sami al-wakeel data transmission and computer networks the physical layer

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Data Transmission and Computer Networks

The Physical Layer

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Continuous Signal

No break or discontinuities in a signal. Example: Speech. A signal is continuous if:

aastsat

)()(lim

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Discrete Signal

A discrete signal takes on only a finite number of values.

Example: binary 1s and 0s.

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Definitions

Sine wave: where x(t) is the signal at time t,

A is the maximum amplitude of the signal,

f represents the number of cycles per second, and

defines the phase of the signal.

)π2sin()( ftAtx

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Definitions

Cosine wave: If the phase shift of a sine wave is –90 degrees (-/2

radian), the same signal can be expressed as a cosine wave instead of sine wave.

)2

ππ2sin()π2cos()( ftAftAtx

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Definitions

Amplitude:– The amplitude is the instantaneous value of a signal

at any time.

Frequency:– The Frequency is the inverse of the period, or the

number of repetitions of periods per second. Its unit is Hertz (Hz).

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Definitions

Phase:– The phase describes the position of the waveform

relative to time zero. The range of shift is within a single period of a signal.

– The phase is a measure in degree or radian (2 = 360o).

– The figure shows two signals that are out of phase by /2 radians.

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Definitions

Relationship between different phases:

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Definitions

Example 2.1:

The electricity that comes into a house is a simple sine wave. The maximum amplitude is approximately 127 volts and the frequency is 60 Hz. Write the mathematical equation.

Solution of Example 2.1:

)sin(377271)(φ)sin(2π)(

condradians/se 377607

222π2

ttxftAtx

f

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Definitions

Example 2.2:Your voice is a summation of sine waves, each sine wave having its own frequency, phase, and amplitude. The range of frequencies is normally between 300 and 3300 Hz. Give a general equation.


with 300 Hz < fi < 3300 Hz. f0 is called the fundamental frequency, and f2, f3 … fn are called the harmonics.

)π2sin()π2sin()π2sin()( 222101 nnn tfAtfAtfAtx

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Definitions

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Periodic Signal

A periodic signal is a signal that repeats itself at equal time interval.

It is made up of a finite series of sinusoidal frequency components.

A signal is periodic if and only if:

ttsTts )()(

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Periodic Signal

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Periodic Signal

The period (T) of the periodic signal determines the fundamental frequency component: reciprocal of the period in seconds yields the frequency in Hz.

The other components have frequencies which are multiples of the fundamental frequency component, and known as the harmonics.

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Periodic Signal

Mathematically, we can express any periodic waveform as follows:

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Periodic Signal

Fourier Analysis

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Basic Binary Signals

1- Unipolar Signal:– The amplitude of a unipolar signal varies between +V and 0 volts. – It is called Return-to-Zero (RZ) signal. – A unipolar signal has mean signal level of V/2 volts.

2- Bipolar Signal:– The amplitude of a bipolar signal varies between +V and -V volts. – It is called Non-Return-to-Zero (NRZ) signal.

– A bipolar signal has mean signal level of zero.

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The mathematical expressions for unipolar and bipolar signals are:

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Time Domain Representation of a Signal

))π(32sin(3

1)π2sin( 00 tftf

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Frequency Domain Representation of a Signal

))3(2sin(3

1)2sin()( 00 tftfts

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Time Domain and Frequency Domain

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The frequency spectrum of a signal is the collection of all component frequencies it contains and is shown using a frequency domain graph.

The bandwidth of a signal is the width of the frequency spectrum. To calculate the bandwidth, subtract the lowest frequency from the highest frequency of the range.

A digital signal contains an infinite number of frequencies with different amplitudes. However, if we send only those components whose amplitudes are significant (above an acceptable threshold), we can still recreate the digital signal with reasonable accuracy at the receiver.

Frequency Spectrum and Bandwidth

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Exact and Significant Spectrums

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Decomposition of a Digital Signal

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Significant Bandwidth

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Example 2.3:

If a periodic signal is decomposed into five sine waves with frequencies of 100, 300, 500, 700, and 900 Hz, what is the bandwidth?


