PowerPoint Presentation by JMBWm22

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									     Chapter 1
Data Communications
              Introduction
• Look at:
  – Multiplexing (1.1)
  – Signaling (1.2)
  – Encoding and Decoding (1.3)
  – Error Detection and Recovery (1.4)
  – Flow Control (1.5)
  – Sliding Window (1.6)
  – Congestion Management (1.7)
            Introduction
• Networking involves much more than
  managing computers; it is
  understanding what makes them work
• We can now send multiple signals
  simultaneously over the same cable
  segment
• Multiplexing combines several
  connections into one larger channel
              Multiplexing

• There are many different types of
  multiplexing:
  – Frequency division multiplexing (FDM) is a
    method of transmission in which numerous
    signals are combined on a single
    communications line or channel
                Multiplexing

• There are many different types of
  multiplexing:
  – Wave division multiplexing (WDM) is a
    form of frequency division multiplexing
    specifically for combining many optical
    carrier signals into a single optical fiber
               Multiplexing

• There are many different types of
  multiplexing:
  – Time division multiplexing (TDM) is
    multiple data streams that are combined in
    a single signal and transmitted over the
    same link by allocating a different time slot
    for the transmission of each channel
              Multiplexing

• There are many different types of
  multiplexing:
  – Pulse code modulation (PCM) converts the
    human signal into a digital representation
  – In PCM a digital signal is obtained by
    sampling the analog signal at various
    points in time
                Signaling
• Signaling is communication of
  information between network nodes by
  using telecommunications signals
• The carrier signal has three major
  characteristics:
  – amplitude
  – frequency
  – phase
                Signaling

• Analog signaling adds information or
  encodes information to an alternating
  current (AC) base signal by modifying
  the frequency or signal strength
• Digital signaling uses discrete steps to
  represent information in binary format
  as zeros (0s) or ones (1s)
               Signaling

• Each binary digit or bit represents one
  piece of information, where a bit being a
  1 means one thing, and a 0 means
  another
• Within the circuitry of the PC, ones and
  zeros are represented by voltage levels
               Signaling

• A signal can be any type of information
• The transition from a 0 to a 1 is called
  the rising edge
• The transition from 1 to 0 is called the
  falling edge
• Signals change as time progresses, and
  this is what enables the flow of data
                  Signaling

• Some common terms used in signaling:
  – Clock - sets the tone for everything that
    happens within a particular electronic
    circuit
  – Cycle -This is a single pass through of the
    signal, from the rising edge through the
    falling edge, until the start of the next rising
    edge
                Signaling

• Some common terms used in signaling:
  – Cycle length - The amount of time required
    for the signal to complete one full cycle.
  – Rise time and fall time - This measures
    how long it takes for the level to change
    from 0 to 1, or 1 to 0.
      Encoding and Decoding

• Encoding is putting electronic data into
  a standard format
• Data is encoded into signals to send
  them from one place to another
• Once the data has been encoded and
  sent along a physical medium, it must
  be decoded on the other end
      Encoding and Decoding

• Because information can be either
  digital or analog, and signals can be
  digital or analog there are several types
  of encoding
  – Analog-to-analog
  – Analog-to-digital
  – Digital-to-analog
  – Digital-to-digital
      Encoding and Decoding
• The various types of encoding include:
  – Amplitude modulation (AM) is the encoding
    of a carrier wave by the changes of its
    amplitude along with the changes in input
    signal
  – Frequency modulation (FM) ) is the method
    of encoding data onto an AC wave by
    changing the instantaneous frequency of
    the wave
      Encoding and Decoding

• The various types of encoding include:
  – Phase shift modulation (PSM) is when
    digital signals are conveyed by shifting
    phases
  – PSM is typically used for digital signaling,
    or satellite communication
  – Multilevel signaling is the use of light or
    sound to encode and transmit information
      Encoding and Decoding

• When converting digital data to digital
  signals common techniques are used :
  – Non-Return to Zero (NRZ) uses two levels
    of signaling or is bipolar
  – The two levels or states can be expressed
    as either on or off or high or low
  – This is the most basic and simplistic
    method of encoding
      Encoding and Decoding

• When converting digital data to digital
  signals common techniques are used :
  – Non-Return to Zero Inverted (NRZ-I), is an
    inverted signal level
  – If the bit is a 1, the signal transitions
  – If the bit is a 0, the level stays current (no
    transition)
      Encoding and Decoding

• When converting digital data to digital
  signals common techniques are used :
  – Manchester is a synchronous clock
    encoding technique used to encode the
    clock and data of a synchronous bit stream
  – It uses the rising or falling edge in the
    middle of each bit time to indicate a zero or
    one
      Encoding and Decoding
• When converting digital data to digital
  signals common techniques are used :
  – Differential Manchester is similar to
    Manchester encoding
  – A 1 bit is indicated by making the first half
    of the signal equal to the last half of the
    previous bit's signal
  – There is no transition at the start of the bit
    time
      Encoding and Decoding

