Unformatted text preview: presenting 1 this is a differential encoding scheme used by IEEE 802.5
used Biphase Pros and Cons
Biphase Con at least one transition per bit time and possibly two
maximum modulation rate is twice NRZ
requires more bandwidth Pros synchronization on mid bit transition (self clocking)
has no dc component
has error detection Modulation Rate
Modulation D = modulation rate, baud
R = data rate, bps
b = number of bits per
signal element Scrambling
Scrambling use scrambling to replace sequences that would
use
produce constant voltage
produce
these filling sequences must produce enough transitions to sync
be recognized by receiver & replaced with original
be same length as original design goals have no dc component
have no long sequences of zero level line signal
have no reduction in data rate
give error detection capability B8ZS and HDB3
B8ZS Digital Data, Analog Signal
Digital main use is public telephone system has freq range of 300Hz to 3400Hz
use modem (modulatordemodulator) encoding techniques Amplitude shift keying (ASK)
Frequency shift keying (FSK)
Phase shift keying (PK) Modulation Techniques
Modulation Amplitude Shift Keying
Amplitude encode 0/1 by different carrier amplitudes usually have one amplitude zero susceptible to sudden gain changes inefficient used for up to 1200bps on voice grade lines
very high speeds over optical fiber Binary Frequency Shift
Keying
Keying most common is binary FSK (BFSK)
two binary values represented by two different
two
frequencies (near carrier)
frequencies
less susceptible to error than ASK
used for up to 1200bps on voice grade lines
high frequency radio
even higher frequency on LANs using coax Multiple FSK
Multiple each signalling element represents more
each than one bit
than more than two frequencies used more bandwidth efficient more prone to error Phase Shift Keying
Phase phase of carrier signal is shifted to
phase represent data
represent binary PSK
binary two phases represent two binary digits differential PSK phase shifted relative to previous transmission
phase
rather than some reference signal
rather Quadrature PSK
Quadrature get more efficient use if each signal
get element represents more than one bit
element eg. shifts of π/2 (90o)
eg.
each element represents two bits
split input data stream in two & modulate onto
split
carrier & phase shifted carrier
carrier can use 8 phase angles & more than one
can amplitude
amplitude 9600bps modem uses 12 angles, four...
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 Fall '11
 DRyinyan

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