Title: CSC 335 Data Communications and Networking
1CSC 335 Data Communications and Networking
- Lecture 4c Communication and Multiplexing
- Dr. Cheer-Sun Yang
2Multiplexing
Carrying multiple signals from multiple sources
over one medium is called multiplexing.
3Where does multiplexing occurs?
- Computer systems one CPU is connected to many
workstations - US mail
- Print spools
- TV programs (why? Explained later)
4Frequency 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.
5Example of FDM
6Frequency 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.
7Frequency 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.
8Time 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.
9Time Division Multiplexing
10What 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.
11Statistical Multiplexing
12Wave Division Multiplexing
- Used in optical communication systems
- Different color is represented by different
spectrum
13Example of WDM
14Contention 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.
15Data 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.
16Huffman 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.
17Example of Huffman Code
18How do we interpret a stream of characters?
19Receiving 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
20No-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.
21How do we find Huffman Code for a group of
characters?Note Huffman Code may not be
unique.
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27Run-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.
28Run-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.
29Run-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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31Another 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.
32Relative 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?
33Lempel-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.
34Image 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.
35Required Reading
- Shay, Section 3.1, 3.2, 3.3,3.5
- Exercises