08-01-2013, 03:59 PM
Data and Computer Communications
Data and Computer.ppt (Size: 1.07 MB / Downloads: 75)
Asynchronous and Synchronous Transmission
timing problems require a mechanism to synchronize the transmitter and receiver
receiver samples stream at bit intervals
if clocks not aligned, drifting will sample at wrong time after sufficient bits are sent
two techniques to synchronize
asynchronous transmission
synchronous transmission
Synchronous Transmission
block of data transmitted, sent as a frame
clocks must be synchronized
can use separate clock line
or embed clock signal in data
need to indicate start and end of block
use preamble and post-amble
more efficient (lower overhead) than asynchronous
Types of Error
an error occurs when a bit is altered between transmission and reception
single bit errors
only one bit altered
caused by white noise
burst errors
contiguous sequence of B bits in which first, last and any number of intermediate bits are in error
caused by impulse noise or by fading in wireless
effect is greater at higher data rates
Cyclic Redundancy Check
one of most common and powerful checks
for block of k bits, transmitter generates an n-k bit frame check sequence (FCS)
Transmits n bits which is exactly divisible by some number
receiver divides frame by that number
if no remainder, assume no error
for math, see Stallings chapter 6