Data Transmission Techniques

1
Data Transmission Techniques

• MSIT 126 Computer-based Comm. Systems and
  Networks
• Lecture 4

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Communication and Signals

• A signal is any change in a measurable condition
  that is used to represent information.
• There is an almost endless number of ways we can
  send signals however, they all require that we
  use some sort of energy to make a change that can
  be detected by someone or something.
• We can make sounds, wave flags, post signs, or
  use hand gestures.
• All of these can convey information, as long as
  both the sender and recipient agree on the
  meaning of the signal.

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Communication and Signals

• If we want to move data between computers, we
  must use a signaling method that machines can
  easily understand and that can span distances.
• Thus, data networks use three types of
  electromagnetic energy--electricity, light, and
  radio waves--to transmit signals.

4
Communication and Signals

• Regardless of the type of energy used to carry a
  signal, all signals can be categorized according
  to one of three basic characteristics
• Analog or digital
• One-way (simplex) or two-way (duplex)
• Timed (synchronous) or not timed (asynchronous)

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Analog and Digital Transmission

• Analog and digital are the two ways we can use a
  flow of energy to carry data.
• As our various communication systems become
  computerized, more and more signals are being
  transmitted digitally.
• However, some key communication systems continue
  to use analog.
• If a data transmission must travel across many
  networks, such as a file downloaded from your
  favorite World Wide Web (Web) site to your home
  computer, the signal may be converted from
  digital to analog, and back again.

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Analog Transmission

• "Analog" means a signal is carried as a pattern
  of continually changing waves.
• For example, the sounds you hear are caused by
  waves through the air.
• Audio, video, and data signals are carried on
  electromagnetic waves.
• Analog transmission is commonly used in
  commercial radio and television broadcasting,
  cable television, and the telephone line that
  serves your home.

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Analog Transmission

• There are two basic qualities of an analog
  signal
• Frequency refers to the number of times per
  second a wave swings back and forth in a cycle
  from its beginning point to its ending point.
  Think of frequency as the number of wave crests
  or cycles that passes a fixed point during a
  particular period of time. Therefore, frequency
  is measured in cycles per second or hertz (Hz).
  In sound waves, frequency determines the pitch of
  a sound. In visible light, frequency determines
  the color.

Frequency
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Analog Transmission

• The electromagnetic spectrum includes a wide
  range of frequencies, from infrared (low
  frequencies) through visible light, to
  ultraviolet, microwaves, and beyond (high
  frequencies).

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Analog Transmission

• Amplitude refers to the height of a wave, or how
  far from the center a wave swings. Generally
  speaking, amplitude describes the power of a
  signal. The frequency does not change, but the
  amplitude does. If this is a sound wave, the
  pitch would remain constant, while the loudness
  would change.

Amplitude
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Modulation

• We transmit analog signals by first establishing
  a constant, consistent wave form called a carrier
  wave.
• We then use the signal pattern to change the
  carrier wave's amplitude, frequency, or both.
• Another device, such as a radio receiver, can
  then extract the signal by detecting the way the
  carrier wave has been changed.
• This process of altering a carrier wave to
  represent information is called "modulation."

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Modulation

• If we hold the frequency constant and vary the
  amplitude, this process is called amplitude
  modulation (AM).
• If we hold the amplitude constant and vary the
  frequency within a fixed range, the process is
  called frequency modulation (FM).

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Analog Bandwidth

• Each frequency or range of frequencies can carry
  a separate signal.
• For example, radio and television stations all
  use radio waves to transmit information however,
  each individual signal is carried over a
  different frequency.

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Analog Bandwidth

• The more frequencies a transmission medium can
  handle, the more channels of information it can
  carry.
• Each assigned slice of frequencies is called a
  "band."
• Therefore, the information-carrying capacity of a
  transmission path is called its "bandwidth."
• The bandwidth of an analog channel is expressed
  as the difference or span between its lowest and
  highest frequencies.

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Analog Bandwidth

• We generally use the term "broadband" to describe
  an analog communication medium that has a wide
  bandwidth.
• A single broadband connection, such as a
  television cable, can support many channels by
  assigning each one to a separate frequency.
• However, not all analog transmission is
  broadband.
• Some analog links, such as the telephone line to
  your home, may be capable of carrying only one
  channel.
• The term is also used to describe high-capacity
  digital connections.

