Chapter 10: Transmission Efficiency
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Transcript of Chapter 10: Transmission Efficiency
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
Alternate Approaches to Terminal Support
Direct point-to-point links Multidrop lineMultiplexer Integrated MUX function 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)
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”
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.
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, …
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.
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
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
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
Video Compression
Requires high compression levelsThree common standards used: M-JPEG ITU-T H.261 MPEG