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Topic 4: Physical Layer- Chapter 10: 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 to Terminal Support

Direct point-to-point links Multidrop lineMultiplexer 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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Frequency Division Multiplexing (FDM)

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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 (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”

POTS

Upstream Downstream

0 20 25 200 250

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DSL Modems

Upload Speed:16-640 Kbps

Download Speed:1.5-9 Mbps

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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.

Frequency

Bits/Hertz

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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, …

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TDM

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*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.

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*SONET/SDH Signal Hierarchy

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*STS-1 and STM-N Frames

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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

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STDM

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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 ModemsUpload Speed:400 Kbps

Download Speed:10-30 Mbps

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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 count

Used 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 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

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Lempel-Ziv Encoding

Used in V.42 bis, ZIPbuffer strings at transmitter and

receiver replace strings with pointer to

location of previous occurrencealgorithm creates a tree-based

dictionary of character strings

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Lempel-Ziv Example

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*Video Compression

Requires high compression levelsThree common standards used:

M-JPEG ITU-T H.261 MPEG

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*MPEG Processing Steps

Preliminary scaling and color conversion

Color subsamplingDiscrete cosine transformation (DCT)QuantizationRun-length encodingHuffman coding Interframe compression