Chapter 6 Modem Fundamentals

1
Chapter 6Modem Fundamentals

• Part II Understanding Internet Access
  Technologies

2
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 dial-up 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
8
Figure 6-4
9
Figure 6-5
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Data Flow

• Data communication networks, including
  modem-to-modem 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

• EIAs 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 signals 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
19
Figure 6-17
Figure 6-19
20
Figure 6-20
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Phase ModulationFigure 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 modems 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 modems 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 todays 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 todays 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 41 compression using the
  Lempel Ziv algorithm
• MNP Class 5 --- supports 1.31 and 21 ratios
  (via Huffman encoding and run-length encoding)
• MNP Class 7 up to 31 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 receivers error-check matches the
  senders, 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
37
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
41
Table 6-10
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Error Recovery

• Automatic repeat request (ARQ) is the most widely
  used error-recovery 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 6Modem Fundamentals

• Part II Understanding Internet Access
  Technologies