Transcript Business Data Communications and Networking

Business Data
Communications and
Networking, 6th ed.
FitzGerald and Dennis
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Chapter 4
Physical Layer: Data
Transmission
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Objectives of Chapter 4
Understand
 digital transmission of digital data,
 analog transmission of digital data,
 digital transmission of analog data,
 how analog/digital modems work.
Become familiar with…
 modems and several standard types of
modems.
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INTRODUCTION
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Introduction
There are two fundamentally different types of
data:
 Digital -Computer produced signals that are
binary, either on or off.
 Analog
- Electrical signals which are
shaped like the sound waves they transfer.
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Introduction
Data can be transmitted through a circuit in
the same form they are produced.
Data can also be converted from one form
into the other for transmission over network
circuits.
Likewise, it is possible to translate analog
voice data into digital form for transmission
over digital computer circuits using a device
called a codec.
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Introduction
Digital transmission offers five key benefits over
analog transmission.
• Digital transmission produces fewer errors than analog
transmission.
• Digital transmission is more efficient.
• Digital transmission permits higher maximum
transmission rates.
• Digital transmission is more secure because it is easier
to encrypt.
• Finally, and most importantly, integrating voice, video
and data on the same circuit is far simpler with digital
transmission.
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DIGITAL TRANSMISSION OF
DIGITAL DATA
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Digital Transmission of Digital
Data
All computers produce binary data. For this
data to be understood by both the sender
and receiver, both must agree on a
standard system for representing the
letters, numbers, and symbols that
comprise the messages.
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Coding
A character is 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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Coding
A byte is a group of consecutive bits that is
treated as a unit or character.
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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Transmission Modes
Parallel Mode is the way the internal transfer
of binary data takes place inside a
computer.
Serial Mode is the predominant method of
transferring information in data
communications.
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Transmission Modes
Parallel Mode
Serial Mode
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Baseband Transmission
Digital transmission is the transmission of
electrical pulses. Digital information is
binary in nature in that it has only two
possible states 1 or 0.
Digital signals are commonly referred to as
baseband signals.
In order to successfully send and receive a
message, both the sender and receiver
have to agree how often the sender can
transmit data (data rate).
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Baseband Transmission
With unipolar signaling technique, the voltage
is always positive or negative (like a dc
current).
In bipolar signaling, the 1’s and 0’s vary from
a plus voltage to a minus voltage (like an ac
current).
In general bipolar signaling experiences fewer
errors than unipolar signaling because the
signals are more distinct.
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Baseband Transmission
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Baseband Transmission
Manchester encoding is a special type of
unipolar signaling in which the signal is
changed from a high to low or low to high in
the middle of the signal.
Manchester encoding is less susceptible to
having errors go undetected, because if
there is no transition, the receiver knows
that an error must have occurred.
Manchester encoding is commonly used in
local area networks (ethernet, token ring).
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ANALOG TRANSMISSION
OF DIGITAL DATA
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Analog Transmission of Digital
Data
Telephone networks were originally built for
human speech rather than data.
Analog Transmission occurs when the signal
sent over the transmission media
continuously varies from one state to
another in a wave-like pattern.
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The North American
Telephone System
The telephone system (commonly called
POTS for plain old telephone service)
enables voice communication between any
two telephones within its network.
Houses and offices are connected to a
telephone company end office (central
office class 5) by a set of two twisted pair
wires (called the local loop).
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The North American
Telephone System
The end office is connected to a central office
class 4 by a trunk line.
The central offices are arranged in a
hierarchy; a class 4 is connected to a class
3 office which is connected to a class 2 etc.
The telephone system was originally
designed as an analog system, but today,
most trunk lines are digital.
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Bandwidth on a Voice Circuit
Every sound wave has two parts, half above
the zero point (positive), and half below
(negative) and three important
characteristics.
• The height of the wave is called amplitude.
• The length of the sound wave is expressed as
the number of waves per second or frequency,
expressed in Hertz (Hz).
• The phase is the direction in which the wave
begins.
Bandwidth refers to a range of frequencies.
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Bandwidth on a Voice Circuit
Frequency: 1 Period/Sec = 1 Hertz
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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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Modulation
Modulation is the technique that modifies the
form of a digital electrical signal so the
signal can carry information on a
communications media.
