Transcript Chapter 8: Data Communication Fundamentals

Chapter 8:
Data Communication
Fundamentals
Business Data Communications,
4e
Three Components of Data
Communication
Data
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Analog: Continuous value data (sound, light,
temperature)
Digital: Discrete value (text, integers, symbols)
Signal
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Analog: Continuously varying electromagnetic
wave
Digital: Series of voltage pulses (square wave)
Transmission
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Analog: Works the same for analog or digital
signals
Digital: Used only with digital signals
Analog Data-->Signal Options
Analog data to analog signal
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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
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Requires a codec (encoder/decoder)
Allows use of digital telephony, voice mail
Digital Data-->Signal Options
Digital data to analog signal
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Requires modem (modulator/demodulator)
Allows use of PSTN to send data
Necessary when analog transmission is used
Digital data to digital signal
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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
Analog transmission
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only transmits analog signals, without regard for data
content
attenuation overcome with amplifiers
signal is not evaluated or regenerated
Digital transmission
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transmits analog or digital signals
uses repeaters rather than amplifiers
switching equipment evaluates and regenerates
signal
Data, Signal, and Transmission
Matrix
A
Data
D
D
A
A
D
Signal
Transmission
System
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
Why Use Analog Transmission?
Already in place
Significantly less expensive
Lower attentuation rates
Fully sufficient for transmission of voice
signals
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)
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
1
0
0
1
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
1
1
0
1
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
0
0
1
1
Differential Phase Shift Keying
(DPSK)
0
1
1
0
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
Voice Grade Modems
Cable Modems
DSL Modems
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
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
Codec
Coder/Decoder
Converts analog signals into a digital
form and converts it back to analog
signals
Where do we find codecs?
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Sound cards
Scanners
Voice mail
Video capture/conferencing
Digital Encoding of 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
NRZ-L
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
NRZI
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
Advantages
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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
Manchester Code
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
Differential Manchester
Digital Encoding Illustration
Asynchronous and Synchronous
Transmission
Asynchronous Transmission
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Data are transmitted one character at a time.
Timing (synchronization) is maintained within each
character, by the use of start elements and stop
elements.
Synchronous Transmission
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A block of bits is transmitted in a steady stream
without start and stop codes.
Synchronization
 A separate clock line
 To embed the clocking information in the data signal
Asynchronous Transmission
Synchronous Transmission
To determine the beginning and end of a
block of bits
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Begins with a preamble bit pattern
Ends with a postamble bit pattern
Frame =
preamble + control + data + postamble
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
Generic Communications
Interface Illustration
DTE and DCE
DTE
interface
interface
modem
host computer
DTE
modem
DCE
terminal
Four Characteristics of Interfaces
Mechanical: The actual physical
connection of the DTE to the DCE.
Electrical: Voltage levels and timing of
voltage changes.
Functional: The functions that are
performed.
Procedural: The sequence of events for
transmitting data.
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
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
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
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electrical/mechanical specs covered by RS422 & RS-423
Functional Specifications
Specifies the role of the individual circuits
Data circuits in both directions allow fullduplex communication
Timing signals allow for synchronous
transmission (although asynchronous
transmission is more common)
Functional
specification
Table 8.6
(Page 200)
Procedural Specifications
Multiple procedures are specified
Simple example: exchange of
asynchronous data on private line
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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
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
RS-232 DB-25 Connectors
DB-25 Female
DB-25 Male
RS-232 DB-25 Pinouts
RS-232 DB-9 Connectors
Limited RS-232
RS-422 DIN-8
Found on Macs
DIN-8 Male
DIN-8 Female
Electrical Specifications
Specifies signaling between DTE and DCE
Uses NRZ-L encoding
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Voltage < -3V = binary 1
Voltage > +3V = binary 0
Rated for <20Kbps and <15M
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greater distances and rates are theoretically
possible, but not necessarily wise
RS-232 Signals (Asynch)
Even Parity
Odd Parity
No Parity