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William Stallings
Data and Computer
Communications
7th Edition
Chapter 6
Digital Data Communications
Techniques
Asynchronous and Synchronous
Transmission
•Timing problems require a mechanism to
synchronize the transmitter and receiver
•Two solutions
—Asynchronous
—Synchronous
Asynchronous
•Data transmitted on character at a time
—5 to 8 bits
•Timing only needs maintaining within each
character
•Resynchronize with each character
Asynchronous (diagram)
Asynchronous - Behavior
•In a steady stream, interval between characters
is uniform (length of stop element)
•In idle state, receiver looks for transition 1 to 0
•Then samples next seven intervals (char length)
•Then looks for next 1 to 0 for next char
•Simple
•Cheap
•Overhead of 2 or 3 bits per char (~20%)
•Good for data with large gaps (keyboard)
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)
