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بنام خداوند جان وخردمهندسی اینترنت

ضوابط کالس :به طور جدی 16/4 قانون حضور مرتب درکالس-- 1

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بود لذا تقاضا ممنوع. های ارائه شده در کالس ویا :PPمرجع درس - 4مرجع کتابی درس:- 5

•Data and Computer Communicatinnos, WilliamStallings, 7th edition, Prentice Hall.

Hrh. PhD. UMIST_ 85/86

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What will you be learning?◆ What is internet and The Internet?

◆ What is the infrastructure of the Internet?◆ Information processing and data transmission in the Internet.

◆ The Internet transmission medium.◆ The Internet transmission technoques.

◆ The Internet routing and protocoles. ◆The routing algorithms.

◆ The basics of DNS (Domain Name Service).◆ The basics of WEB Servers.

◆ Advanced materials on the future of the Internet.

نخست آغاز –درس Introductionسر

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The Internet A Hierarchy of Networks

Every computer that is connected to the Internet is part of a network. For example, you may use a modem and dial a local number to connect to an ISP (Internet service provider).

At work, you may be part of a LAN, but you most likely still connect to the Internet using an ISP that your company has contracted with. When you connect to an ISP, you become part of their network. The ISP may then connect to a larger network and become part of their network. The Internet is simply a network of networks.

Most large communications companies (ICTs) have their own dedicated backbones connecting various regions. In each regionional ICT, is called a POP. The POP is a place for local users to access the ICT's network, often through a local phone number or dedicated line.

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Star LAN

A review of computer networks.

IEEE 802.3 or 802.5 or 802.11 protocoles.

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BUS LAN

CSMA/CD_(802.3) or CSMA/CA or TCP/IP.

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RING LAN

IEEE 802.5_Token ring.

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Internetworks

Fast Ethernets or Internetworks

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The Internet Infrastructure

ICT utilities

Dedicated Fast Speed Digital lines.

High Capacity Optical Networks i.e: SONET/ SDH…etc.

Optical Carriers OCs

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Information processing and data transmission in the Internet.Analog and Digital Data Transmission

•Data—Entities that convey meaning

•Signals—Electric or electromagnetic representations of data

•Transmission—Communication of data by propagation and processing of

signalsAnalog and Digital Data

•Analog—Continuous values within some interval

—e.g. sound, video •Digital

—Discrete values—e.g. text, integers

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Analog and Digital Signals •Means by which data are propagated

•Analog—Continuously variable

—Various media •wire, fiber optic, space

—Speech bandwidth 100Hz to 7kHz—Telephone bandwidth 300Hz to 3400Hz

—Video bandwidth 4MHz •Digital

—Use two DC componentsAdvantages & Disadvantages of Digital

•Cheaper •Less susceptible to noise

•Greater attenuation—Pulses become rounded and smaller

—Leads to loss of information

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Components of Speech •Frequency range (of hearing) 20Hz-20kHz

—Speech 100Hz-7kHz •Easily converted into electromagnetic signal for

transmission •Sound frequencies with varying volume

converted into electromagnetic frequencies withvarying voltage

•Limit frequency range for voice channel—300-3400Hz

Video Components Let us calculate a required Bandwidth for a TV picture:

•TV Picture Information - 525 lines scanned per frame at frame rate of 30 frames per second.

—525 lines but 42 lines lost during vertical retrace therefore usable lines is 483 per frame.

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•So 525 lines x 30 scans = 15750 lines per second —63.5µs per line, (11µs for retrace, so 52.5 µs per video line)

•Max frequency if line alternates black and white •Horizontal resolution is about 450 lines giving 225 cycles of wave in

52.5 µs •Therefore Max frequency of a TV line is 225/52.5 = 4.2MHz

Binary Digital Data •From computer terminals etc.

•Two dc components •Bandwidth depends on data rate

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Data and Signals

•Usually use digital signals for digital data andanalog signals for analog data

•Can use analog signal to carry digital data—Modem

•Can use digital signal to carry analog data—Compact Disc audio

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Analog Transmission •Analog signal transmitted without regard to

content •May be analog or digital data

•Attenuated over distance •Use amplifiers to boost signal (overcome Attenuation) but:

•Also amplifies noiseDigital Transmission

•Concerned with content •Integrity endangered by noise, attenuation etc. To overcome the

problem use Repeaters. •Repeaters

—Repeater receives signal—Extracts bit pattern

—Retransmits—Attenuation is overcome

—Noise is not amplified

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Advantages of Digital Transmission

•Digital technology —Low cost LSI/VLSI technology

•Data integrity —Longer distances over lower quality lines

•Capacity utilization —High bandwidth links economical

—High degree of multiplexing easier with digital techniques •Security & Privacy

—Encryption •Integration

—Can treat analog and digital data similarly

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Transmission Impairments •Signal received may differ from signal

transmitted •Analog - degradation of signal quality

•Digital - bit errors •Caused by

—Attenuation and attenuation distortion—Delay distortion

—NoiseAttenuation

•Signal strength falls off with distance •Depends on medium

•Received signal strength:—must be enough to be detected

—must be sufficiently higher than noise to be received without error •Attenuation is an increasing function of frequency.