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Digital (Baseband) Transmission

• Instead of a pattern of continually changing
  waves, digital signals are transmitted in the
  form of binary bits information represented as a
  series of 1s and 0s.
• The term "binary" refers to the fact that there
  are only two values for a bit on or off, high or
  low.
• "On" bits are depicted as 1 and "off" bits are
  depicted as 0.

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Digital (Baseband) Transmission

• When bits are transmitted over wires, a 0 is
  represented by the absence of electricity or a
  low voltage.
• A 1 is represented by the presence of electricity
  or a higher voltage.
• A digital signal can travel in any direction down
  a wire.

Baseband transmission
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Digital Signals Over Analog Links

• We can also transmit digital data over an analog
  transmission system by representing binary bits
  as different patterns of analog waves.
• For example, a sudden change of amplitude or
  frequency could represent a "1."
• Either type of modulation (FM or AM) can be used
  to represent digital bits.

Analog modulation
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Digital Signals Over Analog Links

• If you connect to the Internet over a standard
  telephone line, your modem converts (modulates)
  your computer's digital data patterns to the
  analog signals used on your telephone line.
• (The term "modem" is the shortened version of
  modulator-demodulator.)

Digital and analog signal
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Digital Signals Over Analog Links

• Data networks, both small and large, use purely
  digital transmission.
• Baseband is relatively simple, less costly than
  broadband (analog), and yet still very fast.
• Thus, it is far more widely used than broadband.
• Although potentially much faster and able to span
  longer distances than baseband, broadband
  requires a modem at each end of a link, which
  increases the cost of every device attached to a
  local area network (LAN).

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Digital Bandwidth

• Unlike a broadband link, which can transmit
  multiple analog signals (each on a different
  frequency), a baseband link can usually only
  transmit a single signal at any particular
  moment.
• Thus, the bandwidth of a digital channel measures
  the number of bits it can carry per second.
• Digital bandwidth is commonly measured in
  kilobits (thousands of bits) per second (Kbps),
  megabits (millions of bits) per second (Mbps),
  and gigabits (billions of bits) per second (Gbps).

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One-Way or Two-Way Transmission

• Thus, we have categorized communication according
  to whether it uses digital or analog signals.
• A second characteristic of communication
  describes whether signals may flow in one or two
  directions simultaneously.
• In data networking, we use three terms to
  describe the way signals flow across a channel

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One-Way or Two-Way Transmission

• Simplex--A signal may flow in only one direction.
  A commercial radio station uses simplex
  communication, because listeners cannot transmit
  radio signals back to the station.
• Half-duplex--Signals may flow in either
  direction, but not simultaneously. For example, a
  two-way radio allows only one user to speak at
  any one time.
• Full-duplex--Signals may flow in both directions
  simultaneously. A telephone is the best example
  of full-duplex communication. One wire brings a
  signal from the telephone company to your home,
  and a second wire carries a separate signal from
  your home back to the telephone company. Thus,
  you can both hear and speak simultaneously.

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Signal Timing

• We can also categorize communication according to
  the way it is organized or coordinated
• Synchronous--The communicating parties or
  endpoints use time to organize communication. In
  a data network, two communicating computers use
  each clock "tick" to detect whether a digital bit
  is a 1 or 0. For example, as each time interval
  passes, a computer checks the network cable to
  see whether the voltage is high (binary 1) or low
  (binary 0). Because the timing determines the
  meaning of the signal, it is essential that the
  endpoints synchronize their clocks before sending
  signals.

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Signal Timing

• Asynchronous--Bits are not transmitted on any
  strict timetable. For example, a computer modem
  typically uses 8 bits to represent one character
  or letter. To indicate the start of each
  character, the sending modem transmits a "start"
  bit to alert the receiving end to the incoming
  signal. After the final bit of the character is
  transmitted, the sending modem transmits a "stop"
  bit to indicate the end of the character. This
  process is repeated for each character the modem
  transmits. Thus, the receiving modem must stay
  synchronized to the signal only for the length of
  time it takes to transmit 8 bits. If the sending
  and receiving clocks are slightly out of
  synchronization, these short data transfers will
  still be successful.