• Additional methods of encoding include:
  – 4B/5B this takes data in four-bit codes and
    maps it to corresponding five-bit codes
  – This is done to guarantee no more than
    three 0s in a row so that synchronization is
    more adequate is TCP/IP
      Encoding and Decoding

• Additional methods of encoding include:
  – MLT-3 works much in the same in way that
    NRZ-I works
  – It encodes based on presence of a
    transition or lack of a transition
  – This is a three-level form of data encoding
    used to concentrate the signal power
    below 30 MHz
      Encoding and Decoding

• Additional methods of encoding include:
  – 8B/6T the value of the data byte is
    compared to the values in the 8B/6T table
  – The remapping table has nine symbols
    used for starting and ending delimiters and
    control characters
     Encoding and Decoding

• Additional methods of encoding include:
  – 8B/10B is an encoding scheme in which 8-
    bit binary data values are represented by
    10-bit symbols
  – The data octet is split up into the three
    most significant bits and the five least
    significant bits
      Encoding and Decoding
• Additional methods of encoding include:
  – 4D-PAM5 is a four-dimensional, five-level
    pulse amplitude modulation.
  – This is a way of encoding bits on copper
    wires to get a 1 GB per second transfer
    rate when the maximum rate of a single
    wire is 125 MHz
  – A five-level signal called pulse amplitude
    modulation 5 is used
            Error Recovery

• Error detection and recovery is
  accomplished using:
  – Parity checking ensures when data is
    transmitted from one device to another
    there is a way to recover lost transactions
  – Parity checks are used during transmission
    of data to detect errors that are caused by
    interference or noise
            Error Recovery

• Error detection and recovery is
  accomplished using :
  – Checksums are a simple error-detection
    scheme
  – Each message has a value based on the
    number of bits in the message
  – If the value matches, it's assumed that the
    complete transmission was received
            Error Recovery

• Error detection and recovery is
  accomplished using :
  – Cyclic redundancy checking is a technique
    for checking errors in data that has been
    transmitted on a communications link
  – It is substantially reliable in detecting
    transmission errors and is commonly used
    in modems
            Error Recovery

• Error detection and recovery is
  accomplished using :
  – Error correction is a sophisticated form of
    checking where errors are corrected when
    they are detected
  – Error-correcting code is an algorithm for
    expressing a sequence of numbers
            Flow Control

• Flow control is a method by which the
  data flow between devices is managed
  so that the data can be handled at an
  efficient pace
• Flow control can take place at various
  levels such as user process to user
  process, host to host, and router to
  router
             Flow Control
• Flow control occurs when the receiving
  system tells the sending system to stop
  because it has nowhere to put the data
• Two types of flow control are generally
  supported:
  – Hardware uses two of the wires that are
    connected between the machine and the
    modem
  – Software uses control bytes
             Sliding Window
• Windowing is a flow-control method whereby
  the source requires an acknowledgement
  from the destination after a certain amount of
  data has been transmitted
• It requires the receiving device to
  communicate with the sending device by
  sending back an acknowledgement when it
  receives data
           Sliding Window

• Sliding Window Protocols use two
  different methods:
  – In data acknowledgement, both sender and
    receiver keep a window of
    acknowledgment
  – In the Stop-and-Wait or One Bit, the
    sender transmits one data unit then waits
    for an acknowledgment before sending the
    next data unit
     Congestion Management
• Congestion can be managed in several
  ways:
  – Preallocation schemes try to prevent
    congestion from happening by requiring
    that resources be preallocated before any
    data can be sent
  – This guarantees that the resources will be
    available to process the data when it is
    received
     Congestion Management

• Congestion can be managed in several
  ways:
  – Data discard is a simple but inefficient way
    to handle congestion
  – If insufficient resources are present to
    process existing data, the queued data is
    simply discarded
     Congestion Management

• Congestion can be managed in several
  ways:
  – Isarithmic congestion control is an
    approach to congestion avoidance
  – It sets an upper limit on the number of data
    packets allowed to be present on the
    network at any one time
     Congestion Management
• Congestion can be managed in several
  ways:
  – Choke packets ask the sender to cut back
    traffic voluntary.
  – Traffic shaping is a range of techniques
    used to prioritize the transmission of data
    over a network link.
  – The idea behind shaping is to change
    bursts of traffic to uniform, regular traffic.
     Congestion Management

• Congestion can be managed in several
  ways:
  – Load shedding is a process of
    systematically reducing the demand by
    temporarily decreasing the load in
    response to shortages
  – Jitter control a method to make sure traffic
    gets through the network smoothly

								
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