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Relate the following with Computer Networks

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Transmission Media

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Classes of transmission media

In telecommunications, transmission media can be divided into two broad categories:guided and unguided. Guided media include twisted-pair cable, coaxial cable, and

fiber-optic cable. Unguided medium is free space.

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GUIDED MEDIA

Twisted-Pair Cable

Coaxial Cable

Fiber-Optic Cable

Topics discussed in this section:

Guided media, which are those that provide a conduit from one device to another,

include twisted-pair cable, coaxial cable, and fiber-optic cable.

A signal traveling along any of these media is directed and contained by the

physical limits of the medium.

Twisted-pair and coaxial cable use metallic (copper) conductors that accept

and transport signals in the form of electric current.

Optical fiber is a cable that accepts and transports signals in the form of light.

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Twisted-pair cable

A twisted pair consists of two conductors (normally copper), each with its

own plastic insulation, twisted together, as shown in below Figure.

One of the wires is used to carry signals to the receiver, and the other is used only as a

ground reference. The receiver uses the difference between the two. In addition to the

signal sent by the sender on one of the wires, interference (noise) and crosstalk mayaffect both wires and create unwanted signals.

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 relative to the noise or crosstalk

sources (e,g., one is closer and the other is 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 is farther; in the next twist,the reverse is true. Twisting makes it probable that both wires are equally affected by

external influences (noise or crosstalk). This means that the receiver, which calculates

the difference between the two, receives no unwanted signals. The unwanted signals

are mostly canceled out. From the above discussion, it is clear that the number of

twists per unit of length (e.g., inch) has some effect on the quality of the cable.

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UTP and STP cables

The most common twisted-pair cable used in communications is referred to as

unshielded twisted-pair (UTP). IBM has also produced a version of twisted-pair

cable for its use called shielded twisted-pair (STP). STP cable has a metal foil orbraided mesh covering that encases each pair of insulated conductors. Although

metal casing improves the quality of cable by preventing the penetration of noise or

crosstalk, it is bulkier and more expensive. The following Figure shows the

difference between UTP and STP.

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Categories of unshielded twisted-pair cables

ApplicationsTwisted-pair cables are used in telephone lines to provide voice and data channels.

The local loop-the line that connects subscribers to the central telephone office-

commonly consists of unshielded twisted-pair cables.

The DSL lines that are used by the telephone companies to provide high-data-rate

connections also use the high-bandwidth capability of unshielded twisted-pair

cables.

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Coaxial cable

Coaxial cable (or coax) carries signals of higher frequency ranges than those in

twistedpair cable, in part because the two media are constructed quite differently.

Instead of having two wires, coax has a central core conductor of solid or strandedwire (usually copper) enclosed in an insulating sheath, which is, in turn, encased

in an outer conductor of metal foil, braid, or a combination of the two. The outer

metallic wrapping 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 the whole cable is protected by a plastic cover (see below

Figure).

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Categories of coaxial cables

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BNC connectors

To connect coaxial cable to devices, we need coaxial connectors. The most

common type of connector used today is the Bayonet-Neill-Concelman (BNC),

connector. The following Figure shows three popular types of these connectors: the

BNC connector, the BNC T connector, and the BNC terminator.

The BNC connector is used to connect the end of the cable to a device, such as a

TV set. The BNC T connector is used in Ethernet networks to branch out to a

connection to a computer or other device. The BNC terminator is used at the end of

the cable to prevent the reflection of the signal.

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Appl icat ions

Coaxial cable was widely used in analog telephone networks wherea single coaxial network could carry 10,000 voice signals.

Later it was used in digital telephone networks where a singlecoaxial cable could carry digital data up to 600 Mbps. However,coaxial cable in telephone networks has largely been replaced todaywith fiber-optic cable. Cable TV networks also use coaxial cables.

In the traditional cable TV network, the entire network used coaxialcable. Later, however, cable TV providers replaced most of themedia with fiber-optic cable; hybrid networks use coaxial cable onlyat the network boundaries, near the consumer premises. Cable TVuses RG-59 coaxial cable.

Another common application of coaxial cable is in traditionalEthernet LANs. Because of its high bandwidth, and consequentlyhigh data rate, coaxial cable was chosen for digital transmission in

early Ethernet LANs.

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Fiber-optic cable

A fiber-optic cable is made of glass or plastic and transmits signals in the form of

light. To understand optical fiber, we first need to explore several aspects of the

nature of light. Light travels in a straight line as long as it is moving through a single

uniform substance. If a ray of light traveling through one substance suddenly enters

another substance (of a different density), the ray changes direction. The following

figure shows how a ray of light changes direction when going from a more dense to a

less dense substance.

As the figure shows, if the angle of incidence I (the angle the ray makes with the line

perpendicular to the interface between the two substances) is less than the critical

angle, the ray refracts and moves closer to the surface. If the angle of incidence is

equal to the critical angle, the light bends along the interface. If the angle is greater than

the critical angle, the ray reflects (makes a turn) and travels again in the denser

substance. Note that the critical angle is a property of the substance, and its valuediffers from one substance to another.

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Optical f iber

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 the

core is reflected off the cladding instead of being refracted into it. See the

following Figure.

