Chapter 10: Transmission Efficiency


Topic 4: Physical Layer Chapter 10: Transmission Efficiency Business Data Communications, 4e 1

Transmission Efficiency: Multiplexing  Several data sources share a common transmission medium simultaneously  Line sharing saves transmission costs  Higher data rates mean more cost effective transmissions  Takes advantage of the fact that most individual data sources require relatively low data rates 2

Multiplexing Diagram 3

Alternate Approaches to Terminal Support  Direct point-to-point links  Multidrop line  Multiplexer  Integrated MUX function in host 4

Direct Point-to-Point 5

Multidrop Line 6

Multiplexer 7

Integrated MUX in Host 8

Frequency Division Multiplexing     Requires analog signaling & transmission Total bandwidth = sum of input bandwidths + guardbands Modulates signals so that each occupies a different frequency band Standard for radio broadcasting, analog telephone network, and television (broadcast, cable, & satellite) 9

Frequency Division Multiplexing (FDM) 10

FDM Example: ADSL   ADSL 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 (Plain old telephone service) 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” Upstream Downstream POTS 0 20 25 200 250 11

DSL Modems Upload Speed: 16-640 Kbps Download Speed: 1.5-9 Mbps 12

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

Bits/Hertz Frequency 13

Synchronous Time-Division Multiplexing (TDM)      Used in digital transmission Requires data rate of the medium to exceed data rate of signals to be transmitted Signals “take turns” over medium Slices of data are organized into frames 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, … 14

TDM 15

*SONET/SDH    SONET (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-T Specifications for taking advantage of the high-speed digital transmission capability of optical fiber.


*SONET/SDH Signal Hierarchy 17

*STS-1 and STM-N Frames 18

Statistical Time Division Multiplexing (STDM)  “Intelligent” TDM  Data rate capacity required is well below the sum of connected capacity  Digital only, because it requires more complex framing of data  Widely used for remote communications with multiple terminals 19


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 21

Cable Modems Upload Speed: 400 Kbps Download Speed: 10-30 Mbps 22

Cable Modem Scheme 23

*Transmission Efficiency: Data Compression     Reduces the size of data files to move more information with fewer bits Used for transmission and for storage Combines w/ multiplexing to increase efficiency Works on the principle of eliminating redundancy    Codes are substituted for compressed portions of data Lossless: reconstituted data is identical to original (ZIP, GIF) Lossy: reconstituted data is only “perceptually equivalent” (JPEG, MPEG) 24

*Run Length Encoding  Replace long string of anything with flag, character, and count  Used in GIF to compress long stretches of unchanged color, in fax transmissions to transmit blocks of white space 25

*Run-Length Encoding Example 26

Huffman Encoding    Length of each character code based on statistical frequency in text Tree-based dictionary of characters Encoding is the string of symbols on each branch followed. String Encoding TEA 10 00 010 SEA 011 00 010 TEN 10 00 110 27

Lempel-Ziv Encoding  Used in V.42 bis, ZIP  buffer strings at transmitter and receiver  replace strings with pointer to location of previous occurrence  algorithm creates a tree-based dictionary of character strings 28

Lempel-Ziv Example 29

*Video Compression  Requires high compression levels  Three common standards used:    M-JPEG ITU-T H.261


*MPEG Processing Steps  Preliminary scaling and color conversion  Color subsampling  Discrete cosine transformation (DCT)  Quantization  Run-length encoding  Huffman coding  Interframe compression 31