Business Data Communications & Networking Lecture 6 More Data Communications

Business Data Communications & Networking

Lecture 6

More DataCommunications


Data Link ControlSpecified flow and error control for

synchronous communicationData link module arranges data into

frames, supplemented by control bitsReceiver checks control bits, if data

is intact, it strips them


High-Level Data Link Control

On transmitting side, HDLC receives data from an application, and delivers it to the receiver on the other side of the link

On the receiving side, HDLC accepts the data and delivers it to the higher level application layer

Both modules exchange control information, encoded into a frame


HDLC Frame Structure

Flag: 01111110, at start and end

Address: secondary station (for multidrop configurations)

Information: the data to be transmitted

Frame check sequence: 16- or 32-bit CRC

Control: purpose or function of frame Information frames:

contain user data Supervisory frames:

flow/error control (ACK/ARQ)

Unnumbered frames: variety of control functions (see p.131)


HDLC Operation

Initialization: S-frames specify mode and sequence numbers, U-frames acknowledge

Data Transfer: I-frames exchange user data, S-frames acknowledge and provide flow/error control

Disconnect: U-frames initiate and acknowledge


Transmission Efficiency: Multiplexing

Several data sources share a common transmission medium, with each source having its own channel

Line sharing saves transmission costsHigher data rates mean more cost-

effective transmissionsMost individual data sources require

relatively low data rates (p. 142)


Transmission Efficiency: Data compression

Reduces the size of data files to move more information with fewer bits

Used for transmission and for storage ZIP Stuffit

Often combined with multiplexing to increase efficiency


Alternate Approaches to Terminal Support

Direct point-to-point links Multidrop lineMultiplexer Integrated MUX function in host


Direct Point-to-Point


Multidrop Line


Multiplexer


Integrated MUX in Host


Frequency Division Multiplexing (FDM)

Requires analog signaling & transmissionBandwidth = sum of inputs + guardbandsModulates signals so that each occupies a

different frequency bandStandard for radio broadcasting, analog

telephone network, and television (broadcast, cable, & satellite)

http://members.tripod.com/tx_line/businessdataco/images/f6-3.pdf


Synchronous Time-Division Multiplexing (TDM)

Used in digital transmissionRequires data rate of the medium to

exceed data rate of signals to be transmitted

Signals “take turns” over mediumSlices of data are organized into

frames



Synchronous TDM and PSTN

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, ...

Data rate of 1.544MbpsUses PCM to digitize voice transmission

at 8K/sec, frame length of 193bits


SONET: Synchronous Optical Network

Specification for high-speed digital transfer via optical fiber

Rates from 51.84Mbps to 13.2GbpsUses Synchronous TDM



Statistical Time Division Multiplexing

requires digital signaling & transmissiondata rate capacity required is well below

the sum of connected capacitysame concepts as synchronous TDMuses memory buffers to avoid loss of datawidely used for remote communications

with multiple terminalssimilar to medium-sharing done by LANs


Data Compression

Works on the principle of eliminating redundancy

Codes are substituted for compressed portions of data

Lossless: reconstituted data is identical to original (GIF, ZIP)

Lossy: reconstituted data is only “perceptually equivalent” (JPEG, MPEG)


Run Length Encoding

Replace string of anything with flag, character, and count


Huffman Encoding

Length of each character code based on statistical frequency in text

Modified: Group III Fax Encodes runs of black or white 4 million pixels to < .5 million bits full page < 1 minute @ 9.6kb/s


Lempel-Ziv Encoding

Used in V.42 bis, ZIPbuffer strings at transmitter and

receiverreplace strings with pointer to

location of previous occurrencealgorithm creates a tree-based

dictionary of character strings


Lossy Algorithms (JPEG/MPEG)

Scaling and color conversion (to YUV)Color subsampling (reduces hue info)Discrete cosine transformationQuantizationRun-length encodingHuffman coding (lossless compression)Interframe compressions (MPEG only)

Documents

Business Data Communications & Networking Lecture 6 More Data Communications