There are three fundamental methods of
analog modulation of an analog signal:
 Amplitude Modulation (AM)
 Frequency Modulation (FM)
 Phase Modulation(PM)
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Modulation
There are also three fundamental methods of
analog modulation of a digital signal:
 Amplitude Shift Keying (ASK)
 Frequency Shift Keying (FSK)
 Phase Shift Keying (PSK)
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Amplitude Modulation and ASK
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Frequency Modulation and FSK
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Phase Modulation and PSK
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Sending Multiple Bits
Simultaneously
Each of the three modulation techniques can
be refined to send more than one bit at a
time. It is possible to send two bits on one
wave by defining four different amplitudes.
This technique could be further refined to
send three bits at the same time by defining
8 different amplitude levels or four bits by
defining 16, etc. The same approach can
be used for frequency and phase
modulation.
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Sending Multiple Bits
Simultaneously
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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.
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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Bits Rate Versus Baud Rate
Versus Symbol Rate
The terms bit rate (the number of bits per
second) and baud rate are used incorrectly
much of the time. They are not the same.
A bit is a unit of information, a baud is a unit of
signaling speed, the number of times a signal
on a communications circuit changes. ITU-T
now recommends the term baud rate be
replaced by the term symbol rate.
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Bits Rate Versus Baud Rate
Versus Symbol Rate
The bit rate and the symbol rate (or baud
rate) are the same only when one bit is sent
on each symbol. If we use QAM or TCM,
the bit rate would be four to eight times the
baud rate.
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Capacity of a Voice Circuit
The capacity of a voice circuit (the maximum
data rate) is the fastest rate at which you
can send your data over the circuit.
The maximum symbol rate in any circuit
depends upon the bandwidth available and
the signal to noise ratio.
Voice grade lines provide a bandwidth of
3000 Hz.
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MODEMS
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Modems
Modem is an acronym for Modulator/
Demodulator, and takes digital electrical
pulses from a computer, terminal, or
microcomputer and converts them into a
continuous analog signal, for transmission
over an analog voice grade circuit.
It then re-converts the analog signal to its
original digital format.
Most modems accept commands from a
microcomputer keyboard.
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Modem Standards
There are many different types of modems
available today.
Most modems support several standards so that
they can communicate with a variety of
different modems.
Better modems can change data rates during
transmission to reduce the rate in case of
noisy transmission (fast retrain).
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Modem Standards
Modem
Standard
V.22
V.32
V.32bis
V.34
V.34+
Maximum
Symbol
Rate
Maximum
Bits per
Symbol
1200
2400
2400
2400
3429
3429
1
1
4
6
8.4
9.8
Maximum
Data Rate
(bps)
1200
2400
9600
14,400
28,800
33,600
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Modem Standards
 V.22
• 1200-2400 baud/bps, FSK
 V.32
and V.32bis
• full duplex at 9600 bps (2400 baud at QAM)
• bis uses TCM to achieve 14,400 bps.
 V.34
and V.34bis
• for phone networks using digital transmission
beyond the local loop.uses rates above the
2400 baud. Up to 28,800 bps (TCM)
• bis up to 36,600 with TCM
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Modem Standards
 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.
• bis 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
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data rate.
Modem Standards
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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DIGITAL TRANSMISSION OF
ANALOG DATA
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Digital Transmission of Analog
Data
Analog voice data can be sent over digital
networks using a pair of special devices
called CODECs (Coder/Decoder).
Operation is very similar to how modems
function.
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Pulse Amplitude Modulation
Analog voice data must be translated into a
series of binary digits before they can be
transmitted.
With Pulse Amplitude Modulation, 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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Pulse Amplitude Modulation
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Pulse Amplitude Modulation
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Pulse Amplitude Modulation
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Pulse Amplitude Modulation
For standard voice grade circuits, the
sampling of 3300 Hz at an average of 2
samples/second would result in a sample
rate of 6600 times per second.
There are two ways to reduce quantizing error
and improve the quality of the PAM signal.
• Increase the number of amplitude levels
• Sample more frequently (oversampling).
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Pulse Code Modulation
Pulse Code Modulation is the most commonly
used technique in the PAM family and 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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ANALOG/DIGITAL MODEMS
(56K MODEMS)
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Analog/Digital Modems (56k
Modems)
The V.34+ modem is probably the fastest
analog modem that will be developed.
The basic idea behind 56K modems (V.pcm)
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.
Subtract the one bit in the PCM symbol
used for control, and the maximum data
rate becomes 56K.
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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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End of Chapter 4
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