Let fh be the highest frequency, fl be the lowest frequency, and B be the bandwidth. Then,

B = fh - fl = 900 – 100 = 800 Hz


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Example 2.4:

A signal has a bandwidth of 20 KHz. The highest frequency is 60 KHz. What is the lowest frequency?


Let fh be the highest frequency, fl be the lowest frequency, and B be the bandwidth. Then,

B = fh - fl fl = 60 – 20 = 40 KHz


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A transmission medium has a limited bandwidth, which means that it can transfer only some ranges of frequencies.

A transmission medium with particular bandwidth is capable of transmitting only digital signals, whose bandwidth is less than the bandwidth of the medium.

If the signal is sent on a transmission medium whose bandwidth is less than the required significant bandwidth, the signal may be so distorted that is not recognized at the receiver.

The channel capacity is the data rate, in bit per second (bps), at which data can be communicated.

Medium Bandwidth and Channel Capacity

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Example 2.5:

What bandwidth is required for data being sent at a rate of 10 bps?


In the worst-case scenario, the data consist of alternating 0s and 1s. This is the situation that will require the largest bandwidth. Each 1 and 0 bit combination can be considered one cycle. Therefore, the required bandwidth is 5 Hz.


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Example 2.6:

We want to transmit 10 pictures per second. Each picture is made by 5-by-5 pixels (picture elements). What is the required bandwidth?


Each picture is made of 25 pixels. We assume that we send one bit per pixel (1 for black, 0 for white). Therefore, we can send 25 bit per picture and 250 bit per second. Therefore, the required bandwidth is 125 Hz.


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Example 2.7:

A television screen composed of a grid of 525 lines by 700 columns (total of 367,500 pixels). A pixel can be black and white. Thirty complete frames are scanned in one scanned. What is the bandwidth required?


The number of bits that must be sent per second is 30 × 367,500 = 11,025,000 bps. If one bit is sent per pixel, then the required bandwidth is 5,513,500 Hz (6 MHz approximately).


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Attenuation and Distortion

Transmitted electrical signals are attenuated and distorted by the transmission medium.

The extent of attenuation and distortion is strongly influenced by:– The type of transmission medium. – The bit rate of the data being transmitted. – The distance between the two communicating devices.

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Attenuation and Distortion

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Attenuation And Distortion

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1- Attenuation

As a signal propagates a long a transmission line, its amplitude decreases. Therefore, a limit for the cable length must be set.

If the cable is longer, one or more repeaters (amplifiers) must be inserted.

We measure both attenuation and amplification (gain) in decibels (dB).

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1- Attenuation

If we denote transmitted signal power level by P1 and the received power by P2, then

dBP

PnAttenuatio

2

110log10

dBP

PionAmplificat

1

210log10

)( 21 PP

)( 21 PP

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1- Attenuation

Example 2.8:A transmission channel between two DTEs is made up of three section. The first introduces an attenuation of 16 dB, the second an amplification of 20 dB, and the third an attenuation of 10 dB. Assuming a mean transmitted power level of 400 mW, determine the mean output power of the channel.

16 dBAttenuation

20 dBAmplification

P2 10 dBAttenuation

P4P1 = 400 mW P3

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1- Attenuation

Solution of example 2.8:

First section:

2

110log10P

PnAttenuatio

210

400log1016

P

210

400log6.1

P

210

400log

6.1 1010 P

2

40081.39

P

mWP 0475.1081.39

4002

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1- Attenuation

Solution of example 2.8 (Continued):

Second section:

2

310log10P

PionAmplificat

0475.10log1020 3

10

P

0475.10log

23

10

1010P

mWP 75.10041000475.103

0475.10log2 3

10

P

0475.10100 3P

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1- Attenuation


Third section:

4

310log10P

PnAttenuatio

410

75.1004log1010

P

410

75.1004log1

P

410

75.1004log

1 1010 P

4

75.100410

P

mWP 475.10010

75.10044

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1- Attenuation


Or, Overall attenuation channel = (16 - 20) + 10 = 6 dB

4

110log10P

PnAttenuatio

410

400log106

P

410

400log

6.0 1010 P

4

400981.3

P

mWP 475.100981.3

4004

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2- Limited Bandwidth

Channel Bandwidth specifies the sinusoidal frequency components from 0 up to some frequency fc that will be transmitted by the channel undiminished. All frequencies above this cutoff frequency are strongly attenuated.

