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ITC242 – Introduction to Data Communications

Week 9

Topic 14 Data TransmissionTopic 15 Data Communication

FundamentalsReading 3

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Last Week

Reading 2 – Wide Area and Large-Scale Networks

• Describe the basic concepts associated with wide area networks

• Identify the uses, benefits, and drawbacks of WAN technologies such as ATM, FDDI, SONET, SMDS

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Topic 14 – Data Transmission

Learning Objectives

• Describe the difference between analogue and digital signals

• Discuss the various transmission impairments that affect signal quality and transfer rate

• Use Shannon’s formula to calculate the capacity of a channel

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Electromagnetic Signals

• Analog Signal – signal intensity varies in a smooth fashion over

time. In other words, there are no breaks or discontinuities in the signal

• Digital Signal – signal intensity maintains a constant level for some

period of time and then changes to another constant level

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Analog Sine Wave

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Digital Square Wave

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Signal Characteristics

• Peak Amplitude (A)– Maximum signal value, measured in volts

• Frequency (f)– Repetition rate– Measured in cycles per second or Hertz (Hz)

• Period (T)– Amount of time it takes for one repetition, T=1/f

• Phase ()– Relative position in time, measured in degrees

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Frequency Domain Concepts

• Spectrum of a signal is the range of frequencies that it contains

• Absolute bandwidth of a signal is the width of the spectrum

• Effective bandwidth contained in a relatively narrow band of frequencies, where most of signal’s energy is found

• The greater the bandwidth, the higher the information-carrying capacity of the signal

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Bandwidth

• Width of the spectrum of frequencies that can be transmitted– if spectrum=300 to 3400Hz,

bandwidth=3100Hz

• Greater bandwidth leads to greater costs

• Limited bandwidth leads to distortion

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Voice/Audio Analog Signals

• Easily converted from sound frequencies (measured in loudness/db) to electromagnetic frequencies, measured in voltage

• Human voice has frequency components ranging from 20Hz to 20kHz

• For practical purposes, the telephone system has a narrower bandwidth than human voice, from 300 to 3400Hz

• Actual bandwidth used is 4kHz to isolate audio signal from adjacent bandwidths.

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Voice Bandwidth

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Digital Text Signals

• Transmission of electronic pulses representing the binary digits 1 and 0

• How do we represent letters, numbers, characters in binary form?

• Earliest example: Morse code (dots and dashes)

• Most common current forms: ASCII, UTF

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e.g. ASCII Character Set Control Characters

Binary Oct Dec Hex Abbr PR[1] CS[2] CEC[3] Description

0000 0000 000 0 00 NUL ␀ ^@ Null character

0000 0001 001 1 01 SOH ␁ ^A Start of Header

0000 0010 002 2 02 STX ␂ ^B Start of Text

0000 0011 003 3 03 ETX ␃ ^C End of Text

0000 0100 004 4 04 EOT ␄ ^D End of Transmission

0000 0101 005 5 05 ENQ ␅ ^E Enquiry

0000 0110 006 6 06 ACK ␆ ^F Acknowledgment

0000 0111 007 7 07 BEL ␇ ^G \a Bell

0000 1000 010 8 08 BS ␈ ^H \b Backspace[4][8]

0000 1001 011 9 09 HT ␉ ^I \t Horizontal Tab

0000 1010 012 10 0A LF ␊ ^J \n Line feed

0000 1011 013 11 0B VT ␋ ^K Vertical Tab

0000 1100 014 12 0C FF ␌ ^L \f Form feed

0000 1101 015 13 0D CR ␍ ^M \r Carriage return[7]

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e.g. ASCII Character Set Printable Characters

Binary Dec Hex Glyph

0100 0000 64 40 @

0100 0001 65 41 A

0100 0010 66 42 B

0100 0011 67 43 C

0100 0100 68 44 D

0100 0101 69 45 E

0100 0110 70 46 F

Binary Dec Hex Glyph

0110 0000 96 60 `

0110 0001 97 61 a

0110 0010 98 62 b

0110 0011 99 63 c

0110 0100 100 64 d

0110 0101 101 65 e

0110 0110 102 66 f

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Transmission Media

• Physical path between transmitter and receiver (“channel”)

• Design factors affecting data rate– bandwidth– physical environment– number of receivers– impairments

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Impairments and Capacity

• Impairments exist in all forms of data transmission

• Analog signal impairments result in random modifications that impair signal quality

• Digital signal impairments result in bit errors (1s and 0s transposed)

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Transmission Impairments:Guided Media

• Attenuation– loss of signal strength over distance

• Attenuation Distortion– different losses at different frequencies

• Delay Distortion– different speeds for different frequencies

• Noise– distortions of signal caused by interference

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Transmission Impairments:Unguided (Wireless) Media

