guided transmission media 2

8/8/2019 Guided Transmission Media 2

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1. INTRODUCTION

A transmission media are the material substance which can propagate signals. Guided

Transmission Media uses a "cabling" system that guides the data signals along a specific path.

The data signals are bound by the "cabling" system. Guided Media is also known as Bound

Media. Cabling is meant in a generic sense in the previous sentences and is not meant to beinterpreted as copper wire cabling only.

2. TWISTED PAIR CABLE

The wires in Twisted Pair cabling are twisted together in pairs. Each pair would consist of a wireused for the positive data signal and a wire used for the negative data signal. Any noise that

appears on 1 wire of the pair would occur on the other wire. Because the wires are opposite

polarities, they are 180 degrees out of phase. When the noise appears on both wires, it cancels or

nulls itself out at the receiving end. Twisted Pair cables are most effectively used in systems thatuse a balanced line method of transmission: polar line coding (Manchester Encoding) as opposed

to unipolar line coding (TTL logic).

Straight copper wires tend to act as antennas and pick up extraneous signal. The twisting helps

reduce the amount of interferences. Twisted pairs often are bundled together and wrapped in

protective coating. There are two types of twisted pair cable; Unshielded and Shielded twistedpair.

Cables without a shield are called Unshielded Twisted Pair or UTP. Twisting the wires together

results in a characteristic impedance for the cable. A typical impedance for UTP is 100 ohm for


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Coaxial Cable consists of 2 conductors. The inner conductor is held inside an insulator with the

other conductor woven around it providing a shield. An insulating protective coating called a

jacket covers the outer conductor.

The outer shield protects the inner conductor from outside electrical signals. The distance

between the outer conductor (shield) and inner conductor plus the type of material used for

insulating the inner conductor determine the cable properties or impedance. Typical impedancesfor coaxial cables are 75 ohms for Cable TV, 50 ohms for Ethernet Thinnet and Thicknet. The

excellent control of the impedance characteristics of the cable allow higher data rates to be

transferred than Twisted Pair cable.

There are two types of coax cable; Baseband and Broadband.

Baseband: In this, the frequency band occupied by a single or composite signal in its original or

unmodulated form. The cable id dedicated for only one channel.

Broadband: It uses high frequency transmission over medium, several streams of data can be

transmitted at different frequencies simultaneously.

Table 7.2 Categories of coaxial cables


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4. OPTICAL FIBER

Optical Fiber consists of thin glass fibers that can carry information at frequencies in the visible

light spectrum and beyond. The typical optical fiber consists of a very narrow strand of glass

called the Core. Around the Core is a concentric layer of glass called the Cladding. A typicalCore diameter is 62.5 microns (1 micron = 10-6 meters). Typically Cladding has a diameter of

125 microns. Coating the cladding is a protective coating consisting of plastic, it is called the

Jacket.


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If the angle of incidence is small, the light rays are reflected and do not pass into the water. If the

angle of incident is great, light passes through the media but is bent or refracted.

Optical Fibers work on the principle that the core refracts the light and the cladding reflects thelight. The core refracts the light and guides the light along its path. The cladding reflects any

light back into the core and stops light from escaping through it - it bounds the media!

Optical Transmission Modes:

a) Step Indexb) Grade Index

c) Single Mode

Step Index has a large core the light rays tend to bounce around, reflecting off the cladding,inside the core. This causes some rays to take a longer or shorted path through the core. Sometake the direct path with hardly any reflections while others bounce back and forth taking a

longer path. The result is that the light rays arrive at the receiver at different times. The signal

becomes longer than the original signal. LED light sources are used. Typical Core: 62.5 microns.


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Grade Index has a gradual change in the Core's Refractive Index. This causes the light rays to

be gradually bent back into the core path. This is represented by a curved reflective path in the

attached drawing. The result is a better receive signal than Step Index. LED light sources areused. Typical Core: 62.5 microns.

Both Step Index and Graded Index allow more than one light source to be used (different colourssimultaneously!). Multiple channels of data can be run simultaneously!

Single Mode has separate distinct Refractive Indexes for the cladding and core. The light ray

passes through the core with relatively few reflections off the cladding. Single Mode is used for asingle source of light (one colour) operation. It requires a laser and the core is very small: 9microns.

Table 7.3 Fiber types

Advantages of Optical Fiber:

Noise immunity: RFI and EMI immune (RFI - Radio Frequency Interference, EMI-ElectroMagnetic Interference)


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Security: cannot tap into cable.

Large Capacity due to BW (bandwidth)

No corrosion

Longer distances than copper wire

Smaller and lighter than copper wire

Faster transmission rate


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Disadvantages of Optical Fiber:

Physical vibration will show up as signal noise!

Limited physical arc of cable. Bend it too much & it will break!

Difficult to splice

The cost of optical fiber is a trade-off between capacity and cost. At higher transmission

capacity, it is cheaper than copper. At lower transmission capacity, it is more expensive.

5. CONNECTORS USED

Connector used with Twisted Pair Cable is RJ-45.

Coaxial Cables use BNC Connector.


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Fiber Optics cable use SC, ST, MTRJ Connectors etc.

