transmission efficiency

7/30/2019 Transmission Efficiency

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Transmission Efficiency

Business Data Communications, 4e


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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


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Multiplexing Diagram


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Alternate Approaches toTerminal Support

Direct point-to-point links

Multidrop line

Multiplexer

Integrated MUX function in host


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Direct Point-to-Point


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Multidrop Line


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Multiplexer


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Integrated MUX in Host


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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)


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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 bandUse FDM within the upstream and downstream bands,

using discrete multitone


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Synchronous Time-DivisionMultiplexing (TDM)

Used in digital transmission

Requires data rate of the medium to exceed data rateof 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,


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SONET/SDH

SONET (Synchronous Optical Network) is an

optical transmission interface proposed by BellCoreand 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.


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SONET/SDH Signal Hierarchy


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STS-1 and STM-N Frames


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Statistical Time DivisionMultiplexing

Intelligent TDM

Data rate capacity required is well below thesum of connected capacity

Digital only, because it requires more

complex framing of data

Widely used for remote communications with

multiple terminals


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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


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Cable Modem Scheme


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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)


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Run Length Encoding

Replace long string of anything with flag,

character, and countUsed in GIF to compress long stretches of

unchanged color, in fax transmissions to

transmit blocks of white space


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Run-Length Encoding Example


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Huffman EncodingLength 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 EncodingTEA 10 00 010

SEA 011 00 010

TEN 10 00 110


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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


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Lempel-Ziv Example


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MPEG Processing Steps

Preliminary scaling and color conversion

Color subsamplingDiscrete cosine transformation (DCT)

Quantization

Run-length encoding

Huffman coding

Interframe compression

transmission efficiency

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