Explain Wired and Wireless Transmission Media for mobile computing.
Wired Transmission Media :
1) Twisted Pair Cable
A twisted pair consists of two insulated copper wires, typically about 1 mm thick.
The wires are twisted together in a helical form, just like a DNA molecule.
Twisting is done because two parallel wires constitute a fine antenna.
When the wires are twisted, the waves from different twists cancel out, so the wire radiates less effectively.
Why cable is twisted?
If the two wires are parallel, the effect of these unwanted signals is not the same in both wires because they are at different locations relatives to the noise or crosstalk sources.
This results in a difference at the receiver.
By twisting the pair, a balance is maintained.
Types of Twisted-Pair Cable
A) Unshielded twisted-pair (UTP)
Twisted pair cabling comes in several varieties, two of which are important for computer networks.
Category 3 twisted pairs consist of two insulated wires gently twisted together.
Category 5 is the more advanced twisted pairs were introduced.
They are similar to category 3 pairs, but with more twists per centimeter, which results in less crosstalk and a better-quality signal over longer distances, making them more suitable for high-speed computer communication.
Latest categories are 6 and 7, which are capable of handling signals with bandwidths of 250 MHz and 600 MHz, respectively (versus a mere 16 MHz and 100 MHz for categories 3 and 5, respectively).
B) Shielded twisted-pair (STP)
STP cable has a metal foil or braided mesh covering that encases each pair of insulated conductors.
Metal casing improves the quality of cable by preventing the penetration of noise or crosstalk.
It is bulkier and more expensive.
Applications:
Used in telephone lines to provide voice and data channels.
The DSL lines uses by telephone companies use the high-bandwidth capability of UTP cables.
LANs, such as 10Base-T, 100Base-T, also uses twisted-pair cables.
Fig. Shielded twisted-pair Cable
2) Coaxial Cable
It has better shielding than twisted pairs, so it can span longer distances at higher speeds.
Two kinds of coaxial cable are widely used. One kind, 50-ohm cable, is commonly used when it is intended for digital transmission from the start.
The other kind, 75-ohm cable, is commonly used for analog transmission and cable television but is becoming more important with the advent of Internet over cable.
A coaxial cable consists of a stiff copper wire as the core, surrounded by an insulating material.
The insulator is encased by a cylindrical conductor, often as a closely-woven braided mesh.
The outer conductor is covered in a protective plastic sheath.
The construction and shielding of the coaxial cable give it a good combination of high bandwidth and excellent noise immunity.
The bandwidth possible depends on the cable quality, length, and signal-to-noise ratio of the data signal. Modern cables have a bandwidth of close to 1 GHz.
Coaxial cables used to be widely used within the telephone system for long-distance lines but have now largely been replaced by fiber optics on long-haul routes.
3) Fiber Optics
A fiber-optic cable is made of glass or plastic and transmits signals in the form of light.
Optical fibers use reflection to guide light through a channel.
A glass or plastic core is surrounded by a cladding of less dense glass or plastic.
The difference in density of the two materials must be such that a beam of light moving through a core is reflected off the cladding instead of being refracted into it.
Fig. Fiber Optic Cable
Fiber optic cables are similar to coax, except without the braid.
Figure shows a single fiber viewed from the side. At the center is the glass core through which the light propagates.
The core is surrounded by a glass cladding with a lower index of refraction than the core, to keep all the light in the core.
Next comes a thin plastic jacket to protect the cladding. Fibers are typically grouped in bundles, protected by an outer sheath. Figure shows a sheath with three fibers.
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