Chapter 10: Transmission Efficiency

1. Chapter 10:Transmission Efficiency Business Data Communications, 4e

2. 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

3. Multiplexing Diagram

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

5. Direct Point-to-Point

6. Multidrop Line

7. Multiplexer

8. Integrated MUX in Host

9. 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)

10. Frequency Division Multiplexing FDM

11. CATV Channel Frequency Allocation

12. 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 • 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”

13. FDM In ADSL

14. 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.

16. 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.

17. TDM

18. SynchronousTime-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, …

19. Synchronous TDM

20. 1 1 2 2 N N TDM Frames and Channels Frame … Channel (Time Slot)

21. Digital Carrier Systems

22. DS-1 Transmission Format

23. 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

24. 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.

25. SONET/SDH Signal Hierarchy

26. STS-1 and STM-N Frames STM-N

27. SONET STS-1 Frame Structure

29. Statistical Time Division Multiplexing • “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

30. 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

31. Cable Modem Scheme

32. 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) Transmission Efficiency: Data Compression

33. 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

35. Run-Length Encoding Example

36. 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

37. 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

38. Lempel-Ziv Example

39. http://www.data-compression.com/lempelziv.html Dictionary

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

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