Transcript chapter six

Chapter 6
Modem Fundamentals
Part II: Understanding Internet Access Technologies
Topics Addressed in Chapter 6
Dial-up access via ISPs
Data codes
Transmitting encoded data
Interfaces and interface standards
Signal representation and modulation
Modem capabilities
Error detection and correction
Modem/computer communications
Special-purpose modems
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Dial-Up Access Via ISPs
Consumers and businesses typically gain Internet access via ISPs.
Many ISPs provide a variety of connection interfaces including:
Dial-in modem connections
ISDN
xDSL
Cable modems
T-n and fractional T-n
Wireless service providers (WSPs) provide wireless Internet access for
users with wireless modems, smart phones, and Web-enabled PDAs, or
handheld computers
Despite increasing use of DSL and cable modems, dial-in access over
voice-grade analog circuits is the most common form of Internet access
for consumers
Point-to-point (PPP) protocol is the most widely used protocol over dialup connections
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Character Encoding
Encoding is one of the first requirements of a
data communication network (see Figure 6-1)
Character encoding involves the conversion of
human-readable characters to corresponding
fixed-length series of bits
Bits can be represented as discrete signals
and therefore can be easily transmitted or
received over communication media
When bits are represented as discrete signals,
such as different voltage levels, they are in a
digital format
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Data Codes
Several character encoding schemes are widely used
in data communication systems including:
ASCII (American Standard Code for Information
Interchange) – See Table 6-2
EBCDIC (Extended Binary-Coded Decimal Interchange Code)
– See Table 6-3
Unicode (aka ISO 10646)
Touch-tone telephone code
As illustrated in Table 6-1, these vary in the number
of bits used to represent each character as well as
the total number of characters that can be
represented
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Transmitting Encoded Data
The bits that represent encoded characters can be transmitted
simultaneously (parallel transmission) or one at time (serial
transmission) – see Figure 6-2
Serial transmission is more widely used than parallel transmission
for data communication
Parallel transmission is used for communication between
components within a computer
In serial transmission, encoded characters can either be
transmitted one at a time (asynchronous transmission) or in
blocks (synchronous transmission) – see Figure 6-5
Figure 6-4 illustrates asynchronous transmission of a single
character.
UART provides the interface between parallel transmission within
the computer and serial transmission ports. It also plays a key role
in formatting encoded characters for asynchronous transmission
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Figure 6-2
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Figure 6-4
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Figure 6-5
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Data Flow
Data communication networks, including modem-tomodem communications, must have some
mechanism for control over the flow of data between
senders and receivers
Three elementary kinds of data flow are:
Simplex
Half-duplex
Full-duplex
These are illustrated in Figures 6-6 and 6-7
Most modems in use today support both full- and
half-duplex communication
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Figure 6-7
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Interfaces and Interface
Standards
There are two major classes of data communication equipment:
Data communication equipment (DCE): this includes modems,
media, switches, routers, satellite transponders, etc.)
Data terminating equipment (DTE): this includes terminals, servers,
workstations, printers, etc.)
The physical interface is the manner in these two classes are
joined together (see Figure 6-8)
A wide range of interface standards exist including
RS-232-C
RS-422, RS-423, RS-449
A variety of ISO and ITU interfaces
USB and FireWire
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Figure 6-8
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RS-232-C
EIA’s RS-232-C standard is arguably the most important physical
layer standard
It is the most widely accepted standard for transferring encoded
characters across copper wires between a computer or terminal
and a modem
RS-232-C uses voltage levels between –15 and +15 volts (see
Figure 6-9); negative voltages are used to represent 1 bits and
positive voltages are use to represent 0 bits
This standard does not specify size or kind of connectors to be
used in the interface. It does define 25 signal leads (see Table
6-4). 25-pin connectors and 9-pin connectors are most
common, but other kinds of connectors are sometimes used
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Figure 6-9
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Digital Data Transmission
All communication media are capable of
transmitting data in either digital or analog
form.