6. TRANSMISSION MEDIA SELECTION CRITERIA

The selection of the most effective transmission system for a given application must be made in

the context of a number of key design considerations. Such considerations include general

transmission characteristics such as bandwidth and error performance, both of which affect

throughput. Additionally, you must consider the allowable distance between devices as well asissues of propagation delay, security, mechanical strength, physical dimensions, and speed of

deployment. Finally, and perhaps most importantly, consider local availability and cost,

including cost of acquisition, deployment, Operation and Maintenance (O & M), and upgrade orreplacement.

i. Transmission Characteristics

The basic transmission characteristics of a given medium are of primary importance. Those

characteristics include bandwidth, error performance, and distance between network elements.

These three dimensions of a transmission system, in combination, determine the effectivethroughput, or the amount of information you can put through, that is, send over, the system.


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Bandwidth, in this context, refers to the raw amount of bandwidth, or capacity, the medium

supports. Error performance refers to the number or percentage of errors introduced in the

process of transmission. Distance refers to the minimum and maximum spatial separationbetween devices over a single link, rather than over a complete, end - to - end circuit. Clearly, the

attractiveness of any given transmission system increases to the extent that you realize greater

available bandwidth, fewer errors, and a greater maximum distance between various networkelements such as amplifiers and repeaters.

ii. Propagation Delay

Propagation delay refers to the length of time required for a signal to travel from transmitter toreceiver across a transmission system. Factors impacting propagation delay include the distance

between transmitter and receiver and the density of the medium.

iii. Security

Security, in the context of transmission systems, addresses the protection of data from

interception as it transverses the network. Clearly, increasing amounts of sensitive data are beingtransmitted across public networks, well outside the range of physical protection on the users

premises. Therefore, security is of greater concern than ever before. The airwave systems (e.g.,

microwave and satellite) are inherently insecure, as access to the signal is easily accomplishedand virtually undetectable through an antenna properly tuned and in proximity to the signal path.

It is much more difficult to physically tap a wire line circuit. Also the digital systems offer much

greater security potential than analog systems by virtue of the fact that application software canquite effectively encrypt, or encode, the data to conceal its true meaning.

iv. Mechanical Strength

Mechanical strength applies especially to wired systems. Installers must physically manipulate

twisted pair, coaxial, and fiber optic cables while deploying and reconfiguring them. Clearly,

each type of wire, fiber, and cable has certain physical limits to the maximum severity of thebend it can tolerate (bend radius) without cracking or breaking and the amount of bending and

twisting (flex strength) it can tolerate. There also are limits to the amount of weight or

longitudinal stress a cable or wire can support (tensile strength) without suffering deformation orbreaking (break strength). Strength members improve the tensile strength of OutSide Plant (OSP)

aerial cables and inside riser cables. Fiber optic cables are used for improved strength and

increased protection of the fragile glass fiber from physical damage.

v. Physical Dimensions

The physical dimensions of a transmission system must be considered as well. Certainly, youmust consider the sheer weight of a cable system. The bulk (diameter) of the cable is important,


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as conduit and raceway space often is at a premium. The physical dimensions of airwave systems

are no less important, as the size and weight of a reflective microwave or satellite dish and

mounting system (e.g., bracket and tower) may require support, particularly in locations thatexperience high winds.

vi. Speed of Deployment

Speed can be of the essence at times. Wired connections take some time, even under the best of

circumstances and even using the most pliable wires and quickest connectors. Radio antennasmay take some time to install, but once they are in place, the time required to configure and

reconfigure the connections between them can take little time, if any. If fact, the antennas can

even establish and maintain connections while in motion. Portability and even mobility are key

wireless advantages. It is said that time is money, which leads us to consider cost.

vii. Cost

Ultimately, financial considerations rule, and media selection is no exception. Costconsiderations include acquisition, deployment, operation and maintenance (O & M), and

upgrade or replacement. Without getting involved in a lengthy discussion of each cost issue atthis point, it certainly is particularly worth pausing for a moment to compare the deployment

costs of wireline and wireless media.

Wired transmission systems require securing legal rights - of - way and digging trenches, boringtunnels, planting poles, placing conduits and manholes, pulling and splicing cables, placing

amplifiers or repeaters, and so on. Such costs, clearly, are not trivial. Wireless systems, on the

other hand, require securing rights - of - way, erecting towers, mounting antennas, securingspectrum licenses, and so on. While it is difficult to make hard - and - fast generalizations, the

deployment of wired systems certainly involves a set of cost issues that can be problematic.

Further, wired systems tend to be more susceptible to the forces of man (e.g., cable - seeking

backhoes, posthole diggers, and trains) and nature (e.g., earthquakes and floods). Whether caused

by man or nature, catastrophic failures add repair costs to the equation.

Twisted Pair Coaxial Optical Fiber

Cost Low Medium High

Speed Low Medium High

Distance Upto 1 mile

(1-2 Mbps for 1 mile

10 Mbps for 100 m)

2-3 miles 20-30 miles

Expandability Normal Normal Difficult

Security Yes Yes Yes


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Noise Immunity Low Medium-High High

Interference Electrical Electrical Immune

guided transmission media 2

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