business data communications & networking lecture 6 more data communications
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Business Data Communications & Networking
Lecture 6
More DataCommunications
Business Data Communications & Networking
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
Business Data Communications & Networking
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
Business Data Communications & Networking
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)
Business Data Communications & Networking
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
Business Data Communications & Networking
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)
Business Data Communications & Networking
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
Business Data Communications & Networking
Alternate Approaches to Terminal Support
Direct point-to-point links Multidrop lineMultiplexer Integrated MUX function in host
Business Data Communications & Networking
Direct Point-to-Point
Business Data Communications & Networking
Multidrop Line
Business Data Communications & Networking
Multiplexer
Business Data Communications & Networking
Integrated MUX in Host
Business Data Communications & Networking
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)
Business Data Communications & Networking
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
Business Data Communications & Networking
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
Business Data Communications & Networking
SONET: Synchronous Optical Network
Specification for high-speed digital transfer via optical fiber
Rates from 51.84Mbps to 13.2GbpsUses Synchronous TDM
Business Data Communications & Networking
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
Business Data Communications & Networking
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)
Business Data Communications & Networking
Run Length Encoding
Replace string of anything with flag, character, and count
Business Data Communications & Networking
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
Business Data Communications & Networking
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
Business Data Communications & Networking
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)
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