Chapter 10: Transmission Efficiency Business Data Communications, 4e

Transmission Efficiency: Multiplexing 8Several data sources share a common transmission medium simultaneously 8Line sharing saves transmission costs 8Higher data rates mean more cost-effective transmissions 8Takes advantage of the fact that most individual data sources require relatively low data rates

Multiplexing Diagram

Alternate Approaches to Terminal Support 8Direct point-to-point links 8Multidrop line 8Multiplexer 8Integrated MUX function in host

Direct Point-to-Point

Multidrop Line

Multiplexer

Integrated MUX in Host

Frequency Division Multiplexing 8Requires analog signaling & transmission 8Total bandwidth = sum of input bandwidths + guardbands 8Modulates signals so that each occupies a different frequency band 8Standard for radio broadcasting, analog telephone network, and television (broadcast, cable, & satellite)

FDM Example: ADSL 8ADSL uses frequency-division modulation (FDM) to exploit the 1-MHz capacity of twisted pair. 8There are three elements of the ADSL strategy 8Reserve lowest 25 kHz for voice, known as POTS 8Use echo cancellation 1 or FDM to allocate a small upstream band and a larger downstream band 8Use FDM within the upstream and downstream bands, using “discrete multitone”

Discrete Multitone (DMT) 8Uses multiple carrier signals at different frequencies, sending some of the bits on each channel. 8Transmission band (upstream or downstream) is divided into a number of 4-kHz subchannels. 8Modem 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) 8Used in digital transmission 8Requires data rate of the medium to exceed data rate of signals to be transmitted 8Signals “take turns” over medium 8Slices of data are organized into frames 8Used in the modern digital telephone system 8US, Canada, Japan: DS-0, DS-1 (T-1), DS-3 (T-3),... 8Europe, elsewhere: E-1, E3, …

SONET/SDH 8SONET (Synchronous Optical Network) is an optical transmission interface proposed by BellCore and standardized by ANSI. 8Synchronous Digital Hierarchy (SDH), a compatible version, has been published by ITU-T 8Specifications 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 8“Intelligent” TDM 8Data rate capacity required is well below the sum of connected capacity 8Digital only, because it requires more complex framing of data 8Widely used for remote communications with multiple terminals

STDM: Cable Modems 8Cable TV provider dedicates two channels, one for each direction. 8Channels are shared by subscribers, so some method for allocating capacity is needed\-- typically statistical TDM

Cable Modem Scheme

Transmission Efficiency: Data Compression 8Reduces the size of data files to move more information with fewer bits 8Used for transmission and for storage 8Combines w/ multiplexing to increase efficiency 8Works on the principle of eliminating redundancy 8Codes are substituted for compressed portions of data 8Lossless: reconstituted data is identical to original (ZIP, GIF) 8Lossy: reconstituted data is only “perceptually equivalent” (JPEG, MPEG)

Run Length Encoding 8Replace long string of anything with flag, character, and count 8Used in GIF to compress long stretches of unchanged color, in fax transmissions to transmit blocks of white space

Run-Length Encoding Example

Huffman Encoding 8Length of each character code based on statistical frequency in text 8Tree-based dictionary of characters  Encoding is the string of symbols on each branch followed. String Encoding TEA SEA TEN

Lempel-Ziv Encoding 8Used in V.42 bis, ZIP 8buffer strings at transmitter and receiver 8replace strings with pointer to location of previous occurrence 8algorithm creates a tree-based dictionary of character strings

Lempel-Ziv Example

Video Compression 8Requires high compression levels 8Three common standards used: 8M-JPEG 8ITU-T H.261 8MPEG

MPEG Processing Steps 8Preliminary scaling and color conversion 8Color subsampling 8Discrete cosine transformation (DCT) 8Quantization 8Run-length encoding 8Huffman coding 8Interframe compression