chapter 8: data communication fundamentals
DESCRIPTION
Chapter 8: Data Communication Fundamentals. Business Data Communications, 4e. Three Components of Data Communication. Data Analog: Continuous value data (sound, light, temperature) Digital: Discrete value (text, integers, symbols) Signal Analog: Continuously varying electromagnetic wave - PowerPoint PPT PresentationTRANSCRIPT
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Chapter 8:Data Communication
FundamentalsBusiness Data Communications, 4e
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Three Components of Data Communication
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 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
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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
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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
DA
A
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 attentuation 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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Analog Channel Capacity: BPS vs. Baud
Baud=# of signal changes per second BPS=bits per second In early modems only, baud=BPS Each signal change can represent more than one bit,
through complex modulation of amplitude, frequency, and/or phase
Increases information-carrying capacity of a channel without increasing bandwidth
Increased combinations also leads to increased likelihood of errors
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Voice Grade Modems
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Cable Modems
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DSL Modems
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Digital Encoding of Analog Data
Primarily used in retransmission devices 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.
8000 samples/sec sufficient for 4000hz
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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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Generic CommunicationsInterface Illustration
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DTE and DCE
DTE DTE
host com puter term inal
in terface in terface
m odem m odem
DCE
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RS-232C (EIA 232C)
EIA’s “Recommended Standard” (RS) Specifies mechanical, electrical, functional,
and procedural aspects of the interface Used for connections between DTEs and
voice-grade modems, and many other applications
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EIA-232-D
new version of RS-232-C adopted in 1987 improvements in grounding shield, test and
loop-back signals the prevalence of RS-232-C in use made it
difficult for EIA-232-D to enter into the marketplace
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RS-449 EIA standard improving on capabilities of RS-232-C provides for 37-pin connection, cable lengths up to
200 feet, and data rates up to 2 million bps covers functional/procedural portions of R-232-C
electrical/mechanical specs covered by RS-422 & RS-423
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Functional Specifications
Specifies the role of the individual circuits Data circuits in both directions allow full-
duplex communication Timing signals allow for synchronous
transmission (although asynchronous transmission is more common)
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Procedural Specifications Multiple procedures are specified Simple example: exchange of asynchronous data on
private line Provides means of attachment between computer and modem Specifies method of transmitting asynchronous data between
devices Specifies method of cooperation for exchange of data
between devices
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Mechanical Specifications
25-pin connector with a specific arrangement of leads
DTE devices usually have male DB25 connectors while DCE devices have female
In practice, fewer than 25 wires are generally used in applications
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DB-25 Female
DB-25 Male
RS-232 DB-25 Connectors
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RS-232 DB-25 Pinouts
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RS-232 DB-9 Connectors
Limited RS-232
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RS-422 DIN-8
Found on Macs
DIN-8 Male DIN-8 Female
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Electrical Specifications
Specifies signaling between DTE and DCE Uses NRZ-L encoding
Voltage < -3V = binary 1 Voltage > +3V = binary 0
Rated for <20Kbps and <15M greater distances and rates are theoretically
possible, but not necessarily wise
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RS-232 Signals (Asynch)
Odd Parity
Even Parity
No Parity