• Free-Space Loss– Signals disperse with distance

• Atmospheric Absorption– Water vapor and oxygen contribute to signal loss

• Multipath– Obstacles reflect signal creating multiple copies

• Refraction• Thermal Noise

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Types of Noise

• Thermal (aka “white noise”)– Uniformly distributed, cannot be eliminated

• Intermodulation– When different frequencies collide (creating

“harmonics”)

• Crosstalk– Overlap of signals

• Impulse noise– Irregular spikes, less predictable

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Channel Capacity

• The rate at which data can be transmitted over a given path, under given conditions

• Four concepts– Data rate– Bandwidth– Noise– Error rate

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Shannon’s Equation

• The signal to noise ratio (SNR) sets the upper bound on the achievable data rate.

• Shannon’s equation provides a theoretical maximum channel capacity given SNR.

• C = B log2 (1 + SNR)

– B = Bandwidth of channel in Hertz – C= Channel capacity in bps– SNR = Signal-to-noise ratio

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Shannon’s Equation

• What is the capacity of a signal operating at 5kHz with a SNR of 3?C = B log2 (1 + SNR)

C = 5000 Hz x log2(1+3)

C = 5000 Hz x log24

C = 5000 Hz x 2

C = 10000 bps

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Review

• Describe the difference between analogue and digital signals

• Transmission impairments – attenuation and noise affect signal quality

• Shannon’s formula provides a theoretical estimate of maximum channel capacity

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Topic 15 – Data Communication Fundamentals

Learning Objectives• Explain the difference between analogue and

digital transmission• Describe digital and analogue encoding

techniques for the transmission of data• Explain the difference between asynchronous

and synchronous transmission and when each technique is used

• Describe the process of error detection

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Data Communication Components

• Data– Analog: Continuous value data (sound, light,

temperature)– Digital: Discrete value (text, integers, symbols)

• Signal– Analog: Continuously varying electromagnetic wave– Digital: Series of voltage pulses (square wave)

• Transmission– Analog: Works the same for analog or digital signals– Digital: Used only with digital signals

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Analog DataSignal Options• Analog data to analog

signal– Inexpensive, easy conversion

(eg telephone)– Data may be shifted to a

different part of the available spectrum (multiplexing)

– Used in traditional analog telephony

• Analog data to digital signal– Requires a codec

(encoder/decoder)– Provides benefits of digital

transmission (e.g Audio CD)

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Digital DataSignal Options

• Digital data to analog signal– Requires modem

(modulator/demodulator)– Allows use of PSTN to send

data– Necessary when analog

transmission is used

• Digital data to digital signal– Requires CSU/DSU (channel

service unit/data service unit)– Less expensive when large

amounts of data are involved– More reliable because no

conversion is involved

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Transmission Choices

• Analog transmission– only transmits analog signals, without regard to data

content– attenuation overcome with amplifiers– signal is not evaluated or regenerated

• Digital transmission– transmits analog or digital signals– uses repeaters rather than amplifiers– switching equipment evaluates and regenerates

signal

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Advantages of Digital Transmission

• The signal is exact

• Signals can be checked for errors

• Noise/interference are easily filtered out

• A variety of services can be offered over one line

• Higher bandwidth is possible with data compression

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Why Use Analog Transmission?

• Already in place

• Significantly less expensive

• Lower attenuation rates

• Sufficient for transmission of voice signals

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Analog Encoding of Digital Data

• Data encoding and decoding technique to represent data using the properties of analog waves

• Modulation: the conversion of digital signals to analog form

• Demodulation: the conversion of analog data signals back to digital form

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Methods of Modulation

• Amplitude modulation (AM) or amplitude shift keying (ASK)

• Frequency modulation (FM) or frequency shift keying (FSK)

• Phase modulation or phase shift keying (PSK)

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Amplitude Shift Keying (ASK)

• In radio transmission, known as amplitude modulation (AM)

• The amplitude (or height) of the sine wave varies to transmit the ones and zeros

• Major disadvantage is that telephone lines are very susceptible to variations in transmission quality that can affect amplitude

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1 0 0 1

ASK Illustration

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Frequency Shift Keying (FSK)

• In radio transmission, known as frequency modulation (FM)

• Frequency of the carrier wave varies in accordance with the signal to be sent

• Signal transmitted at constant amplitude• More resistant to noise than ASK• Less attractive because it requires more

analog bandwidth than ASK

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1 1 0 1

FSK Illustration

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Phase Shift Keying (PSK)

• Also known as phase modulation (PM)• Frequency and amplitude of the carrier

signal are kept constant• The carrier signal is shifted in phase

according to the input data stream• Each phase can have a constant value, or

value can be based on whether or not phase changes (differential keying)

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0 0 1 1

PSK Illustration

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Modulation Example

Asymmetric Digital Subscriber Line (ADSL)• Telephone exchange can provide support for a number of

ISPs,

• At the exchange a combined data/voice signal is transmitted over a subscriber line

• At subscriber’s site, twisted pair is split and routed to both a PC and a telephone– At the PC, an ADSL modem demodulates the data signal for the PC.

