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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

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
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
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
  • 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)
amplitude shift keying ask
Business Data Communications, 5eAmplitude 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
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
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)
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
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.
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
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
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
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
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%)