1. Business Data Communications & Networking Lecture 6 More Data Communications

2. Business Data Communications & Networking Data Link Control zSpecified flow and error control for synchronous communication zData link module arranges data into frames, supplemented by control bits zReceiver checks control bits, if data is intact, it strips them

3. Business Data Communications & Networking High-Level Data Link Control zOn transmitting side, HDLC receives data from an application, and delivers it to the receiver on the other side of the link zOn the receiving side, HDLC accepts the data and delivers it to the higher level application layer zBoth modules exchange control information, encoded into a frame

4. Business Data Communications & Networking HDLC Frame Structure zFlag: 01111110, at start and end zAddress: secondary station (for multidrop configurations) zInformation: the data to be transmitted zFrame check sequence: 16- or 32- bit CRC zControl: purpose or function of frame yInformation frames: contain user data ySupervisory frames: flow/error control (ACK/ARQ) yUnnumbered frames: variety of control functions (see p.131)

5. Business Data Communications & Networking HDLC Operation zInitialization: S-frames specify mode and sequence numbers, U-frames acknowledge zData Transfer: I-frames exchange user data, S-frames acknowledge and provide flow/error control zDisconnect: U-frames initiate and acknowledge

6. Business Data Communications & Networking Transmission Efficiency: Multiplexing zSeveral data sources share a common transmission medium, with each source having its own channel zLine sharing saves transmission costs zHigher data rates mean more cost- effective transmissions zMost individual data sources require relatively low data rates (p. 142)

7. Business Data Communications & Networking Transmission Efficiency: Data compression zReduces the size of data files to move more information with fewer bits zUsed for transmission and for storage yZIP yStuffit zOften combined with multiplexing to increase efficiency

8. Business Data Communications & Networking Alternate Approaches to Terminal Support zDirect point-to-point links zMultidrop line zMultiplexer zIntegrated MUX function in host

9. Business Data Communications & Networking Direct Point-to-Point

10. Business Data Communications & Networking Multidrop Line

11. Business Data Communications & Networking Multiplexer

12. Business Data Communications & Networking Integrated MUX in Host

13. Business Data Communications & Networking Frequency Division Multiplexing (FDM)FDM zRequires analog signaling & transmission zBandwidth = sum of inputs + guardbands zModulates signals so that each occupies a different frequency band zStandard for radio broadcasting, analog telephone network, and television (broadcast, cable, & satellite)

14. Business Data Communications & Networking Synchronous Time-Division Multiplexing (TDM)TDM zUsed in digital transmission zRequires data rate of the medium to exceed data rate of signals to be transmitted zSignals “take turns” over medium zSlices of data are organized into frames

15. Business Data Communications & Networking Synchronous TDM and PSTN zUsed in the modern digital telephone system yUS, Canada, Japan: DS-0, DS-1 (T-1), DS-3 (T-3),... yEurope, elsewhere: E-1, E3,... zData rate of 1.544Mbps zUses PCM to digitize voice transmission at 8K/sec, frame length of 193bits

16. Business Data Communications & Networking SONETSONET: Synchronous Optical Network zSpecification for high-speed digital transfer via optical fiber zRates from 51.84Mbps to 13.2Gbps zUses Synchronous TDM

17. Business Data Communications & Networking Statistical Time Division Multiplexing zrequires digital signaling & transmission zdata rate capacity required is well below the sum of connected capacity zsame concepts as synchronous TDM zuses memory buffers to avoid loss of data zwidely used for remote communications with multiple terminals zsimilar to medium-sharing done by LANs

18. Business Data Communications & Networking Data Compression zWorks on the principle of eliminating redundancy zCodes are substituted for compressed portions of data zLossless: reconstituted data is identical to original (GIF, ZIP) zLossy: reconstituted data is only “perceptually equivalent” (JPEG, MPEG)

19. Business Data Communications & Networking Run Length Encoding zReplace string of anything with flag, character, and count

20. Business Data Communications & Networking Huffman Encoding zLength of each character code based on statistical frequency in text zModified: Group III Fax yEncodes runs of black or white y4 million pixels to <.5 million bits yfull page < 1 minute @ 9.6kb/s

21. Business Data Communications & Networking Lempel-Ziv Encoding zUsed in V.42 bis, ZIP zbuffer strings at transmitter and receiver zreplace strings with pointer to location of previous occurrence zalgorithm creates a tree-based dictionary of character stringsbased dictionary

22. Business Data Communications & Networking Lossy Algorithms (JPEG/MPEG) zScaling and color conversion (to YUV) zColor subsampling (reduces hue info) zDiscrete cosine transformation zQuantization zRun-length encoding zHuffman coding (lossless compression) zInterframe compressions (MPEG only)