In general, channel bandwidth refers to the width of the range of frequencies that channel can transmit, and not the frequency themselves.

If the lowest frequency a channel can transmit is f1 and the highest is f2, the the bandwidth is: f2 – f1.

Because the telephone line can transmit frequencies from approximately 300 to 3300 Hz, its bandwidth is 3 KHz.

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LIMITED BANDWITH

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The sequence 101010… generates the highest-frequency components, while a sequence of all 1s or all 0s is equivalent to a zero frequency of the appropriate amplitude.

The channel capacity is the data rate, in bit per second (bps), at which data can be communicated.

In 1928, Nyquest developed the relationship between bandwidth (W) and the channel capacity (C) in noise-free environment. The Nyquest relationship is:

WC 2

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Example 2.9:

A binary signal of rate 500 bps is to be transmitted over a communication channel. Derive the minimum bandwidth required assuming:

(a) The fundamental frequency only,

(b) The fundamental and third harmonic, and

(c) The fundamental, third, and fifth harmonic of the worst-case sequence are to be received.

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Solution of Example 2.9:The worst case sequence 101010… at 500 bps has a fundamental frequency component of 250 Hz. Hence the third harmonic is 750 Hz and the fifth harmonic is 1250Hz.

The bandwidth required in each case is as follows:

(a) 0-250 Hz.

(b) 0-750 Hz.

(c) 0-1250 Hz.

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We can transmit more than one bit with each change in the signal amplitude, therefore increasing the data bit rate.

With multilevel signaling in noise-free environment, the Nyquest formulation becomes:

Where C is the channel capacity in bps.

W is the bandwidth of the channel in Hz.

M is the number of levels per signaling elements.

MWC 2log2

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For Limited-bandwidth channel such as PSTN, we can often use more than two levels. This means that each signal element can represent more than a single binary digit.

In general, if the number of signal levels is M, the number of bits per signal element m, is given by:

The rate of change of signal is known as the signaling rate (Baud rate) (Rs), and measures in baud.

Mm 2log

WRs 2

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It is related to the data bit rate, R, by the following expression:

The signaling element time period, Ts, is given

by:

The time duration of each bit, Tb, is:

mRR s

ss R

T1

RTb

1

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sRRmM 12

sRRmM 224

sRRmM 338

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The bandwidth efficiency of transmission channel, B, is defined as:

mW

RB 2

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Example 2.10:

Data is to be transmitted over the PSTN using a transmission scheme with eight levels per signaling element. If the bandwidth of the PSTN is 3000 Hz, determine the Nyquest maximum data transfer rate (C) and the bandwidth efficiency (B).


MWC 2log2bpsC 18000330002

mB 2HzbpsB /632

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3- Delay Distortion

The rate of propagation of a sinusoidal signal along a transmission line varies with the frequency of the signal.

When we transmit a digital signal with various frequency components, making up the signal, arrive at the receiver with varying delays, resulting in delay distortion of the received signal.

Note that:

f

v

fv

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3- Delay Distortion

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4- Noise

There are several sources for the channel noise:

I. Crosstalk:

It is caused by unwanted electrical coupling between adjacent lines. This coupling results in a signal that is being transmitted in one line being picked up by adjacent lines as a small noise signal.

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4- Noise

II. Impulse Noise: Impulse noise is a sharp spike of energy for a

small time duration.

Example: A lightning discharge.

If the duration of impulse noise is 0.01 second, it will destroy 48 bits of data being transmitted at 4800 bps.

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4- Noise

III. Thermal Noise: At all temperatures above absolute zero, all transmission media

experience thermal noise, where absolute zero = 0 kelvin (K) = - 273 ْ C.

The amount of thermal noise to be found in a bandwidth of 1 Hz in any conductor is:

where No is the noise power density for one Hz (watts/Hz),

k is Boltzmann’s constant (1.3803 x 10-23 joule K-1), and

T is the temperature in Kelvin (K). The thermal noise in watts present in a bandwidth of W Hz can be

expressed by:

kTNo

oNWN

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4- Noise

There are random perturbations on a transmission line even no signal is being transmitted.