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Delay Distortion •Only in guided media

•Propagation velocity varies with frequency. Different frequency components of a signal are attenuated

differently, and travel at different speeds through guided media .This may lead to delay distortion

Noise •Additional signals inserted between transmitter and receiver

•Thermal—Due to thermal agitation of electrons

—Uniformly distributed—White noise

•Intermodulation—Signals that are the sum and difference of original frequencies

sharing a medium.

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Noise Continues •Crosstalk

—A signal from one line is picked up by another •Impulse

—Irregular pulses or spikes—e.g. External electromagnetic interference

—Short duration—High amplitude

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Channel Capacity •Data rate:

—In bits per second, bps (not Bps)—Rate at which data can be communicated

. Baud Rate:

# __of changes per second , in a given medium. __Data rate = baud rate * # of bits.

•Bandwidth:

—In cycles per second of Hertz, Hz.—Constrained by transmitter and medium.

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Nyquist Bandwidth •If rate of signal transmission is 2B then signal with frequencies

no greater than B are sufficient to carry signal rate, why?

—Assume we have a square wave of repeating 101010.If a positive pulse is a 1 and a negative pulse is 0, then each pulse lasts 1/2 T1 (T1 = 1/f1) and the data rate is 2f1 bits per second.

•If we limit the components to a maximum frequency (restrict the bandwidth) we need to make sure the signal is accurately represented.

•Based on the accuracy we require, the bandwidth can carry a particular data rate. The theoretical maximum communication limit is given by the Nyquist formula:

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C = 2Blog2M

C = 2*4*106log22= 8MHz

C = Medium capacity or data transfer rate in bps

B = bandwidth (in hertz)

M = number of possible signaling levelsE.g. If channell bandwidth is 4 MHz and information is in binary format then C is:

where:

Nyquist Bandwidth Continues:

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Signal Strength

•An important parameter in communication is thestrength of the signal transmitted. Even more important is the strength being received.

•As signal propagates it will be attenuated (decreased) •Amplifiers are inserted to increase signal strength

•Gains, losses and relative levels of signals are expressed in decibels.dB = 10 log x1/x2

–This is a logarithmic scale, but strength usually falls logarithmically

–Calculation of gains and losses involves simple addition and subtraction

•Decibel measure of difference in two power levels is.

Bell

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Physical Aspects

•Signal Attenuation and Distortion—As a signal propagates across a transmission medium its

amplitude decreases. This is known as signal attenuation.

—A typical signal consists of a composition of many frequency components (Fourier Analysis). Due to the limited transmission bandwidth of a medium, the higher frequency components may not be able to be transmitted.

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•Recall the Nyquist formula:

Shannon capacity—A transmission line may experience interference from a number of

locations, called noise. Noise is measured in terms of signal to noise power ratio, expressed in decibels:

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—The effects of noise on channel capacity can beseen using the Shannon-Hartley Law:

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Signal Delay—There exists a transmission propagation delay in any medium

•Speed of light and Electromagnetic waves in free space is : 3 x 10^8 m/s

•Speed of EM in cable/wire is half of what is in free space i.e: 1.5 x 10^8 m/s

—Important parameter is :1 .Round-trip-delay (time from first bit sent to last bit

acknowledged)2 .Propagation delay Tp and transmission delay Tx:

Tp = d / VTx = n / R

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d = distance in meters V = EM speed n = number of bits transmitted R = link bit rate in bits per second

e.g: An e-mail of 1kB is transmitted with the speed of 128 kbps to a distance of 1500 km what is the propagation delay Tp and transmission delay Tx? ( medium is free space).

n = 1k * 8 = 8k bit V = 3 * 10^8 m/s

Tp = d/V = 1500 * 10^3 m / 3 * 10^8 m/s = 5 * 10^-3 s or

Tp = 5 ms

Tx = n / R Tx = 8k bit / 128k bps

Tx = 0.0625 s or Tx= 62.5 ms

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Appendix

dB = 10 log X1/X2

dBw= 10log X1 /1W

dBm = 10log X1/1mW

---dB and dBw could be operated mathematically.

---dB and dBm The same.