CIS 325 - Data Communications 1

CIS-325Data Communication

Dr. L. G. Williams, Instructor

CIS 325 - Data Communications 2

Chapter Six

Transmission Efficiency

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Why Efficiency?

Transmission Service (the phone bill) is most expensive part of data communications

To reduce these costs– put more info on at one time (multiplexing)– make shorter calls (compression)

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Transmission Efficiency: MultiplexingSeveral data sources share a common

transmission medium, with each source having its own channel

Line sharing saves transmission costsHigher data rates mean more cost-effective

transmissionsMost individual data sources require

relatively low data rates (p. 142)

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Transmission Efficiency: Data compressionReduces the size of data files to move more

information with fewer bitsUsed for transmission and for storage

– ZIP– Stuffit

Often combined with multiplexing to increase efficiency

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Alternate Approaches to Terminal SupportDirect 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 & transmissionBandwidth = sum of inputs + guardbandsModulates signals so that each occupies a

different frequency bandStandard for radio broadcasting, analog

telephone network, and television (broadcast, cable, & satellite)

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Synchronous Time-Division Multiplexing (TDM)Used in digital transmissionRequires data rate of the medium to exceed

total data rate of signals to be transmittedSignals “take turns” over mediumSlices of data are organized into frames

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Synchronous TDM

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, ...

Data rate of 1.544MbpsUses PCM to digitize voice transmission at

8K/sec, frame length of 193bits

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SONET: Synchronous Optical Network

Specification for high-speed digital transfer via optical fiber

Rates from 51.84Mbps to 13.2GbpsUses Synchronous TDM

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Statistical Time Division Multiplexing dynamic allocation of slots data rate capacity required is well below the sum

of connected capacity same concepts as synchronous TDM uses memory buffers to avoid loss of data widely used for host-to-terminal communications

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Data Compression

Works on the principle of eliminating redundancy

Codes are substituted for compressed portions of data

Lossless: reconstituted data is identical to original (GIF, ZIP)

Lossy: reconsituted data is only “perceptually equivalent” (JPEG, MPEG)

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Run Length Encoding

Replace string of anything with flag, character, and count

e.g. aaaaaa becomes Sca6

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Lempel-Ziv Encoding

Used in V.42 bis, ZIPUses a dictionary of existing stringsreplace strings with a specific code know to

both xmtr and rcvr dictionaryxmtr plugs in code, rcvr replaces code with

original stringAlgorithm is adaptable - codes change as

needed

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Video Compression

Full motion videoRequires tremendous data capacityhigh redundancy of ‘frame’ makes

compression work well

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Video Compression Standards

3 popular standards– M-JPEG - intraframe compression– ITU-T H.261 - intra- and interframe– MPEG - intra- and interframe

These are ‘lossy’ algorithms– not exactly the same as original

Fractal Compression– 100:1 compression ratio