Digital Data Communications Techniques Digital Data Communications Techniques A conversation forms a twoway communication link there is a measure of symmetry between the two parties and messages pass to and fro There is a continual stimulusresponse cyclic action remarks call up other re ID: 241472
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Slide1
Data and Computer Communications
Digital
Data Communications TechniquesSlide2
Digital Data Communications Techniques
A conversation forms a two-way communication link; there is a measure of symmetry between the two parties, and messages pass to and fro. There is a continual stimulus-response, cyclic action; remarks call up other remarks, and the behavior of the two individuals becomes concerted, co-operative, and directed toward some goal. This is true communication.
—On Human Communication
, Colin CherrySlide3
Asynchronous and Synchronous Transmission
timing problems require a mechanism to synchronize the transmitter and receiver
receiver samples stream at bit intervals
if clocks are not precisely aligned, drifting will sample at wrong time after sufficient bits are senttwo solutions to synchronizing clocks:Slide4
Asynchronous Transmission
data are transmitted one character at a time
each character is 5 to 8 bits in length
receiver has the opportunity to resynchronize at the beginning of each new character simple and cheaprequires overhead of 2 or 3 bits per character (~20%)the larger the block of bits, the greater the cumulative timing error
good
for data with large gaps (keyboard)Slide5
Asynchronous TransmissionSlide6
Synchronous Transmission
b
lock
of bits transmitted in a steady stream without start and stop codesclocks must be synchronized to prevent driftcan use separate clock lineembed clock signal in dataneed to indicate start and end of block
use preamble and
postamble bit patterns
frame is data plus preamble, postamble, and control information
more efficient
than
asynchronous for large blocks of data Slide7
Types of Error
a
n
error occurs when a bit is altered between transmission and receptionbinary 1 is transmitted and binary 0 is received or binary 0 is transmitted and binary 1 is receivedSlide8
Error Detection
regardless of design you will have errors
can detect errors by using an error-detecting code added by the transmitter
code is also referred to as check bitsrecalculated and checked by receiverstill chance of undetected errorparityparity bit set so character has even (even parity) or odd (odd parity) number of oneseven number of bit errors goes undetectedSlide9
Parity Check
the simplest error detecting scheme is to append a parity bit to the end of a block of data
if any even number of bits are inverted due to error, an undetected error occursSlide10
Error Detection ProcessSlide11
Cyclic Redundancy Check (CRC)
one of most common and powerful checks
for block of
k bits transmitter generates an n bit frame check sequence (FCS)transmits k+n bits which is exactly divisible by some predetermined numberreceiver divides frame by that numberif no remainder, assume no errorSlide12
Error Correction
correction of detected errors usually requires data block to be retransmitted
not appropriate for wireless applications
bit error rate is high causing lots of retransmissionspropagation delay long (satellite) compared with frame transmission time, resulting in retransmission of frame in error plus many subsequent framesneed to correct errors on basis of bits receivedcodewordon the transmission end each k-bit block of data is mapped into an n-bit block (n > k) using a forward error correction (FEC) encoderSlide13
Error Correction ProcessSlide14
How Error Correction Works
a
dds
redundancy to transmitted messageredundancy makes it possible to deduce original message despite some errorsblock error correction codeSlide15
Line Configuration - Topology
Slide16
Line Configuration - TopologySlide17
Line Configuration - Duplex
data exchanges classified as
half or full
duplexhalf duplex (two-way alternate)only one station may transmit at a timerequires one data path
full duplex (two-way simultaneous
)
simultaneous
transmission and reception between two
stations
requires
two data
paths
separate
media or frequencies used for each directionSlide18
Summary
asynchronous & synchronous transmission
asynchronous
data transmitted one character at a timesynchronousblock of bits transmitted in steady stream without start and stop codeserror detection and correctionsingle bit error and error bursterror detecting codesparity and cyclic redundancy check (CRC)line configurationstopologyfull duplex and half duplex