– At the telephone, a microfilter passes the 4-kHz voice signal.

• The data and voice signals are combined on the twisted pair line using frequency-division-multiplexing techniques

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DSL Modem Layout

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Digital Encoding of Analog Data

• Evolution of telecommunications networks to digital transmission and switching requires voice data in digital form

• Best-known technique for voice digitization is pulse-code modulation (PCM)

• The sampling theorem: Exact reconstruction of a continuous-time baseband signal from its samples is possible if the signal is bandlimited and the sampling frequency is greater than twice the signal bandwidth

• Good-quality voice transmission can be achieved with a data rate of 8 kbps

• Some videoconference products support data rates as low as 64 kbps

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Converting Samples to Bits

• Quantizing• Similar concept to pixelization• Breaks wave into pieces, assigns a value

in a particular range• 8-bit range allows for 256 possible sample

levels• More bits means greater detail, fewer bits

means less detail

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Converting Samples to Bits

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Codec

• Coder/Decoder• Converts analog signals into a digital form

and converts it back to analog signals• Where do we find codecs?

– Sound cards– Scanners– Voice mail– Video capture/conferencing

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Digital Encodingof Digital Data

• Most common, easiest method is different voltage levels for the two binary digits

• Typically, negative=1 and positive=0• Known as NRZ-L, or nonreturn-to-zero level,

because signal never returns to zero, and the value during a bit time is a level voltage

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Differential NRZ• Differential version is NRZI

(NRZ, invert on ones)• A bit time: a constant-

voltage pulse• A signal transition at the

beginning of the bit time: Change=1, no change=0

• Advantage of differential encoding is that it is more reliable to detect a change in polarity than it is to accurately detect a specific level

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Problems With NRZ

• Difficult to determine where one bit ends and the next begins

• In NRZ-L, long strings of ones and zeroes would appear as constant voltage pulses

• Timing is critical, because any drift results in lack of synchronization and incorrect bit values being transmitted

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Biphase Alternatives to NRZ

• Require at least one transition per bit time, and may even have two

• Modulation rate is greater, so bandwidth requirements are higher

• Advantages– Synchronization due to predictable transitions– Error detection based on absence of a

transition

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Manchester Code

• Transition in the middle of each bit period

• Transition provides clocking and data

• Low-to-high=1 , high-to-low=0

• Used in Ethernet

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Differential Manchester

• Midbit transition is only for clocking

• Transition at beginning of bit period=0

• Transition absent at beginning=1

• Has added advantage of differential encoding

• Used in token-ring LANs

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Digital Encoding Illustration

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Analog Encoding of Analog Information

• Voice-generated sound wave can be represented by an electromagnetic signal with the same frequency components, and transmitted on a voice-grade telephone line.

• Modulation can produce a new analog signal that conveys the same information but occupies a different frequency band– A higher frequency may be needed for effective

transmission– Analog-to-analog modulation permits frequency-division

multiplexing (Chapter 17)

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Asynchronous and Synchronous Transmission

• For receiver to sample incoming bits properly, it must know arrival time and duration of each bit that it receives

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Asynchronous Transmission• uses start and stop bits to signify the

beginning ait ASCII character

• 01000001010010000101011000011

• Data transmitted one character at a time, where each character is 5 to 8 bits in length.

• Timing or synchronization must only be maintained within each character; the receiver has the opportunity to resynchronize at the beginning of each new character.

• Simple and cheap but requires an overhead of 2 to 3 bits per character

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Synchronous Transmission• Block of bits transmitted in a steady stream without

start and stop codes. • Clocks of transmitter and receiver must somehow be

synchronized– Provide a separate clock line between transmitter and

receiver; works well over short distances, – Embed the clocking information in the data signal.

• Each block begins with a preamble bit pattern and generally ends with a postamble bit pattern

• The data plus preamble, postamble, and control information are called a frame

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Review

• The difference between analogue and digital transmission

• Digital and analogue encoding techniques:– ASK, FSK, PSK– NRZ-L, NRZI, Manchester, Differential

Manchester

• Asynchronous transmission

• Synchronous transmission