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CSC 335 Data Communications and Networking

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Dr. Cheer-Sun Yang. Multiplexing ... FDM: two or more signals that uses different carrier frequencies can be ... Commonly used in television and radio transmission. ... – PowerPoint PPT presentation

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Title: CSC 335 Data Communications and Networking


1
CSC 335 Data Communications and Networking
  • Lecture 4c Communication and Multiplexing
  • Dr. Cheer-Sun Yang

2
Multiplexing
Carrying multiple signals from multiple sources
over one medium is called multiplexing.
3
Where does multiplexing occurs?
  • Computer systems one CPU is connected to many
    workstations
  • US mail
  • Print spools
  • TV programs (why? Explained later)

4
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.

5
Example of FDM
6
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.

7
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.

8
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.

9
Time Division Multiplexing
10
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.

11
Statistical Multiplexing
12
Wave Division Multiplexing
  • Used in optical communication systems
  • Different color is represented by different
    spectrum

13
Example of WDM
14
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.

15
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.

16
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.

17
Example of Huffman Code
18
How do we interpret a stream of characters?
19
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
20
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.

21
How do we find Huffman Code for a group of
characters?Note Huffman Code may not be
unique.
22
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23
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24
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25
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26
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27
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.

28
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.

29
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.

30
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31
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.

32
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?

33
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.

34
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.

35
Required Reading
  • Shay, Section 3.1, 3.2, 3.3,3.5
  • Exercises
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