Chapter 16 data communication fundamentals
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Chapter 16: Data Communication Fundamentals. Business Data Communications, 5e. 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, 5e


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

Business Data Communications, 5e


Analog data signal options

Analog DataSignal 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

Business Data Communications, 5e


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

Business Data Communications, 5e


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

Business Data Communications, 5e


Data signal and transmission matrix

A

Data

D

D

Transmission

System

A

A

D

Signal

Data, Signal, and Transmission Matrix

Business Data Communications, 5e


Advantages of digital transmission

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

Business Data Communications, 5e


Why use analog transmission

Why Use Analog Transmission?

  • Already in place

  • Significantly less expensive

  • Lower attenuation rates

  • Fully sufficient for transmission of voice signals

Business Data Communications, 5e


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

Business Data Communications, 5e


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

Business Data Communications, 5e


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)

Business Data Communications, 5e


Amplitude shift keying ask

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

Business Data Communications, 5e


Ask illustration

ASK Illustration

1

0

0

1

Business Data Communications, 5e


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

Business Data Communications, 5e


Fsk illustration

FSK Illustration

1

1

0

1

Business Data Communications, 5e


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)

Business Data Communications, 5e


Psk illustration

PSK Illustration

0

0

1

1

Business Data Communications, 5e


Differential phase shift keying dpsk

Differential Phase Shift Keying (DPSK)

0

0

1

1

Business Data Communications, 5e


Voice grade modems

Voice Grade Modems

Business Data Communications, 5e


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 (see Chapter 17)

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

Business Data Communications, 5e


Cable modem layout

Cable Modem Layout

Business Data Communications, 5e


Asymmetric digital subscriber line adsl

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)

Business Data Communications, 5e


Dsl modem layout

DSL Modem Layout

Business Data Communications, 5e


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

Business Data Communications, 5e


Converting samples to bits

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

Business Data Communications, 5e


Codec

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

Business Data Communications, 5e


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

Business Data Communications, 5e


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

Business Data Communications, 5e


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

Business Data Communications, 5e


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

  • Advantages

    • Synchronization due to predictable transitions

    • Error detection based on absence of a transition

Business Data Communications, 5e


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

Business Data Communications, 5e


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

Business Data Communications, 5e


Digital encoding illustration

Digital Encoding Illustration

Business Data Communications, 5e


Digital interfaces

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

Business Data Communications, 5e


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 (Chapter 17)

Business Data Communications, 5e


Asynchronous and synchronous transmission

Asynchronous and Synchronous Transmission

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

Business Data Communications, 5e


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

Business Data Communications, 5e


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

Business Data Communications, 5e


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

    • Error correction

Business Data Communications, 5e


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%)

Business Data Communications, 5e


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%)

Business Data Communications, 5e


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