The Signal-to-Noise Ratio (SNR) is expressed in decibels as:

where S is the average power in a received signal, and N is noise power. High SNR means a high power signal relative to the

prevailing noise level, resulting in a good-quality signal.

dBN

SSNR )(log10 10

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4- Noise

In 1948, Shannon calculated the theoretical maximum bit rate capacity of a channel of bandwidth W as

where C is the channel capacity in bps, W is the bandwidth of the channel in Hz, S is the average signal power in watts, and N is the thermal noise power in watts.

Note that:

)1(log2 N

SWC

2ln

lnlog2

xx

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4- Noise

Example 2.11:

Assuming that a PSTN has a bandwidth of 3000 Hz and a signal-to-noise ratio of 20 dB, determine the maximum theoretical data rate that can be achieved.


)(log10 10 N

SSNR )(log1020 10 N

S 100102

N

S

)1(log2 N

SWC

)1001(log3000 2 C bpsC 199632ln

101ln3000

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Transmission Media

The transmission medium is the physical path between transmitter and receiver in data transmission system.

The transmission media may be classified as guided or unguided media.

With guided media, the waves are guided along the physical path; example of guided media are twisted pair, coaxial cable, and optical fiber.

Unguided media provide a means for transmitting electromagnetic waves, but do not guide them; example propagation through air, vacuum and seawater.

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Transmission Media

For unguided media, at lower frequencies, signals are omnidirectional; that is, the signal propagates in all directions from the antenna. At higher frequencies, it is possible to focus the signal into a directional beam.

Three most important unguided transmission techniques: terrestrial an satellite microwave, and radio.

Microwave frequencies cover a range of about 2 to 40 GHz. At these frequencies, higher directional beam are possible.

Signals in range 30 MHz to1 GHz are radio waves. Omnidirectional transmission is used and signals at these frequencies are suitable for broadcast operations.

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Transmission Media

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Transmission Media

1. Guided Media

I. Two-Wire Open Lines: It is adequate for connecting equipment that is up to 50 meters apart. Bit rate is less than 19.2 kbps. Problems of Two-wire open Lines:

– Crosstalk: It is caused by cross-coupling of electrical signals between adjacent wires in the same cable.

– It can pick-up noise signals from other electrical sources caused by electromagnetic radiation.

These problems contribute to the limited length of line and bit rates.

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Transmission Media

I. Two-Wire Open Lines (Continued):

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Transmission Media

II. Twisted Pair: The most common transmission media for both analog and digital

data is twisted pair. A twisted pair consists of two insulated copper wires arranged in a

regular spiral pattern. The proximity the signal and ground reference wires means that

any interference signal is picked up by both wires reducing its effect on the difference signal.

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The UTP has several categories. These categories define the quality level of the cable. The categories are:

– Category 1: This is a common telephone cable.

– Category 2: CAT2 cable was first networking UTP but is now considered obsolete. It supports data rate up to 1Mbps.

– Category 3: CAT3 is seldom used outside an IBM environment and has been – replaced by CAT5. CAT3 is used in 10Mbps Ethernet LANs and 4Mbps Token

Ring LANs. - Category 4: CAT4 has been specified for up to 20Mbps and is specific for 16Mbps Token Ring LANs.

– Category 5: CAT5 has been specified for data rates up to 100Mbps. This is the preferred cabling installation for Ethernet and Token Ring.

– Category 6: CAT6 has been specified for data rates up to 1000Mbps.

Unshielded Twisted Pair (UTP):

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Transmission Media

II. Twisted Pair (Continued): EIA/TIA-568A/B compliant refers to which of the four pairs in the UTP

cable are designated as transmit, and which are designated as receive. Use the following as a guide:

– EIA/TAI-569A: Devices transmit over pair 3, and receive over pair 2.

– EIA/TAI-569B: Devices transmit over pair 2, and receive over pair 3.

It is important to terminate all cables at a location to the same standard, but not both at the same facility.

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Transmission Media

III. Coaxial Cable:

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Transmission Media

III. Coaxial Cable (Continued): Networking coaxial cable has a maximum data rate of 10Mbps,

and fair noise immunity. However, it is more expensive to install than twisted wire.

