19-10-2012, 12:46 PM
DIGITAL TRANSMISSION FUNDAMENTALS
Digital_Communication.ppt (Size: 1.34 MB / Downloads: 46)
Bits, numbers, information
Bit: number with value 0 or 1
n bits: digital representation for 0, 1, … , 2n
Byte or Octet, n = 8
Computer word, n = 16, 32, or 64
n bits allows enumeration of 2n possibilities
n-bit field in a header
n-bit representation of a voice sample
Message consisting of n bits
The number of bits required to represent a message is a measure of its information content
More bits → More content
Compression
Information usually not represented efficiently
Data compression algorithms
Represent the information using fewer bits
Noiseless: original information recovered exactly
e.g., zip, compress, GIF, fax
Noisy: recover information approximately
JPEG
Tradeoff: # bits vs. quality
Compression Ratio
#bits (original file) / #bits (compressed file)
Bit Rate of Digitized Signal
Bandwidth Ws Hertz: how fast the signal changes
Higher bandwidth → more frequent samples
Minimum sampling rate = 2 x Ws
Representation accuracy: range of approximation error
Higher accuracy
→ smaller spacing between approximation values
→ more bits per sample
Video Signal
Sequence of picture frames
Each picture digitized & compressed
Frame repetition rate
10-30-60 frames/second depending on quality
Frame resolution
Small frames for videoconferencing
Standard frames for conventional broadcast TV
HDTV frames
Transmission of Stream Information
Constant bit-rate
Signals such as digitized telephone voice produce a steady stream: e.g., 64 kbps
Network must support steady transfer of signal, e.g., 64 kbps circuit
Variable bit-rate
Signals such as digitized video produce a stream that varies in bit rate, e.g., according to motion and detail in a scene
Network must support variable transfer rate of signal, e.g., packet switching or rate-smoothing with constant bit-rate circuit
Stream Service Quality Issues
Network Transmission Impairments
Delay: Is information delivered in timely fashion?
Jitter: Is information delivered in sufficiently smooth fashion?
Loss: Is information delivered without loss? If loss occurs, is delivered signal quality acceptable?
Applications & application layer protocols developed to deal with these impairments
Analog Long-Distance Communications
Each repeater attempts to restore analog signal to its original form
Restoration is imperfect
Distortion is not completely eliminated
Noise & interference is only partially removed
Signal quality decreases with # of repeaters
Communications is distance-limited
Still used in analog cable TV systems
Analogy: Copy a song using a cassette recorder