Asynchronous Transmission

1
Asynchronous Transmission
1
0
1
0
1
1
0
0
Idle
Idle
Start bit
Stop bit
Data bit 0
Data bit 7

• Receiver uses Start bit to synchronize to bit
  timing
• of incoming signal

2
Amplitude Modulation versus Frequency Modulation
Binary data
Amplitude
Frequency
3
Synchronous Transmission

• Clock is maintained at receiver with the same
  frequency
• as the transmitters clock and same phase as
  incoming data
• Transmit a clock on a duplicated link ?
• - second link used to transmit a clock
• - clock signal subjected to the same delays as
  data
• Drawback extra cost of the communication medium
• - baseband - cost may not increase significantly
• - broadband - extra costs

4

• Encode data so that the reference clock can be
  extracted
• from received signal along with the data
• Many methods available for encoding data
• - Only those with at least one transition
• (low to high or high to low) of the
  transmitted
• signal per bit time can perform clock
  recovery
• Encoding schemes suitable for baseband signaling
• Non-Return-to-Zero (NRZL)
• Non-Return-to-Zero Inverted (NRZI)
• Manchester Encoding
• Differential Manchester

5
1
0
1
0
1
1
0
0
NRZ
Manchester
Bipolar
Delay
6
Data Encoding Schemes
NRZ Positive voltage 1 Absence of
voltage0 NRZI Transition from low to high or
high to low at the beginning of bit time
denotes 1 No transition 0 ManchesterTiming
transition always occur in the middle of each
bit Mid-bit transition serves as
clocking and data Low to high 1
High to low 0 Logical 1 represented as
a square wave cycle with mark high level
appearing first half cycle and 0
represented by having mark in second half cycle
7
Differential Manchester Mid-bit transition used
only to provide clocking
Presence of a transition at the beginning
of a bit period 0 Absence of a
transition at the beginning of a bit
period 1 Bipolar Always a transition at
the start of each bit time
Additional transition in the middle of bit
time if data is a 1 Delay Has only
half of line transitions as Manchester or
biphase