Cable TV networks can utilize coaxial cable with a 400MHz (or higher) bandwidth. Therefore, 52 TV channels can be carried on a single cable.

The disadvantages to the coaxial cable is the cost and lack of compatibility with twisted wires.

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Transmission Media

III. Coaxial Cable (Continued): The usable bandwidth of a coaxial cable can be as much as 350

MHz (or higher). We can utilize the high bandwidth in one of the two ways:

– Baseband mode, in which all the available bandwidth is used to derive a single high bit rate transmission channel.

– Broadband mode, in which the available bandwidth of the cable is divided into a number of smaller subfrequency bands or channels.

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Transmission Media

III. Coaxial Cable (Continued):

Types of the coaxial cables:

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Transmission Media


a. Thicknet (10BASE5): The diameter of Thicknet is 0.5 inch with a maximum segment

length 500 meter. It supports 100 transceivers on each segment. The number of

connections is limited to prevent signal attenuation. The transceiver is a device to send and receive data to and from

the cable. The minimum spacing of transceiver taps is 2.5 meters.

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Transmission Media


b. Thinnet (10BASE2): The diameter of Thinnet is 0.25 inch with a maximum

segment length 185 meter. It supports 30 transceivers on each segment. The minimum spacing of transceiver taps is no

closer than 0.5 meters. To connect a transceiver, the cable is cut and the

ends prepared for BNC connectors.

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Transmission Media

IV. Optical Fiber

An optical fiber is a thin (2 to 125 m), flexible medium capable of conducting an optical ray.

Various glasses and plastics can be used to make optical fibers.

An optical fiber cable has a cylindrical shape and consists of three concentric sections: the core, the cladding, and the jacket.

The core is the innermost section and consists of one or more very thin fibers made of glass or plastic.

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Transmission Media

IV. Optical Fiber (Continued):

Each fiber is surrounded by its own cladding, a glass or plastic coating that has optical properties different from those of the core.

The outermost layer, surrounding one or a bundle of cladded fibers, is the jacket. The jacket is composed of plastic and other materials to protect against moisture, abrasion, and crushing

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Transmission Media


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Transmission Media


Light: Light is a form of electromagnetic energy. The speed of light depends on the density of the medium which it is

travelling (the higher the density, the slower the speed). It travels at it fastest in a vacuum: 3×108 m/s. The speed decreases

as the medium becomes denser.

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Transmission Media


Advantages of Optical Fibers: Great bandwidth. Smaller size and lighter weight. Lower attenuation. Electromagnetic isolation:

– Fiber-optic uses light for transmission rather that electricity. – External light is the only possible interference and can be blocked by

the outer jacket.– The system is not vulnerable to interference, impulse noise, or

crosstalk. Greater repeater spacing.

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Transmission Media


Disadvantages of Optical Fibers: Cost. Installation/maintenance:

– All connections must be perfectly aligned and matched the core size. – The connection must be complete and not overly tight. A gap between

two cores results in a dissipated signal, and overly tight connection can compress the two core and alter the angle of refraction.

Fragility: Glass fiber is more easily broken than metallic wire.

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Transmission Media

2. Unguided Media:

Unguided media transport electromagnetic waves without using a physical conductor.

Signals are broadcast through air.

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Transmission Media

2. Unguided Media:Radio Frequency Allocation The electromagnetic spectrum is divided into eight

ranges, called bands, each regulated by governmental authorities.

These bands are rated from very low frequency (VLF) to extremely high frequency (EHF).

Radio Communication Radio, microwave, satellite

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Transmission Media

2. Unguided Media:

Radio Frequency Allocation:

Radio Communication Radio, microwave, satellite

3 KHz 300 GHz

VLF LF MF HF VHF UHF SHF EHF

3 KHz

30 KHz 300 KHz

3 MHz

30 MHz 300 MHz 3 GHz 30 GHz 300 GHz

Surface Troposphere IonosphereSpace and

line-of-sightSpace

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Transmission Media

2. Unguided Media:Radio Frequency Allocation:

VLF Very Low Frequency

LF Low Frequency

MF Middle Frequency

HF High Frequency

VHF Very High Frequency

UHF Ultra High Frequency

SHF Super High Frequency

EHF Extremely High Frequency

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Transmission Media

2. Unguided Media:

Propagation of Specific Signals:

I. Very Low Frequency (VLF): VLF waves are propagated as surface waves. Subject to high levels of of atmospheric noise (heat and electricity). Used for long-range radio navigation and for submarine

communications.II. Low Frequency (LF): LF waves are also propagated as surface waves. Used for long-range radio navigation and for radio bacons or

navigational locators.

