CSC 335 Data Communications and Networking

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CSC 335 Data Communications and Networking

• Lecture 4c Communication and Multiplexing
• Dr. Cheer-Sun Yang

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Multiplexing
Carrying multiple signals from multiple sources
over one medium is called multiplexing.
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Where does multiplexing occurs?

• Computer systems one CPU is connected to many
  workstations
• US mail
• Print spools
• TV programs (why? Explained later)

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Frequency Division Multiplexing

• FDM two or more signals that uses different
  carrier frequencies can be transmitted over a
  single medium simultaneously without
  interference.
• Commonly used in television and radio
  transmission.
• A multiplexor receives analog signals from
  multiple sources, each of which has a specific
  bandwidth.
• The signals are combined into another, more
  complex signal with a much larger bandwidth.

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Example of FDM
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Frequency Division Multiplexing

• Engineers have used FDM to build a variety of
  network technologies that permit independent
  communications to proceed simultaneously over a
  single underlying medium.
• For example, the transmitters and receivers used
  with some wireless networks can be tuned to a
  specific channel, making it possible to have two
  independent sets of computers in a single room
  communicating at the same time.

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Frequency Division Multiplexing

• Baseband all the available bandwidth is used to
  derive a single high bit rate transmission path
  (channel).
• Broadband technology the available bandwidth is
  divided into a number of lower bandwidth
  subchannels on one cable.

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Time Division Multiplexing

• TDM is used to share the available capability of
  a baseband channel. Two types of TDM are used
• Synchronized (or fix cycle) Each user has access
  to the channel at precisely defined
  (synchronized) time intervals.
• Asynchronous (or on demand) Users have random
  access to the channel and, once a user has
  acquired access, is the sole user of the channel
  for the duration of the transmission.

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Time Division Multiplexing
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What multiplexing techniques does TV uses?

• Frequency Division Multiplexing Signals of
  different frequencies can be transmitted
  simultaneously.
• Time Division Multiplexing Commercials and
  regular programs are transmitted at different
  time period. The two sub-channels are regular
  program and advertising sub-channels.

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Statistical Multiplexing
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Wave Division Multiplexing

• Used in optical communication systems
• Different color is represented by different
  spectrum

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Example of WDM
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Contention Protocol

• This section (Sec.3.4) is misplaced.
• Contention protocol is used for accessing media
  and usually is called media access sub-layer
  (MAC).
• It should be placed after Chapter 5.

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Data Compression

• Data compression can be done as part of the
  Presentation Layer before messages are passed to
  Transport Layer at the sender side or before
  messages are passed to Application Layer before
  the Application Layer. It can also be done at the
  Physical Layer.
• We discuss Huffman Code, and Run Length Code,
  Relative Encoding, and Lempel-Ziv Encoding
  briefly.

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Huffman Code

• Usually, we can use ASCII code (a fixed length
  coding) to encode characters prior to sending a
  file.
• However, if the file contains only some
  characters whose frequencies of appearance in
  the file are known, we can use Huffman Code to
  shorten the total length of data bits to be
  transmitted.

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Example of Huffman Code
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How do we interpret a stream of characters?
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Receiving and Interpreting a Huffman-Code Message
What characters are we sending/receiving?
Bit stream transmission
(011100011101101101111)
First character sent
Last character sent
A B E C A D B C
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No-Prefix Property

• Huffman Code works due to this property.
• The code for any character never appears as the
  prefix of another code.
• For example, the Huffman Code for A is 01, so no
  other code starts with a 01.

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How do we find Huffman Code for a group of
characters?Note Huffman Code may not be
unique.
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Run-Length Code

• Used in compress 0s in an image file.
• Usually, the image file contains only 0s and 1s
  to indicate that there is a dot or there is no
  dot in the image.

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Run-Length Code Rules

• Rule 1 express the number of 0s with a decimal
  number using four bits. The receiver need to
  append a one bit after inserting 0s.
• Rule 2 if the number of zeros is greater than
  what four bits can represent, use four more bits.
  The receiver need to know that if 1111 is
  received, it is part of the next four bits.

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Run-Length Code Rules (contd)

• Rule 3 How about 11? Use 0000 to indicate that
  there are no 0s in between two 1s.
• Rule 4 What if a stream starts with a 1? Pad
  0000 in front of the stream.

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Another Run-Length Code

• If the data bits include more than just 0s and
  1s, what can we do?
• We can send the character along with the run
  length.

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Relative Encoding

• How about a video transmission where images may
  be very complex in contrast to black and white
  transmission of a fax or a text file?
• We can transmit the first frame and, then, the
  difference between the first frame and the second
  frame instead of the second frame itself.
• What is the advantage?

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Lempel-Ziv Encoding

• If we can find some repeated letters or sequences
  of letters, such as the, them, ing, we can send
  special characters ?, ?, and ?, respectively.
• At the receiving end, the correlations will be
  used as well.
• Details ignored.

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Image Compression

• JPEG (Joint Photographic Experts Group) reduces
  the spatial redundancy found in many still
  pictures.
• MPEG (Moving Pictures Experts Group) must deals
  with not only spatial redundancy found in many
  still pictures, but also the temporal redundancy
  across consecutive frames.

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Required Reading

• Shay, Section 3.1, 3.2, 3.3,3.5
• Exercises