Business data communications 4e chapter 10

Chapter 10:Transmission Efficiency Business Data Communications, 4e... Transmission Efficiency: Multiplexing  Several data sources share a common transmission medium simultaneously  Li

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

Direct Point-to-Point

Multidrop Line

Multiplexer

Integrated MUX in Host

Frequency Division Multiplexing

 Requires analog signaling & transmission

 Total bandwidth = sum of input bandwidths +

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 band

 Use FDM within the upstream and downstream bands, using “discrete multitone”

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

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 (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

SONET/SDH Signal Hierarchy

STS-1 and STM-N Frames

Statistical Time Division

Cable Modem Scheme

Transmission Efficiency:

Data Compression

 Reduces the size of data

files to move more

information with fewer bits

 Used for transmission and

 Lossless: reconstituted data

is identical to original (ZIP, GIF)

 Lossy: reconstituted data is only “perceptually

equivalent” (JPEG, MPEG)

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 Encoding

TEA 10 00 010

SEA 011 00 010

TEN 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