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Propagation modes

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Modes

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F iber construction

The following Figure shows the composition of a typical fiber-optic cable. The outer

jacket is made of either PVC or Teflon. Inside the jacket are Kevlar strands to

strengthen the cable. Kevlar is a strong material used in the fabrication of

bulletproof vests. Below the Kevlar is another plastic coating to cushion the fiber.

The fiber is at the center of the cable, and it consists of cladding and core.

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F iber-optic cable connectors

The subscriber channel (SC) connector is used for cable TV. It uses a push/pull

locking system. The straight-tip (ST) connector is used for connecting cable to

networking devices. It uses a bayonet locking system and is more reliable than

SC. MT-RJ is a connector that is the same size as RJ45.

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Appl icat ions

Fiber-optic cable is often found in backbone networks

because its wide bandwidth is cost-effective. Today, withwavelength-division multiplexing (WDM), we can transferdata at a rate of 1600 Gbps.

Some cable TV companies use a combination of opticalfiber and coaxial cable, thus creating a hybrid network.Optical fiber provides the backbone structure whilecoaxial cable provides the connection to the userpremises. This is a cost-effective configuration since thenarrow bandwidth requirement at the user end does not

justify the use of optical fiber. Local-area networks such as 100Base-FX network (Fast

Ethernet) and 1000Base-X also use fiber-optic cable.

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Advantages and Disadvantages of Op tical Fiber

Advantages Fiber-optic cable has several advantages over metallic cable (twisted pairor coaxial).

Higher bandwidth. Fiber-optic cable can support dramatically higher bandwidths(and hence data rates) than either twisted-pair or coaxial cable. Currently, data ratesand bandwidth utilization over fiber-optic cable are limited not by the medium but bythe signal generation and reception technology available.

Less signal attenuation. Fiber-optic transmission distance is significantly greaterthan that of other guided media. A signal can run for 50 km without requiringregeneration. We need repeaters every 5 km for coaxial or twisted-pair cable.

Immunity to electromagnetic interference. Electromagnetic noise cannot affect fiber-optic cables.

Resistance to corrosive materials. Glass is more resistant to corrosive materialsthan copper.

Light weight. Fiber-optic cables are much lighter than copper cables.

Greater immunity to tapping. Fiber-optic cables are more immune to tapping thancopper cables. Copper cables create antenna effects that can easily be tapped.

Disadvantages There are some disadvantages in the use of optical fiber Installation and maintenance. Fiber-optic cable is a relatively new technology. Its

installation and maintenance require expertise that is not yet available everywhere.

Unidirectional light propagation. Propagation of light is unidirectional. If we needbidirectional communication, two fibers are needed.

Cost. The cable and the interfaces are relatively more expensive than those of otherguided media. If the demand for bandwidth is not high, often the use of optical fiber

cannot be justified.

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Text Book : Behrouz A Forouzan, Data Communications

and Networking,Tata McGraw Hill, New Delhi, Fourth

edition, 2007.

Page No : 191 to 203

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Guided Reading

From page no 193 to 195: Twisted Pair cable

From page no 195 to 198: Coaxial cable

From page no 198 to 202: Fiber-optic cable

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Mind Map

Transmission Media

Guided

Media

Unguided

Media

Fiber-opticCoaxial

Twisted Pair

Radio waves

Microwaves

Infrared waves

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Summary

Transmission media lie below the physical layer.

A guided medium provides a physical conduit from one device to another.

Twistedpair cable, coaxial cable, and optical fiber are the most popular types of

guided media.

Twisted-pair cable consists of two insulated copper wires twisted together.

Twistedpair cable is used for voice and data communications.

Coaxial cable consists of a central conductor and a shield. Coaxial cable can

carry signals of higher frequency ranges than twisted-pair cable. Coaxial cable

is used in cable TV networks and traditional Ethernet LANs.

Fiber-optic cables are composed of a glass or plastic inner core surrounded by

cladding, all encased in an outside jacket. Fiber-optic cables carry data signals

in the form of light. The signal is propagated along the inner core by reflection.

Fiberoptic transmission is becoming increasingly popular due to its noise

resistance, low attenuation, and high-bandwidth capabilities. Fiber-optic cable

is used in backbone networks, cable TV networks, and Fast Ethernet networks.

Mult iple Choice Questions

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Mult iple Choice Questions

1. Transmission media are usually categorized as _______.

A) fixed or unfixed

B) guided or unguided

C) determinate or indeterminate

D) metallic or nonmetallic

2. Transmission media lie below the _______ layer.

A) physical

B) network

C) transport

D) application

3. In a fiber-optic cable, the signal is propagated along the inner core by _______.

A) reflection

B) refraction

C) modulation

D) none of the above

4. __________ consists of a central conductor and a shield.

A) Coaxial

B) Fiber-opticC) Twisted-pair

D) none of the above

5. _______ cable consists of an inner copper core and a second conducting outersheath.

A) Twisted-pair

B) Coaxial

C) Fiber-opticD) Shielded twisted-pair

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Two mark Quest ions

1. Name the two major categories of transmission media.2. How do guided media differ from unguided media?

3. What are the three major classes of guided media?

4. What is the purpose of cladding in an optical fiber?

5. Name the advantages of optical fiber over twisted-pair and coaxial

cable.

Desc r ipt ive Quest ion

Describe the functionalities of guided transmission media.

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Learning outcomes

Students can able to

Explain various guided transmission system

Explain the construction of different types of cables

Identify the application of each Discuss the Advantages and Limitations of the different

types of cables