Topic 4: Physical Layer - Chapter 10: Transmission Efficiency

1
Topic 4 Physical Layer- Chapter 10
Transmission Efficiency

• Business Data Communications, 4e

2
Transmission Efficiency Multiplexing

• Several data sources share a common transmission
  medium simultaneously
• Line sharing saves transmission costs
• Higher data rates mean more cost-effective
  transmissions
• Takes advantage of the fact that most individual
  data sources require relatively low data rates

3
Multiplexing Diagram
4
Alternate Approaches to Terminal Support

• Direct point-to-point links
• Multidrop line
• Multiplexer
• Integrated MUX function in host

5
Direct Point-to-Point
6
Multidrop Line
7
Multiplexer
8
Integrated MUX in Host
9
Frequency Division Multiplexing

• Requires analog signaling transmission
• Total bandwidth sum of input bandwidths
  guardbands
• Modulates signals so that each occupies a
  different frequency band
• Standard for radio broadcasting, analog telephone
  network, and television (broadcast, cable,
  satellite)

10
Frequency Division Multiplexing (FDM)
11
FDM Example ADSL

• ADSL uses frequency-division modulation (FDM) to
  exploit the 1-MHz capacity of twisted pair.
• There are three elements of the ADSL strategy
• Reserve lowest 25 kHz for voice, known as POTS
  (Plain old telephone service)
• Use echo cancellation or FDM to allocate a small
  upstream band and a larger downstream band
• Use FDM within the upstream and downstream bands,
  using discrete multitone

Upstream
Downstream
POTS
0 20 25 200 250
12
DSL Modems
Upload Speed 16-640 Kbps Download Speed 1.5-9
Mbps
13
Discrete Multitone (DMT)

• Uses multiple carrier signals at different
  frequencies, sending some of the bits on each
  channel.
• Transmission band (upstream or downstream) is
  divided into a number of 4-kHz subchannels.
• Modem sends out test signals on each subchannel
  to determine the signal to noise ratio it then
  assigns more bits to better quality channels and
  fewer bits to poorer quality channels.

Bits/Hertz
Frequency
14
Synchronous Time-Division Multiplexing (TDM)

• Used in digital transmission
• Requires data rate of the medium to exceed data
  rate of signals to be transmitted
• Signals take turns over medium
• Slices of data are organized into frames
• Used in the modern digital telephone system
• US, Canada, Japan DS-0, DS-1 (T-1), DS-3 (T-3),
  ...
• Europe, elsewhere E-1, E3,

15
TDM
16
SONET/SDH

• SONET (Synchronous Optical Network) is an optical
  transmission interface proposed by BellCore and
  standardized by ANSI.
• Synchronous Digital Hierarchy (SDH), a compatible
  version, has been published by ITU-T
• Specifications for taking advantage of the
  high-speed digital transmission capability of
  optical fiber.

17
SONET/SDH Signal Hierarchy
18
STS-1 and STM-N Frames
19
Statistical Time Division Multiplexing (STDM)

• Intelligent TDM
• Data rate capacity required is well below the sum
  of connected capacity
• Digital only, because it requires more complex
  framing of data
• Widely used for remote communications with
  multiple terminals

20
STDM
21
STDM Cable Modems

• Cable TV provider dedicates two channels, one for
  each direction.
• Channels are shared by subscribers, so some
  method for allocating capacity is
  needed--typically statistical TDM

22
Cable Modems
Upload Speed 400 Kbps Download Speed 10-30 Mbps
23
Cable Modem Scheme
24
Transmission Efficiency Data Compression

• Reduces the size of data files to move more
  information with fewer bits
• Used for transmission and for storage
• Combines w/ multiplexing to increase efficiency
• Works on the principle of eliminating redundancy

• Codes are substituted for compressed portions of
  data
• Lossless reconstituted data is identical to
  original (ZIP, GIF)
• Lossy reconstituted data is only perceptually
  equivalent (JPEG, MPEG)

25
Run Length Encoding

• Replace long string of anything with flag,
  character, and count
• Used in GIF to compress long stretches of
  unchanged color, in fax transmissions to transmit
  blocks of white space

26
Run-Length Encoding Example
27
Huffman Encoding

• Length of each character code based on
  statistical frequency in text
• Tree-based dictionary of characters
• Encoding is the string of symbols on each branch
  followed. String Encoding TEA 10 00 010
  SEA 011 00 010 TEN 10 00 110

28
Lempel-Ziv Encoding

• Used in V.42 bis, ZIP
• buffer strings at transmitter and receiver
• replace strings with pointer to location of
  previous occurrence
• algorithm creates a tree-based dictionary of
  character strings

29
Lempel-Ziv Example
30
Video Compression

• Requires high compression levels
• Three common standards used
• M-JPEG
• ITU-T H.261
• MPEG

31
MPEG Processing Steps

• Preliminary scaling and color conversion
• Color subsampling
• Discrete cosine transformation (DCT)
• Quantization
• Run-length encoding
• Huffman coding
• Interframe compression