Chapter 10:Transmission Efficiency

Business Data Communications, 4e

Transmission Efficiency: Multiplexing

Several data sources share a common transmission medium simultaneouslyLine sharing saves transmission costsHigher data rates mean more cost-effective transmissionsTakes 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 lineMultiplexer Integrated MUX function in host

Direct Point-to-Point

Multidrop Line

Multiplexer

Integrated MUX in Host

Frequency Division MultiplexingRequires analog signaling & transmissionTotal bandwidth = sum of input bandwidths + guardbandsModulates signals so that each occupies a different frequency bandStandard for radio broadcasting, analog telephone network, and television (broadcast, cable, & satellite)

Frequency Division Multiplexing

FDMFDM

CATVCATVChannelChannelFrequencyFrequencyAllocationAllocation

FDM Example: ADSLADSL 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 or FDM to allocate a small

upstream band and a larger downstream band Use FDM within the upstream and downstream

bands, using “discrete multitone”

FDMFDMIn In ADSLADSL

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 (SNR); it then assigns more bits to better quality channels and fewer bits to poorer quality channels.

Time-Division Multiplexing (TDM)

The division of a transmission facility into two or more channels by allotting the common channel to several information channels, one at a time.

Synchronous TDM (or TDM) Time slots are assigned to devices on a fixed,

predetermined basis.

Statistical TDM (Asynchronous TDM, Intelligent TDM) Time slots are assigned to devices on

demand.

TDM

SynchronousTime-Division Multiplexing (TDM)

Used in digital transmissionRequires data rate of the medium to exceed data rate of signals to be transmittedSignals “take turns” over mediumSlices of data are organized into framesUsed in the modern digital telephone system US, Canada, Japan: DS-0, DS-1 (T-1), DS-3 (T-

3), ... Europe, elsewhere: E-1, E3, …

Synchronous TDM

TDM Frames and Channels

1 2 N 1 2 N……

Frame

Channel(Time Slot)

Digital Carrier Systems

DS-1 Transmission Format

T-1 Facilities T-1 carrier: One of carrier systems supported by

AT&T and other companies Data rate: 1.544 Mbps Support DS-1 multiplex format Applications Private voice networks Private data network Video teleconferencing High-speed digital facsimile Internet access

SONET/SDHSONET (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-TSpecifications for taking advantage of the high-speed digital transmission capability of optical fiber.

SONET/SDH Signal Hierarchy

STS-1 and STM-N Frames

STM-NSTM-N

SONET STS-1 Frame Structure

Statistical Time Division Multiplexing

“Intelligent” TDMData rate capacity required is well below the sum of connected capacityDigital only, because it requires more complex framing of dataWidely 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 bitsUsed for transmission and for storageCombines w/ multiplexing to increase efficiencyWorks on the principle of eliminating redundancy

Codes are substituted for compressed portions of dataLossless: 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 countUsed 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 textTree-based dictionary of charactersEncoding 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, ZIPbuffer strings at transmitter and receiverreplace strings with pointer to location of previous occurrencealgorithm creates a tree-based dictionary of character strings

Lempel-Ziv Example

http://www.data-compression.com/lempelziv.html

Dictionary

Video Compression

Requires high compression levelsThree common standards used: M-JPEG ITU-T H.261 MPEG

MPEG Processing Steps

Preliminary scaling and color conversionColor subsamplingDiscrete cosine transformation (DCT)QuantizationRun-length encodingHuffman codingInterframe compression