Voice-grade dial-up circuits are typically
analog, however, relative to analog
transmission, digital transmission has several
advantages:
Lower error rates
Higher transmission speeds
No digital-analog conversion
Security
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Analog Transmission
Data is represented in analog form when transmitted over
analog voice-grade dial-up circuits (see Figure 6-14)
This is done by varying the amplitude, frequency, or phase of
the carrier signal (carrier wave) raised during the handshaking
process at the start of a communication session between two
modems
During handshaking, the two modems raise a carrier signal and
agree on how it will be manipulated to represent 0 and 1 bits
In some modulation schemes, more than one of the carrier signal’s
characteristics are simultaneously manipulated
Modems (modulator/demodulators) are the devices used to
translate the digital signals transmitted by computers into
corresponding analog signals used to represent bits over analog
dial-up circuits (see Figure 6-13)
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Figure 6-13
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Figure 6-17
Figure 6-19
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Figure 6-20
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Phase Modulation
Figure 6-24
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Bit Rates and Bandwidth
The bandwidth of an analog channel is the difference between
the minimum and maximum frequencies it can carry
A voice-grade dial-up circuit can transmit frequencies between 300
and 3400 Hz and thus has a bandwidth of 3100 Hz
For digital circuits, bandwidth is a measure of the amount of
data that can be transmitted per unit. Bits per second (bps) is
the most widely used measure for digital circuits
Over time, bit rates (bps) have also become on of the key
measures of modem performance (e.g. a 56 Kbps modem)
However, modem bit rates are not necessarily an accurate
reflection of their data throughput rates
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Baud
Rate
Baud rate is a measure of the number of discrete signals that
can be transmitted (or received) per unit of time
A modem’s baud rate measures the number of signals that it is
capable of transmitting (or receiving) per second
Baud rate represents the number of times per second that a
modem can modulate (or demodulate) the carrier signal to
represent bits
Although baud rate and bit rate are sometimes used
interchangeably to refer to modem data transfer speeds, these
are only identical when each signal transmitted (or received)
represents a signal bit
A modem’s bit rate is typically higher than its baud rate because
each signal transmitted or received may represent a combination of
two or more bits
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Dibits, Tribits, Quadbits, and
QAM
Dibits are a transmission mode in which each signal conveys
two bits of data
With tribits, each carrier signal modulation represents a 3-bit
combination
Quadbits is a transmission mode in which each signal represents
a 4-bit combination. Sixteen distinct carrier signal modulations
are required for quadbits
Phase modulation is common on today’s modems because it
lends itself well to the implementation of dibits, tribits, and
quadbits (see Figure 6-27)
Quadrature amplitude modulation (QAM) is widely used in
today’s modems. Many versions of QAM represent far more than
4-bits per baud
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Figure 6-27
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Modem Capabilities
Modems differ in several dimensions including:
The type of medium they can be connected to
(copper-based, fiber-optic, wireless)
Speed
Connection options (such as support for call
waiting)
Support for voice-over-data
Data compression algorithms
Security features (such as password controls or
callback)
Error detection and recovery mechanisms
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Modem Speed
Over time, the evolution of modem standards has corresponded
with increases in modem speeds (see Table 6-6)
In 2002, V.92 is the newest modem standard
V.92 is backward compatible with V.90 but is capable of upstream
data rates of 48,000
Like V.90, V.92 modems leverage PCM for downstream links
A variety of factors contribute to modem speed and data
throughput including:
Adaptive line probing
Dynamic speed shifts
Fallback capabilities
Fallforword capabilities
Data compression
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Table 6-6
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Data Compression
Modem data compression capabilities enable modems to have
data throughput rates greater than their maximum bit rates
This is accomplished by substituting large strings of repeating
characters or bits with shorter codes
The data compression process is illustrated in Figure 6-29
Widely supported standards for data compression include (see
Table 6-7):
