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 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/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.

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 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)

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 Color subsampling Discrete cosine transformation (DCT) Quantization Run-length encoding Huffman coding Interframe compression