Transcript Data Encoding Reading Assignment : Stallings Chapter 4

Data Encoding
Reading Assignment :
Stallings Chapter 2, pp. 45 - 60,
Chapter 4, pp. 96 - 101, 103 - 105, 107 - 111
• Terminology
– Data
• entities that convey meaning
– Signals
• electromagnetic representation of data
– Signaling
• the act of propagating the signal along a
suitable medium
– Transmission
• the communication of data by the
propagation and processing of signals.
– Encoding
• converting analog or digital data to analog
or digital signals
• Data
– analog
• continuous values on some interval
• examples
– audio / acoustic
» sound waves perceived by human
» 20 Hz - 20 k Hz
» above 700 Hz -- slight intelligibility
added
– video
» amount of illumination at each point
» time varying analog signal
» horizontal and vertical retrace
» 483 horizontal lines  30 scans / sec
» interlace : 241.5 lines  60 scans / sec
to avoid flickering
– digital
• e.g., text or character strings
• ASCII
– 7 bits + parity bit
• Signals
– analog
• continuously varying electromagnetic wave
• medium : twisted pair, coaxial cable, fiber
optic cable, atmosphere
• examples
– speech spectrum : 20 Hz to 20 k Hz
» telephone transmitter : 300 to 3400 Hz
» Either high frequencies of speech are cut
off, or
» for higher fidelity, convert acoustic signal
into an electromagnetic signal over 300 3400 Hz.
– video signal
» horizontal blanking pulse
» horizontal sync pulse : to maintain
transmitter-receiver synchronization
» vertical blanking pulse
– Digital
• e.g., high voltage for 1, low voltage for 0
• See chapter 4.
– modem (modulator-demodulator)
• digital  analog (carrier frequency) 
digital
– codec (coder-decoder)
• analog  digital  analog
• methods of transmission
– analog
• amplifier
– digital
• repeater : regenerates digital signal
– digital transmission preferred
• cost dropping for integrated circuits (not
as much for analog equipment)
• Effects of noise, etc. are not cumulative
(due to the use of repeaters).
• effective use of large capacity of
medium
• security and privacy
• Transmission impairments
– Attenuation
• The signal strength falls off
logarithmically with distance for guided
medium.
• Attenuation is an increasing function of
frequency.
– Decibel notation : 10 log10 ( P2 / P1 )
• Considerations
– The transmitter generates a signal as strong
as possible without causing distortion.
– The received signal must be well above noise
level.
– Delay distortion
• The signal velocity over a guided
medium varies with frequency.
• critical for digital data
– Intersymbol interference
» Some of the signal components of one
bit position spill over into other bit
positions.
– Noise
• unwanted signals inserted between
transmitter and receiver
• types
– thermal noise
» thermal agitation of electrons in a
conductor
» across entire frequency spectrum
» cannot be eliminated
– intermodulation noise
» generated by two or more frequencies
sharing the same transmission medium
» f1 + f2 or f1 - f2
– crosstalk
» electrical coupling between nearby twisted
pair
» unwanted signals picked up by microwave
antennas
» same order of magnitude as thermal noise
– impulse noise
» noncontinuous, irregular pulses/spikes,
short duration, relatively high amplitude
» electromagnetic disturbances, lightning
» affects digital signals most
• Data encoding techniques
– analog signaling
• carrier signal
• analog medium : usually band limited
• modulation
– the process of encoding source data onto a
carrier signal with frequency fc .
– input : modulating signal (baseband signal)
– transmitted : modulated signal
– four possibilities
• analog/digital data  analog/digital
signal
– digital data, digital signals
• terminology
– signal element
– signaling rate / baud rate / modulation rate
– unipolar signaling
» all voltage levels positive or negative
– polar signaling
» voltage levels both positive and
negative
• Facts
– An increase in data rate increases bit error
rate.
– An increase in S/N ratio decreases bit error
rate.
– An increase in bandwidth allows an
increase in data rate.
• factors affecting receiver in interpreting
digital signals
–
–
–
–
timing of each bit (clocking)
signal level for each bit position
S/N ratio, data rate, bandwidth
encoding scheme
• desired properties in encoding scheme
– signal spectrum
» lack of direct-current (dc) component
» lack of high-frequency components,
i.e., lower bandwidth
» transmitted power concentrated in the
middle of the transmission bandwidth
– synchronization mechanism for clocking
– error detection capability
– noise immunity
• Nonreturn to Zero (NRZ) coding
techniques
– Nonreturn-to-Zero-Level (NRZ-L)
» 1 : constant positive voltage
» 0 : constant negative voltage
– NRZI
» nonreturn to zero, invert on ones
» A transition at the beginning of a bit time
denotes a binary 1; no transition indicates a
binary 0.
– Limitations
» presence of dc component
» lack of synchronization capability
– used for signal input, digital magnetic recording
• differential encoding
– signal decoded by comparing the polarity of
adjacent signal elements
– advantages : easier to detect a transition
– E.g., if the leads of a twisted pair is inverted,
NRZI still gives correct results, but NRZL does
not.
• Biphase coding techniques
– at least one transition per bit time
– Manchester code
» 1 : low-to-high transition
» 0 : high-to-low transition
» used in Ethernet
– Differential Manchester code
» 1 : absence of transition at beginning of bit
period
» 0 : presence of transition at beginning
» differential coding used
» used in token ring
– mid-bit transition : clocking
– advantages
» synchronization
» no dc component
» error detection : absence of an expected
transition  error
• Modulation rate (D)
– rate at which signal elements are generated.
– Data rate (R) : bits per second
– e.g., Manchester encoding
» R <= D <= 2 R
– digital data, analog signals
• carrier frequency fc
• phone lines : 300 - 3400 Hz
• modulation techniques
– Amplitude-shift keying (ASK)
» binary 1 : A cos( 2  fc t )
» binary 0 : 0
» susceptible to sudden gain change
» 1200 bps on voice-grade phone lines
» ASK is used in transmission over
optical fiber.
– Frequency-shift keying (FSK)
» binary 1 : A cos( 2  f1 t )
» binary 0 : A cos( 2  f2 t )
» f1 , f2 : fc  
» used in 1200 bps modems, highfrequency radio transmission
– Phase-shift keying (PSK)
» data represented by shifting phase of
carrier signal
» A cos( 2  fc t + s )
– Two-phase differential PSK
» binary 1 : A cos( 2  fc t +  )
» binary 0 : A cos( 2  fc t )
– Quadrature phase-shift keying (QPSK)
» binary 11 : A cos( 2  fc t + /4 )
» binary 10 : A cos( 2  fc t + 3/4 )
» binary 00 : A cos( 2  fc t + 5/4 )
» binary 01 : A cos( 2  fc t + 7/4 )
– Combining PSK and ASK
» 9600 bps modem, 12 phase angles, 2
amplitude values, on 2400 bps baud
line