CIS Data Communications1 CIS-325 Data Communication Dr. L. G. Williams, Instructor

CIS Data Communications2 Chapter Six Transmission Efficiency

CIS Data Communications3 Why Efficiency? è Transmission Service (the phone bill) is most expensive part of data communications è To reduce these costs –put more info on at one time (multiplexing) –make shorter calls (compression)

CIS Data Communications4 Transmission Efficiency: Multiplexing è Several data sources share a common transmission medium, with each source having its own channel è Line sharing saves transmission costs è Higher data rates mean more cost-effective transmissions è Most individual data sources require relatively low data rates (p. 142)

CIS Data Communications5 Transmission Efficiency: Data compression è Reduces the size of data files to move more information with fewer bits è Used for transmission and for storage –ZIP –Stuffit è Often combined with multiplexing to increase efficiency

CIS Data Communications6 Alternate Approaches to Terminal Support è Direct point-to-point links è Multidrop line è Multiplexer è Integrated MUX function in host

CIS Data Communications7 Direct Point-to-Point

CIS Data Communications8 Multidrop Line

CIS Data Communications9 Multiplexer

CIS Data Communications10 Integrated MUX in Host

CIS Data Communications11 Frequency Division Multiplexing è Requires analog signaling & transmission è Bandwidth = sum of inputs + guardbands è Modulates signals so that each occupies a different frequency band è Standard for radio broadcasting, analog telephone network, and television (broadcast, cable, & satellite)

CIS Data Communications12 Synchronous Time-Division Multiplexing (TDM) è Used in digital transmission è Requires data rate of the medium to exceed total data rate of signals to be transmitted è Signals “take turns” over medium è Slices of data are organized into frames

CIS Data Communications13 Synchronous TDM è 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,... è Data rate of 1.544Mbps è Uses PCM to digitize voice transmission at 8K/sec, frame length of 193bits

CIS Data Communications14 SONET: Synchronous Optical Network è Specification for high-speed digital transfer via optical fiber è Rates from 51.84Mbps to 13.2Gbps è Uses Synchronous TDM

CIS Data Communications15 Statistical Time Division Multiplexing è dynamic allocation of slots è data rate capacity required is well below the sum of connected capacity è same concepts as synchronous TDM è uses memory buffers to avoid loss of data è widely used for host-to-terminal communications

CIS Data Communications16 Data Compression è Works on the principle of eliminating redundancy è Codes are substituted for compressed portions of data è Lossless: reconstituted data is identical to original (GIF, ZIP) è Lossy: reconsituted data is only “perceptually equivalent” (JPEG, MPEG)

CIS Data Communications17 Run Length Encoding è Replace string of anything with flag, character, and count è e.g. aaaaaa becomes S c a6

CIS Data Communications18 Lempel-Ziv Encoding è Used in V.42 bis, ZIP è Uses a dictionary of existing strings è replace strings with a specific code know to both xmtr and rcvr dictionary è xmtr plugs in code, rcvr replaces code with original string è Algorithm is adaptable - codes change as needed

CIS Data Communications19 Video Compression è Full motion video è Requires tremendous data capacity è high redundancy of ‘frame’ makes compression work well

CIS Data Communications20 Video Compression Standards è 3 popular standards –M-JPEG - intraframe compression –ITU-T H intra- and interframe –MPEG - intra- and interframe è These are ‘lossy’ algorithms –not exactly the same as original è Fractal Compression –100:1 compression ratio