Chapter 10:
Transmission Efficiency
Business Data Communications, 4e
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
Multiplexing Diagram
Alternate Approaches to
Terminal Support
Direct point-to-point links
Multidrop line
Multiplexer
Integrated MUX function in host
Direct Point-to-Point
Multidrop Line
Multiplexer
Integrated MUX in Host
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)
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
Use echo cancellation 1 or FDM to allocate a
small upstream band and a larger downstream
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
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, …
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
SONET/SDH Signal Hierarchy
STS-1 and STM-N Frames
Statistical Time Division
Multiplexing
“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
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
Cable Modem Scheme
Transmission Efficiency:
Data Compression
Reduces the size of data Codes are substituted
files to move more
for compressed
information with fewer
portions of data
bits
Lossless: reconstituted
Used for transmission
data is identical to
and for storage
original (ZIP, GIF)
Combines w/
Lossy: reconstituted
multiplexing to
data is only
increase efficiency
“perceptually
equivalent” (JPEG,
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
Run-Length Encoding Example
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
TEA
SEA
TEN
Encoding
10 00 010
011 00 010
10 00 110
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
Lempel-Ziv Example
Video Compression
Requires high compression levels
Three common standards used:
M-JPEG
ITU-T H.261
MPEG
MPEG Processing Steps
Preliminary scaling and color conversion
Color subsampling
Discrete cosine transformation (DCT)
Quantization
Run-length encoding
Huffman coding
Interframe compression