. Light rays propagate along the fiber core by multiple total internal reflections at the interface.

 

# Microwave

It is a type of electromagnetic wave which wavelength is longer than infrared but shorter than radio waves.

  • Frequency below 100 Hz is radio frequencies and higher is microwave frequency.
  • Speed: 16 Giga bits / second
  • It can carry 250,000 and more voice channels at a time.
  • It loses its strength after about 40 Km, so repeater stations are needed at distance of every 40 Km. Repeater is a device which amplifies coming signal before passing to next repeater station or receiver.

Microwaves are used in communication satellite transmissions because microwaves pass easily through the earth’s atmosphere with fewer interfaces than longer wavelength. There is also much more bandwidth in the microwave spectrum that in the rest of the radio spectrum.

 

# Artificial Satellite

 

The possibility of geo-stationary satellite for international communication purposes had been put forward in 1945 by an Englishman Arthur Clark. However, it was only with the lunching of artificial satellite, first by USSR and later by USA, that the original idea of Clark was given serious consideration. Communication between satellite and the earth is through microwaves. Transmitters on the ground modulate the message signal and transmit them to the satellite.

  • Placed at 36,000 Kms above equator. Satellite is geo-stationary type; it revolves around the earth in 24 hours and earth spins around own axis in 24 hours, so satellite looks stationary from earth stations.
  • A single geostationary satellite can covers approximately 40% of earth surface. Three such satellites, spaced at equal interval (120 angular degrees apart) can provide coverage of entire globe.
  • Satellite links use microwave frequencies in the order of 4 -12 Giga-Hz. Satellite contains amplifier that amplify feeble frequencies coming from earth stations.
  • A geo-stationary satellite can be accessing using a dish antenna aimed at the spot in the sky where the satellite hovers.
  • Major drawback of satellite communication is high cost of installing satellite in its orbit. At present, more than 100 satellites are in parking orbit.

Geo-stationary satellites are very expensive proportions. The total satellite systems include a master control station to keep the satellite in its position and ground stations to receive and transmit the message signals. The satellite itself is a marvelous piece of engineering with electronic circuits, antenna and solar panels. Today, geo-stationary satellites have become very popular in the field of communications.

Artifical satellite

Artifical satellite

They are lunched either by the American National Aeronautic Space Administration (NASA) from Cape Kennedy in USA or by European Space Organization, ARIANE from their lunch site in French Guyana in South America. INSAT is the Indian domestic satellite. The INSAT 2A was lunched by ARIANE in South America. Dish antenna can be used for satellite or microwave communication and broadcasts receptions, space communications, radio astrology and radar.

 

# Infrared (IR)

 

Infrared is a type of electromagnetic wave of large wavelength and small frequencies than visible light. Its frequency is low, so it has small penetrating power. It cannot penetrate opaque medium. IR data transmission is also employed in short-range communication among computer peripherals and personal digital assistants (PDAs). These devices usually conform to standards published by IrDA (Infrared Data Association). Remote controls and lrDA devices uses infrared emitting diodes (LEDs) to emit infrared radiation which is focused by a plastic lens into narrow beam. The beam is modulated to encode data.

The receiver uses a silicon photodiode to convert the infra radiation to an electric current. It responds only to the rapidly pulsing signal created by the transmitter, and filters out slowly changing infrared radiation from ambient light. It is used in optic fiber communication, wireless LAN, computers, printers, TV and many other electronic devices.

 

# Bluetooth

 

Bluetooth is a low cost, low-power, wireless radio frequency technology that allows various electrical devices to communicate with each other. These devices operate in the 2.4 GHz ISM (Industrial, Scientific, Medical) band.

Bluetooth

Bluetooth

One of the advantage of Bluetooth over IrDA (Infrared Data Association) is that close proximity between the communicating devices is not required, distance of up 10 meters or 32 feet are allowed. Also, blue tooth does not suffer from any line of sight(omni directional) restrictions.

The Bluetooth is an industrial specification for wireless personal area networks (PANs). It provides a way to connect and exchange information between devices like as PDAs (Personal Digital Assistants), mobile phone, laptops, PCs, printers and digital camera.

The Spec was developed by Ericsson, later formalized by the Bluetooth Special Interest Group (SIG). The Bluetooth wireless technology comprises hardware and software and interoperability requirements. Beyond unleashing devices by replacing cables, Bluetooth wireless technology provides a universal bridge to existing data networks or peripherals interface, and a mechanism to form small private ad, hoc groupings of connected devices away from fixed network infrastructures.

 

# Cellular Radio

 

Cellular radio is the radio communications technology that makes cellular phones possible. It divides a metropolitan area into a honeycomb of cells. This greatly increases the number of frequencies and users that can take advantage of mobile phone service. Each cell has its own low-power transmitter, rather than having one high-powered radio transmitter to serve an entire city. This significantly increases the number of radio frequencies available for mobile phone service.

Cellular radio has become an important communications medium for mobile voice and data communications. For example, Federal Express uses cellular radio for data communication with terminals in each of its thousands of delivery vans as part of its competitive edge.



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