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Signaling Standards

• Imagine for a moment that you are alone in a
  telegraph office a century ago.
• Suddenly the telegraph sounder begins clacking a
  pattern of long and short signals.
• You know a message is coming in, but what does it
  mean?

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Signaling Standards

• If you are like most people today, you do not
  have the slightest idea what the signal means.
• However, if you are a trained telegraph
  operator, you understand Morse Code, which
  represents each letter of the alphabet by a
  different pattern of sounds.
• Beyond that, you can recognize that different
  parts of the signal identify the sender,
  recipient, and body of the message.
• You know that some special-purpose signals
  identify high-priority messages, or simply tell
  all listening telegraph operators when one of
  them steps out for lunch.

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Signaling Standards

• A network device, like a telegraph operator, is
  only aware of the changing patterns of a signal
  coming down the wire.
• If that signal is to carry information, the
  signal must follow some sort of rules that define
  the structure and content of a message.
• Most important, both the sender and receiver of a
  message must agree on what rules to use.

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Signaling Standards

• Data networks use many different types of rules
  for signaling.
• Some low-level rules define how to modulate a
  digital signal onto an analog wave form.
• Rules at higher levels define the communication
  between applications, or the content of
  electronic mail (e-mail) messages.
• These rules are called "protocols.
• Signaling rules and protocols are developed by
  working groups of technical professionals.
• These associations put an enormous amount of
  effort into creating these technical standards
  and specifications, because they make network
  telecommunication possible.

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ITU

• The International Telecommunication Union (ITU)
  is an intergovernmental organization that
  develops international telecommunication
  standards.
• ITU standards, called "recommendations," make it
  possible for many types of electronic
  communications to occur across national borders.
• Because the advantages of standards-based
  interoperability are obvious to all members, ITU
  recommendations are generally adopted by vendors.
• ITU is organized into three sectors, each one
  focusing on a major area of communications
• ITU-R--Radiocommunication Sector
• ITU-D--Telecommunication Development Sector
• ITU-T or ITU-TSS--Telecommunication
  Standardization Sector

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IEEE

• The Institute of Electrical and Electronics
  Engineers (IEEE) is a world-wide society of
  technical professionals. In addition to promoting
  the development of electrical and electronic
  theory and methods, IEEE working committees
  develop and publish many important standards for
  communications, computing, and telecommunications.
  
• One of the best known IEEE standards is the 802
  series that defines the operation of LANs. For
  example, 802.3 defines Ethernet networks, while
  802.5 defines Token Ring networks. Each standard
  describes one aspect of a system's operation,
  such as data transmission protocols, physical
  signaling, or the construction of cables or
  connectors.

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EIA and TIA

• The Electronic Industries Association (EIA) is a
  U.S. national trade organization that publishes
  hardware-oriented standards to guide its member
  companies. EIA consists of various
  product-oriented groups, such as the
  Telecommunications Industry Association (TIA)
  that focuses on electronic products for data
  communications.
• EIA and TIA have taken the lead in providing
  industry-wide open standards for network wiring
  components and installation. Of particular
  relevance are the RS-232 standard for modem
  communication, and the Commercial Building
  Telecommunications Cabling Standard (EIA/TIA-568)
  (being renamed EIA/TIA SP-2840), which provides
  users and vendors precise guidelines for network
  wiring installation.

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ANSI

• The American National Standards Institute (ANSI)
  is a U.S. voluntary organization that develops
  and publishes standards for many types of
  industries, including data communications,
  programming languages, magnetic storage media,
  office systems, and encryption. An ANSI committee
  also develops the National Electrical Code (NEC),
  a set of U.S. safety specifications for
  electrical circuits.
• ANSI members include large companies, government
  agencies, and professional organizations. Like
  ITU recommendations, ANSI standards are
  nonbinding, but widely adopted.

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ISO

• Just as ANSI membership is composed of other
  organizations, ANSI itself represents the United
  States as a member of the International Standards
  Organization (ISO) and International
  Electrotechnical Commission (IEC).
• Thus, ANSI standards are usually similar to ISO
  standards, differing only when unique aspects of
  North American systems need to be considered.

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Different Signals, Different Media

• Data networks transmit signals using three types
  of energy, and each one of these signals travels
  over a particular type of physical medium
• Electrical current over copper wire
• Light pulses over plastic or glass fiber
• Radio waves through free space