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Transmission Media

2. Unguided Media:Propagation of Specific Signals:

III. Middle Frequency (MF): MF signals are propagated in the troposphere. These frequencies are absorbed by the ionosphere. The distance they can cover limited by the angle needed to reflect

the signal within the troposphere without entering the ionosphere. Absorption increases during the daytime. Used for AM radio and emergency frequencies.

AM Radio300 KHz 3 MHz

535 KHz 1.602 MHz

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Transmission Media

2. Unguided Media:


IV. High Frequency (HF): HF signals use ionospheric propagation. These frequencies moves into the ionosphere, which reflects them

back to the earth. Used for international broadcasting, military communication,

telephone, facsimile.

3 MHz 30 MHz

Frequency range for HF

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Transmission Media

2. Unguided Media:


V. Very High Frequency (VHF): VHF signals use line-of-sight propagation.

Used for VHF television, FM radio, and aircraft AM radio.

30 MHz 300 MHz

Frequency range for VHF

TV

Channels 2-6

FM Aircraft TV

Channels 7-13

54

88

108

174

216

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Transmission Media

2. Unguided Media:


VI. Ultra High Frequency (UHF): VHF signals always use line-of-sight propagation. Used for UHF television, mobile telephone, and microwave. Note that microwave communication begins at 1 GHz in the UHF

band.

300 MHz

3 GHz

Frequency range for UHF

UHF TV Microwave

Channels 14-69

1 GHz470 MHz 806 MHz

Mobile telephone

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Transmission Media

2. Unguided Media:


VII. Super High Frequency (SHF): VHF signals use line-of-sight and space propagation. Used for terrestrial and satellite microwave, and radar

communications.

3 GHz 30 GHz

Frequency range for SHF

Microwave

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Transmission Media

2. Unguided Media:


VII. Extremely High Frequency (EHF): EHF signals use space propagation. Used for radar, satellite and experimental communications.

30 GHz 300 GHz

Frequency range for EHF

Microwave

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Transmission Media

2. Unguided Media:

I. Satellites: Data can be transmitted using electromagnetic waves through the

free space. A satellite receives and retransmits (relays) the data to the

predetermined destinations. A typical satellite channel has high bandwidth (500 MHz) and can

provide many hundreds of high bit rate data links using multiplexing technique.

Multiplexing: Total channel capacity is divided into a number of subchannels, each can support high bit rate link.

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Transmission Media

I. Satellites (Continued): Satellites are geostationary, which means that the satellite

orbits the earth once every 24 hours in synchronism with earth’s rotation.

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Transmission Media

I. Satellites (Continued):

Satellite Transmission Modes:

1. Point-to-Point Link:

Long distance

telephone transmission.

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Transmission Media


2. Broadcast Link:

Television distribution.

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Transmission Media

I. Satellites (Continued):3. Multipoints using Very Small Aperture Terminals (VSAT):

– VSATs is used for private business networks.– The concept of VSAT is:

Typically, a computer is connected to each VSAT and can communicate with the central computer connected to the hub.

Normally, the central site broadcasts to all VSATs on a single frequency, while in the reverse each VSAT transmit at a different frequency.

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Transmission Media


To communicate with a particular VSAT, the central site broadcasts the message with the identity of the intended VSAT at the head of the message.

For VSAT-to-VSAT communication, all messages are first sent to the central site – via the satellite- which then broadcasts them to the intended recipients.

Note that direct VSAT-to-VSAT is possible without passing through a central site.

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Transmission Media

I. Satellites:

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Transmission Media

I. Satellites:

Frequency Bands for Satellite Communications:

Each satellite sends and receives over two different bands. Transmission from the earth to the satellite is called uplink. Transmission from the satellite to the earth is called downlink.

