Transcript Chapter 8: Data Communication Fundamentals

Topic 4: Physical Layer
-
Chapter 8: Data Communication Fundamentals
Business Data Communications,
4e
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Outline
 Characteristics of Electromagnetic
Signals
 Data, Signal, and Transmission
 Analog Transmission of Digital Data
 Digital Transmission of Analog Data
 Digital Transmission of Digital Data
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Electromagnetic Signals
 Function of time


Analog (varies smoothly over time)
Digital (constant level over time, followed
by a change to another level)
 Function of frequency (more important)


Spectrum (range of frequencies)
Bandwidth (width of the spectrum)
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Periodic Signal Characteristics
S(t) = A sin(2ft + f)
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Amplitude (A): signal value, measured in volts
Frequency (f): repetition rate, cycles per second or
Hertz
Period (T): amount of time it takes for one
repetition, T=1/f
Phase (f): relative position in time, measured in
degrees
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Bandwidth
 Width of the spectrum of frequencies
that can be transmitted

if spectrum=300 to 3400Hz,
bandwidth=3100Hz
 Greater bandwidth leads to greater
costs
 Limited bandwidth leads to distortion
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Bandwidth on a Voice Circuit
 Human hearing ranges from about 20 Hz to
about 14,000 Hz (some up to 20,000 Hz).
Human voice ranges from 20 Hz to about
14,000 Hz.
 The bandwidth of a voice grade telephone
circuit is 0 to 4000 Hz or 4000 Hz (4 KHz).
 Guardbands prevent data transmissions from
interfering with other transmission when
these circuits are multiplexed using FDM.
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Bandwidth on a Voice Circuit
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Bandwidth on a Voice Circuit
 It is important to note that the limit on
bandwidth is imposed by the equipment used
in the telephone network.
 The actual capacity of bandwidth of the wires
in the local loop depends on what exact
type of wires were installed, and the number
of miles in the local loop.
 Actual bandwidth in North America varies
from 300 KHz to 1 MHz depending on
distance.
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Data
 Analog data
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Voice
Images
 Digital data
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Text
Digitized voice or images
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amplitude (volts)
Analog Signaling
 represented by sine waves
1 cycle
phase
difference
time
(sec)
frequency (hertz)
= cycles per second
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Phase


Phase
Frequency: 1 Period/Sec = 1 Hertz
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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
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Data Transmissions
 Analog Transmission of Analog Data

Telephone networks (PSTN)
 Digital Transmission of Digital Data

A computer system
 Analog Transmission of Digital Data

Uses Modulation/Demodulation (Modem)
 Digital Transmission of Analog Data

Uses Coder/Decoder (CODEC)
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Digital Coding
 Character: A symbol that has a common,
constant meaning.
 Characters in data communications, as in
computer systems, are represented by groups
of bits [1’s and 0’s].
 The group of bits representing the set of
characters in the “alphabet” of any given
system are called a coding scheme, or simply
a code.
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Digital Coding
 A byte consists of 8 bits that is treated as a unit
or character. (Some Asian languages use 2
bytes for each of their characters, such as
Chinese.)
 (The length of a computer word could be 1, 2, 4 bytes.)
 There are two predominant coding schemes in
use today:
 United States of America Standard Code for Information
Interchange (USASCII or ASCII)
 Extended Binary Coded Decimal Interchange Code
(EBCDIC)
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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 attenuation 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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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)
 Differential Phase Shift Keying (DPSK)
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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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Amplitude Modulation and ASK
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Frequency Shift Keying (FSK)
 In radio transmission, known as frequency
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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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Frequency Modulation and FSK
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Phase Modulation and PSK
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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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Differential Phase Shift Keying
(DPSK)
0
1
1
0
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Sending Multiple Bits
Simultaneously
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Sending Multiple Bits
Simultaneously
/2  01
 10
0
00
3/2  11
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Sending Multiple Bits
Simultaneously
In practice, the maximum number of bits that
can be sent with any one of these techniques
is about five bits. The solution is to combine
modulation techniques.
One popular technique is quadrature amplitude
modulation (QAM) involves splitting the signal
into eight different phases, and two different
amplitude for a total of 16 different possible
values.
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Sending Multiple Bits
Simultaneously
Trellis coded modulation (TCM) is an
enhancement of QAM that combines phase
modulation and amplitude modulation. It can
transmits different numbers of bits on each
symbol (6-10 bits per symbol).
The problem with high speed modulation
techniques such as TCM is that they are more
sensitive to imperfections in the
communications circuit.
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Example
 Use a drawing to show how the bit
pattern 11100100 would be sent using
a combination of 1-bit Amplitude
Modulation and 1-bit Phase Modulation
(1AM+1PM).
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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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Modem Standards
 V.22
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1200-2400 baud/bps (FM)
 V.32 and V.32bis
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full duplex at 9600 bps (2400 baud at QAM)
bis uses TCM to achieve 14,400 bps.
 V.34
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for phone networks using digital transmission beyond the local
loop.
59 combinations of symbol rate and modulation technique
symbol rates 3429 baud. Its bit rate is up to 28,800 bps (TCM-8.4)
 V.34+

