25-05-2012, 02:20 PM
William StallingsData and Computer Communications
William Stallings Data and Computer Communications.ppt (Size: 285 KB / Downloads: 17)
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
Resync with each character
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)
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 async