Band Downlink Uplink

C 3.7 - 4.2 GHz5.925 – 6.425

GHz

Ku11.7 - 12.2

GHz14 – 14.5 GHz

Ka17.7 – 21.0

GHz27.5 – 31 GHz

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Transmission Media

II. Terrestrial Microwave:

Terrestrial microwave links are widely used to provide communication links when it is impractical or too expensive to install physical transmission media, for example across a river or desert.

The distance coverable by a line-of-sight signal depends on the height if the antenna: the taller the antenna, the longer sight distance.

Height allows the signal to travel further without being stopped by the curvature of the planet.

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Transmission Media


Microwave signal propagate in one direction at a time, which means that two frequencies are necessary for two-way communication such as telephone conversation.

Transceiver is an single piece equipment, which allows a single antenna to transmit and receive frequencies.

The common type of microwave antenna is the “dish”. A typical size is about 10 ft in diameter. The antenna focuses a narrow beam to achieve line-of-sight

transmission to the receiving antenna.

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Transmission Media

II. Terrestrial Microwave (Continued): Microwave antenna are usually located at substantial heights above

ground to extend the range between antennas and to be able to transmit over intervening obstacles.

The maximum distance between antennas:

where d is the distance between antennas in kilometers, h is the antenna height in meters, and K is the adjustment factor.

Khd 14.7

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Transmission Media


To achieve long-distance transmission, a series of relay microwave towers (repeaters) is used.

Microwave communication through the earth’s atmosphere can be used reliably over distances in excess of 50 km.

Microwave beam travels through the earth’s atmosphere, therefore, it can be disturbed by the weather conditions.

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Transmission Media

III. Cellular Telephony:

Cellular telephony is designed to provide connections between twp moving devices or between one mobile unit and one land unit.

A service provider must locate and track the caller, assign channel to the call, and transfer the signal from channel to channel as the caller moves out of the range of one channel and into the range of another.

Each cellular service area is divided into small regions called cells. Each cell contains an antenna and is controlled by a switching office called a mobile telephone switching office (MTSO).

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Transmission Media


The MTSO coordinates communications between all the cell offices and the telephone central office. It is computerized and responsible for connecting cells, recording call information, and billing.

The typical radius of a cell is 1 to 12 miles.

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Transmission Media


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Transmission Media


Cellular bands: It is assigned two bands for cellular use. The band between 824 and 849 MHz carries

communications that initiate from mobile phones. The band between 869 and 894 MHz carries

communications that initiate from land phones. Carriers are spaced every 30 KHz, allowing each band to

support up to 833 carriers. For full-duplex communication, the required width for each

channel is 60 KHz and leaves only 416 channels per band.

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Transmission Media


Cellular bands:

416 channels 416 channels

824 MHz 849 MHz 869 MHz 894 MHz

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Transmission Media

III. Cellular Telephony:Transmitting:

To place a call from a mobile phone, the caller enters a code of 9 digits and pressed a send buttons.

The mobile phone scans the band, seeking a setup channel. It sends the phone number to the closest cell office using that

channel. The cell office relays the data to the MTSO. The MTSO sends the data to the telephone central office. IF the

called party is available, a connection is made and the result is relayed to the MTSO.

The MTSO assigns an unused voice channel to the call and a connection is established.

The mobile phone automatically adjust its tuning to the new channel an voice communication can begin.

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Transmission Media


Receiving:

When a land phone places a call to a mobile phone, the telephone office sends the number to the MTSO.

The MTSO searches for the location of the mobile phone by sending query signals to each cell in a process called paging.

One the mobile phone is found, the MTSO transmits a ringing signal.

When the mobile phone is answered, assigns a voice channel to the call, allowing voice communication to begin.

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Transmission Media


Handoff:

During the conversation, the mobile phone may move from one cell to another. When it does, the signal may become weak.

To solve this problem, the MTSO, monitors the level of the signal every few seconds.

If the strength of the signal is diminished, the MTSO seeks a new cell that can accommodate the communication better.

The MTSO changes the channel carrying the call (hands the signal off from the old channel to the new one).

sami al-wakeel data transmission and computer networks the physical layer

Documents

sami alwakeel discrete

unipolar signal

bipolar signal

nrz signal

signal level of v2 volts

physical layer slide

periodic waveform

simple sine wave