up to 33.6 kbps with TCM-9.8
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Modem Standards (Cont’d)
 V.42bis
data compression modems, accomplished by run length encoding,
code book compression, Huffman encoding and adaptive Huffman
encoding
 MNP5 - uses Huffman encoding to attain 1.3:1 to 2:1
compression.
 it uses Lempel-Ziv encoding and attains 3.5:1 to 4:1.
 V.42bis compression can be added to almost any modem standard
effectively tripling the data rate.
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Voice Grade Modems
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Data Compression
 How fast if using V.42bis
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V.32 - 57.6kbps
V.34 - 115.2 kbps
V.34+ - 133.4 kbps
V.90 ?
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Data Compression
There are two drawbacks to the use of data
compression:
 Compressing already compressed data
provides little gain.
 Data rates over 100 Kbps place
considerable pressure on the traditional
microcomputer serial port controller that
controls the communications between the
serial port and the modem.
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Analog Channel Capacity: BPS
vs. Baud
 Baud=# of signal changes per second. ITU-T now recommends the
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term baud rate be replaced by the term symbol rate.
BPS=bits per second
In early modems only, baud=BPS. The bit rate and the symbol rate
(or baud rate) are the same only when one bit is sent on each
symbol.
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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Digital Transmission of Analog
Data
 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
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Codec vs. Modem
 Codec is for coding analog data into
digital form and decoding it back. The
digital data coded by Codec are samples
of analog waves.
 Modem is for modulating digital data
into analog form and demodulating it
back. The analog symbols carry digital
data.
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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.
 Pulse-code modulation (PCM)

8000 samples/sec sufficient for 4000hz
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Pulse Code Modulation (PCM)
Analog voice data must be translated into a
series of binary digits before they can be
transmitted.
With Pulse Code Modulation (PCM), the
amplitude of the sound wave is sampled at
regular intervals and translated into a binary
number.
The difference between the original analog
signal and the translated digital signal is
called quantizing error.
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PCM
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PCM
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PCM
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PCM
PCM uses a sampling rate of 8000 samples
per second.
Each sample is an 8 bit sample resulting in a
digital rate of 64,000 bps (8 x 8000).
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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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Analog/Digital Modems (56k
Modems)
 The basic idea behind 56K modems (V.90) is simple. 56K
modems take the basic concepts of PCM and turn them
backwards. They are designed to recognize an 8-bit digital
signal 8000 times per second.
 It is impractical to use all 256 discrete codes, because the
corresponding DAC output voltage levels near zero are just
too closely spaced to accurately represent data on a noisy
loop. Therefore, the V.90 encoder uses various subsets of the
256 codes that eliminate DAC output signals most susceptible
to noise. For example, the most robust 128 levels are used
for 56 Kbps, 92 levels to send 52 Kbps, and so on. Using
fewer levels provides more robust operation, but at a lower
data rate.
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Downstream vs. Upstream
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Downstream vs. Upstream
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Analog/Digital Modems (56k
Modems)
Noise is a critical issue. Recent tests found 56K modems
to connect at less than 40 Kbps 18% of the time, 40-50
Kbps 80% of the time, and 50+ Kbps only 2 % of the
time.
It is easier to control noise in the channel transmitting
from the server to the client than in the opposite
direction.
Because the current 56K technology is based on the PCM
standard, it cannot be used on services that do not use
this standard.
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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
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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
 Used for low speed (64Kbps) ISDN
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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
 E.g. Manchester coding and Differential
Manchester coding
 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
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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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Transmission Timing Asynchronous vs. Synchronous
 Sampling timing – How to make the clocks in
a transmitter and a receiver consistent?
 Asynchronous transmission – sending shorter
bit streams and timing is maintained for each
small data block.
 Synchronous transmission – To prevent timing
draft between transmitter and receiver, their
clocks are synchronized. For digital signal,
this can be accomplished with Manchester
encoding or differential Manchester encoding.
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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 Communications
Interface Illustration
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DTE and DCE
DTE
interface
interface
modem
host computer
DTE
modem
DCE
terminal
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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 RS422 & 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

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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
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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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*RS-232 DB-25 Connectors
DB-25 Female
DB-25 Male
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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
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