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

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

    • 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 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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


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 Treatment of Signals


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

  • 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

  • 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

  • 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


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

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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


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)


PSK Illustration

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Voice Grade Modems

  • Designed for digital transmission over ordinary phone lines

  • Uses 4-kHz bandwidth

  • Adheres to ITU-T standards


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 Modems


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


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


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 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

  • 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

  • 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

  • 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

  • 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


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


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 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 Transmission

  • More efficient than asynchronous transmission

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

  • Introduces the need for error checking


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

  • 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)

  • 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


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