Chapter 16 data communication fundamentals
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Chapter 16: Data Communication Fundamentals. Business Data Communications, 6e. Data Communication Components. Data Analog: Continuous value data (sound, light, temperature) Digital: Discrete value (text, integers, symbols) Signal Analog: Continuously varying electromagnetic wave

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Chapter 16: Data Communication Fundamentals

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Chapter 16 data communication fundamentals

Chapter 16:Data Communication Fundamentals

Business Data Communications, 6e


Data communication components

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


Analog data signal options

Analog DataSignal Options

  • Analog data to analog signal

    • Inexpensive, easy conversion (e.g., 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


Digital data s ignal options

Digital DataSignal 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


Analog and digital signaling

Analog and Digital Signaling


Transmission choices

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


Analog and digital data and signals

Analog and Digital Data and Signals


Analog and digital treatment of signals

Analog and Digital Treatment of Signals


Advantages of digital transmission

Advantages of Digital Transmission

  • Cost – large scale and very large scale integration has caused continuing drop in cost

  • Data Integrity – effect of noise and other impairments is reduced

  • Capacity Utilization – high capacity is more easily and cheaply achieved with time division rather than frequency division

  • Security & Privacy – Encryption possible

  • Integration – All signals (Voice. Video, image, data) treated the same


Analog encoding of digital data

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


Modem

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


Methods of modulation

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)


Modulation of analog signals for digital data

Modulation of Analog Signals for Digital Data


Amplitude shift keying ask

Business Data Communications, 5e

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


Ask illustration

ASK Illustration

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Frequency shift keying fsk

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


Fsk illustration

FSK Illustration

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Phase shift keying psk

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)


Psk illustration

PSK Illustration

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Voice grade modems

Voice Grade Modems

  • Designed for digital transmission over ordinary phone lines

  • Uses 4-kHz bandwidth

  • Adheres to ITU-T standards


Cable modems

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

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


Cable modems1

Cable Modems


Asymmetric digital subscriber line adsl

Asymmetric DigitalSubscriber Line (ADSL)

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

  • 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.


Adsl modem application

ADSL Modem Application


Digital encoding of analog data

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


Pulse code modulation example

Pulse-Code Modulation Example


Digital encoding of digital data

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


Differential nrz

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


Problems with nrz

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


Biphase alternatives to nrz

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; maximum modulation rate is twice NRZ

  • Advantages

    • Synchronization due to predictable transitions

    • Error detection based on absence of a transition


Manchester code

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 and other LANs


Differential manchester

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


Digital signal encoding schemes

Digital Signal Encoding Schemes


Analog encoding of analog information

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


Analog sine wave signals

Analog Sine-Wave Signals


Asynchronous transmission

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


Asynchronous transmission1

Asynchronous Transmission


Synchronous transmission

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


Synchronous transmission1

Synchronous Transmission

  • More efficient than asynchronous transmission

  • Preamble, postamble and control information are typically < 100 bits

  • Introduces the need for error checking


Error control process

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: redundancy introduced so that the occurrence of an error will be detected

    • Error correction: receiver and transmitter cooperate to retransmit frames that were in error


Error detection parity bits

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%)

  • Noise impulses are often long enough to destroy more than one bit


Error detection cyclic redundancy check crc

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%)


Error detection process

Error Detection Process


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