Chapter 16: Data Communication Fundamentals

Chapter 16:Data Communication

FundamentalsBusiness Data Communications, 5e

Business Data Communications, 5e

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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)– Allows use of digital telephony, voice mail


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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 for 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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Data

Signal

TransmissionSystem

A

DD

DAA

Data, Signal, and Transmission Matrix


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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• Fully 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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Modem

• An acronym for modulator-demodulator• Uses a constant-frequency signal known as

a carrier signal• Converts a series of binary voltage pulses

into an analog signal by modulating the carrier signal

• The receiving modem translates the analog signal back into digital data


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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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0 1 1

Differential Phase Shift Keying (DPSK)

0


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Voice Grade Modems

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.


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Cable Modems

• Permits Internet access over cable television networks. • ISP is at or linked by high-speed line to central cable

office • Cables used for television delivery can also be used to

deliver data between subscriber and central location• Upstream and downstream channels are shared among

multiple subscribers, time-division multiplexing technique (see Chapter 17)

• Splitter is used to direct TV signals to a TV and the data channel to a cable modem


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Cable Modem Layout


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Asymmetric DigitalSubscriber Line (ADSL)

• New modem technology for high-speed digital transmission over ordinary telephone wire.

• Telephone central office can provide support for a number of ISPs, • At central office, 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 (Chapter 17)


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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: If a signal is sampled at regular intervals of time and at a rate higher than twice the significant signal frequency, the samples contain all the information of the original signal.

• 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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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 voltage during a bit transmission is level


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Differential NRZ

• Differential version is NRZI (NRZ, invert on ones)

• 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


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Digital Encoding Illustration


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Digital Interfaces

• The point at which one device connects to another

• Standards define what signals are sent, and how

• Some standards also define physical connector to be used


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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

• Avoids timing problem by not sending long, uninterrupted streams of bits

• 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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Error Control Process

• All transmission media have potential for introduction of errors

• All data link layer protocols must provide method for controlling errors

• Error control process has two components– Error detection– Error correction


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Error Detection: Parity Bits

• Bit added to each character to make all bits add up to an even number (even parity) or odd number (odd parity)

• Good for detecting single-bit errors only• High overhead (one extra bit per 7-bit

character=12.5%)


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Error Detection: Cyclic Redundancy Check (CRC)

• Data in frame treated as a single binary number, divided by a unique prime binary, and remainder is attached to frame

• 17-bit divisor leaves 16-bit remainder, 33-bit divisor leaves 32-bit remainder

• For a CRC of length N, errors undetected are 2-N

• Overhead is low (1-3%)

Chapter 16: Data Communication Fundamentals

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