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Transmission Media

Transmission Media

Transmission media are actually located below the physical layer and directly controlled by the physical layer.
We can say that transmission media belong to layer zero.
The figure shows the position of transmission media in relation to the physical layer.
Computers and other telecommunication devices use signals to represent data. These signals are transmitted from one device to another in the form of electromagnetic energy. Which is propagated through transmission media.
Electromagnetic energy, a combination of electric and magnetic fields vibrating in relation to each other includes power, radio waves, infrared light, visible light, ultra-violet light, and X, gamma and cosmic rays. Each of this constitutes a portion of the electromagnetic spectrum.
Not all portions of the spectrum are currently usable for telecommunications; however.
The media to harness those that are usable are also limited to a few types.
For Telecommunications, transmission media can be divided into two broad categories: guided and unguided. Guided media include twisted pair cable, co-axial cable and fiber-optic cable. Unguided medium is usually air.
Guided Media-
Guided media, which are those that provide a physical medium or path from one device to another, there are three commonly used types of physical media:
1. Twisted pair cable
2. co-axial cable
3. fiber-optic cable
Cable characteristics include susceptibility to electrical interference, flexibility, ease of installation, range of signal transmission and transmission rates.
Transmission rates that can se supported on each of these cable types are measured in millions of bits per second (Mbps). Current cabling media transmission rates can vary from 9 to 90 Mbps and beyond.
Twisted-pair cable –
Twisted pair cable consists of two insulated strands of copper wire twisted together. A number oftwisted pair wires are grouped together and enclosed in a protective sheath to form a cable.
One of the two strands is used to carry signals to the receiver and the other one is used only as a ground reference. The receiver uses the difference between the two levels.
If the two wires are parallel, the effect of the unwanted signals is not the same in both wires because they are at different locations relative to the noise or crosstalk sources (e.g. One closer and one farther).
This results in a difference at the receiver. By twisting the pairs, a balance is maintained.
For example suppose in one twist, one wire is closer to the noise source and the other farther. In the next twist, the reverse is true.
This calculates a zero difference between the two and receiver receives no unwanted signals.
This cabling medium is susceptible to electrical interference and can only carry a signal for 90 meters.
Co-axial cable –
Co-axial (coax) cable is a conductive center wire surrounded by an insulating layer, a layer of wire mesh (shielding) and a non-conductive outer layer.
The most popular coaxial cable for networking applications are commonly called Thinnet and Thicknet.
The outer wire mesh serves both as a shield against noise and as the second conductor, which completes the circuit.
This outer conductor is also enclosed in an insulating sheath, and a plastic cover protects the whole cable.
Coaxial cable is resistant to interference and signal weakening and is generally better than twisted pair cable for longer distances.
Fiber-optic cable –
Optical fibers are used to carry digital signals in the form of modulated pulses of light.
An optical fiber consists of an extremely thin cylinder of glass, called the core, surrounded by a concentric layer of glass, Known as the cladding.
There are two fibers per cable – one to transmit and one to receive.
“An optical fiber is a thin, transparent and flexible that consists of a core surrounded by cladding.
The core and the cladding of an optical fiber are made fron the same material (Silica) and they differ only in their refractive indexes.
The difference in refractive indexes can be achieved by doping silica with different dopants.
We applied a third layer of coating over the cladding to protect the entire structure. The coating material is different from the core and cladding material
Advantages of fiber optics over other cables
• Higher Bandwidth
The higher the carrier’s frequency, the greater the channel bandwidth and the higher the information carrying capacity. As optical fiber uses light as the signal carrier and its frequency is very high (in between 914 to 915 Hz.).
• Small Size and Weight
Its thickness is same as the thickness of human’s hair. And its weight is too much small than corresponding copper cable.
• Immunity to interference and crosstalk
Optical fiber form a dielectric waveguide and are therefore free from electromagnetic interference and radiofrequency interference and crosstalk is negligible.
• Low transmission loss
The use of optical fiber cable provide a low attenuation or transmission loss than the perfect copper conductor wire. This feature is the main advantage of optical fiber communication.
Unguided Media: Wireless –
Unguided media transport electromagnetic waves without using a physical conductor, this type if Communication is often referred to as wireless communication.
Signals are normally broadcast through air and thus are available to anyone who has a device capable of receiving them.
Unguided signals can travel from the source to destination in several ways.
Radio Waves –
Although there is no clear-cut demarcation between radio waves and microwaves, electromagnetic waves ranging in frequencies between 3 KHz and 1 GHz are normally called radio waves.
Radio waves, for the most part, are omni directional. When an antenna transmits radio waves, they are propagated in all directions.
This means that the sending and receiving antennas do not have to be aligned.
A sending antenna can send waves that can be received by any receiving antenna. The omni directional property has a disadvantage, too.
The radio waves transmitted by one antenna are susceptible to interference by another antenna that may send signals using the same frequency or band.
Radio waves, particularly those waves that propagate in the sky mode, can travel long distances.
This makes radio waves a good candidate for long-distance broadcasting such as AM radio.
Microwaves –
Electromagnetic waves having frequencies between 1 and 300 GHz are called microwaves.
Microwaves are unidirectional. When an antenna transmits microwave waves, they can be narrowly focused. This means that the sending and receiving antennas need to be aligned.
The unidirectional property has an obvious advantage. A pair of antenna can be aligned without interfering with another pair of antennas.
Microwave propagation is line-of-sight. Since the towers with the mounted antennas need to be in direct sight of each other, towers that are far apart need to be very tall.
The curvatures of the earth as well as other blocking obstacles do not allow two short towers to communicate using microwaves.
Repeaters are often needed for long distance communication.
Infrared –
Infrared signals, with frequencies from 300 GHz to 400 THz (wavelengths from 1 mm to 770 nm), can be used for short-range communication.
Infrared signals, having high frequencies, cannot penetrate walls.
This advantageous characteristic prevents interference between one system and another; a short –range communication system in one room cannot be affected by another system in the next room.
When we use our infrared remote control, we do not interfere with the use of the remote by our neighbors.
However, this same characteristic makes infrared signals useless for long-range communication.
In addition, we cannot use infrared waves outside a building because the sun’s rays contain infrared waves that can interfere with the communication.