Asynchronous and Synchronous Transmission
• Timing problems require a mechanism to
synchronize the transmitter and receiver
• Two solutions
—Asynchronous
—Synchronous
1.a Asynchronous
• Data transmitted one character at a time
—5 to 8 bits
• Timing only needs maintaining within each
character
• Resynchronize with each character
Asynchronous (diagram):
 |
| Asynchronous |
Asynchronous - Behavior :
• In a steady stream, interval between characters
is uniform
• In idle state, receiver looks for start bit
—transition 1 to 0
• Next samples data bits
—e.g. 7 intervals (char length)
• Then looks for next start bit…
—Simple
—Cheap
—Overhead of 2 or 3 bits per char (~20%)
—Good for data with large gaps (keyboard)
1.b Synchronous - Bit Level
• Block of data transmitted without start or stop
bits
• Clocks must be synchronized
• Can use separate clock line
—Good over short distances
—Subject to impairments
• Embed clock signal in data
—Manchester encoding
—Carrier frequency (analog)
Synchronous - Block Level
• Need to indicate start and end of block
• Use preamble and postamble
—e.g. series of SYN (hex 16) characters
—e.g. block of 11111111 patterns ending in 11111110
• More efficient (lower overhead) than asynchronous
Synchronous (diagram):
 |
synchromous2.Types of Error:
• An error occurs when a bit is altered between transmission and
reception
• Single bit errors
— One bit altered
— Adjacent bits not affected
• Burst errors
— Length B
— Contiguous sequence of B bits in which first, last and any number of
intermediate bits are in error
— Impulse noise
— Fading in wireless
— Effect is greater at higher data rates 2.a Error Detection Process 3. Error Control
• The concept behind error control is the prevention of
delivery of incorrect messages (bits) to a higher level in the
communication hierarchy.
• The probability that one bit is in error is called the Bit Error
Rate BER, e.g. BER = 10-13
There are two ways to manage Error Control
• Forward Error Control - enough additional or redundant
information is passed to the receiver, so it can not only
detect, but also correct errors. This requires more
information to be sent and has tradeoffs.
• Backward Error Control - enough information is sent to
allow the receiver to detect errors, but not correct them.
Upon error detection, retransmitted may be requested.
4. Error Correction: —What is needed for error correction?• Ability to detect that bits are in error• Ability to detect which bits are in error —Techniques include:• Parity block sum checking which can correct a single bit error• Hamming encoding which can detect multiple bit errors and correct less (example has hamming distance of 3 can detect up to 2 errors and correct 1) – 00000 00111 11100 11011 |
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