Download Chapter 8: Data Communication Fundamentals

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
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 (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
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
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
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
Data encoding
decoding
technique to
of and
Digital
Data
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=# of signal changes per second
Baud
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?
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
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
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
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 Encoding Illustration
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
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 R232-C
electrical/mechanical specs covered by RS-422 &
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)
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
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
Voltage < -3V = binary 1
Voltage > +3V = binary 0
Rated for <20Kbps and <15M
greater distances and rates are theoretically
possible, but not necessarily wise
RS-232 Signals (Asynch)
Odd Parity
Even Parity
No Parity