Topic 4: Physical Layer - Chapter 8: Data Communication Fundamentals

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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)
• 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, T1/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 spectrum300 to 3400Hz, bandwidth3100Hz
• 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
• Voice
• Images
• Digital data
• Text
• Digitized voice or images

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Analog Signaling

• represented by sine waves

phase difference
1 cycle
amplitude (volts)
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
• 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

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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 1s
  and 0s.
• 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
  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
0
1
1
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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 (1AM1PM).

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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
• 1200-2400 baud/bps (FM)
• V.32 and V.32bis
• full duplex at 9600 bps (2400 baud at QAM)
• bis uses TCM to achieve 14,400 bps.
• V.34
• 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 (Contd)

• 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 1.31 to
  21 compression.
• it uses Lempel-Ziv encoding and attains 3.51 to
  41.
• 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
• 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 term baud rate be replaced by the
  term symbol rate.
• BPSbits per second
• In early modems only, baudBPS. 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?
• 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 Encodingof Digital Data

• Most common, easiest method is different voltage
  levels for the two binary digits
• Typically, negative1 and positive0
• 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)
• Change1, no change0
• 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
• 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-high1 , high-to-low0
• Used in Ethernet

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Differential Manchester

• Midbit transition is only for clocking
• Transition at beginning of bit period0
• Transition absent at beginning1
• 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 CommunicationsInterface Illustration
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DTE and DCE
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RS-232C (EIA 232C)

• EIAs 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 RS-422
  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
• 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
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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 lt -3V binary 1
• Voltage gt 3V binary 0
• Rated for lt20Kbps and lt15M
• 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