V.42bis --- up to 4:1 compression using the Lempel Ziv algorithm
MNP Class 5 --- supports 1.3:1 and 2:1 ratios (via Huffman
encoding and run-length encoding)
MNP Class 7 – up to 3:1 compression
V.44 --- capable of 20% to 100% improvements over V.42bis
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Figure 6-29
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Table 6-7
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Error Detection and Recovery
In order to ensure that data is not changed or lost
during transmission, error-detection and recovery
processes are standard aspects of modem operations
The general process is as follows (see Figure 6-30)
During handshaking, the modem pair determines the error
checking approach that will be used
The sender sends the error-check along with the data
The receiver calculates its own error-check on received data
and compares it to that transmitted by the sender
If the receiver’s error-check matches the sender’s, no error
is detected; a mismatch indicates a transmission error
Detected errors trigger error recovery mechanisms
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Figure 6-30
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Error Sources
There are many sources of data
communication transmission errors including:
Signal attenuation
Impulse noise
Crosstalk
Echo
Phase jitter
Envelope delay distortion
White noise
Electromagnetic interference (EMI)
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Error Impacts
Errors cause bits to be changed (corrupted)
during transmission; without error-detection
mechanisms, erroneous data could be
received and used in application processing
Figure 6-32 illustrates a transmission error
caused by noise
Table 6-8 indicates that longer impulse noises
can corrupt multiple bits, especially as
transmission speed increases
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Figure 6-32
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Table 6-8
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Error Prevention
Error prevention approaches used in data
communications include:
Line conditioning
Adaptive protocols (such as adaptive line probing,
fallback, adaptive size packet assembly)
Shielding
Repeaters and amplifiers
Better equipment
Flow control
RTS/CTS
XON/OFF
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Error Detection Approaches
Error detection processes vary in complexity and
robustness. They include:
Parity checking (see Table 6-9)
Longitudinal redundancy checks (LRC) – see Table 6-10
Checksums
Cyclical redundancy checks (most widely used and robust)
CRC-12
CRC-16
CRC-32
Sequence checks
Other approaches include check digits, hash totals, byte
counts, and character echoing
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Table 6-9
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Table 6-10
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Error
Recovery
Automatic repeat request (ARQ) is the most widely used errorrecovery approach in data communications. In this approach,
the receiver requests retransmission if an error occurs. There
are three major kinds of ARQ:
Discrete ARQ (aka stop-and-wait ARQ). Sender waits for an ACK or
NAK before transmitting another packet
Continuous ARQ (aka go-back-N ARQ). Sender keeps transmitting
until a NAK is returned; sender retransmits that packet and all
others after it
Selective ARQ. Sender only retransmits packets with errors
Forward error correction codes involve sending additional
redundant information with the data to enable receivers to
correct some of the errors they detect. Hamming code and
Trellis Coded Modulation are examples
Error control/recovery standards include MNP Class 4, V.42, and
LAP-M (see Table 6-12)
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Modem/Computer
Communications
One of the roles of communication software is to enable users to view
and modify modem settings (see Figure 6-33) such as:
error control (see Figure 6-33a and Figure 6-33c)
transmission speed (see Figure 6-33b)
flow control (see Figure 6-33c)
data compression (see Figure 6-33c)
UART settings (see Figure 6-33d)
Most communication software issues Hayes AT command set
instructions to modems
When a user wants to establish a communication session over a dial-up
connection, communication software sends a setup string to the
modem.
The setup string specifies what settings are to be used for
communicating with other modems and how the modem and
computer will interact.
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Figure 6-33c
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Special Purpose Modems
A variety of special purpose modems are found in
data communication networks including:
multiport modems
short-haul modems (see Table 6-13)
modem eliminators (see Figure 6-34)
fiber optic modems
cable modems
ISDN modems
DSL modems
CSU/DSUs
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Chapter 6
Modem Fundamentals
Part II: Understanding Internet Access Technologies