A) Noise and
Errors (173 - 196)
a. White Noise – also called thermal noise
or Gaussian noise, relatively continuous type of noise and is much like the
static you hear when a radio is being tuned between two stations
b. Impulse noise – also called noise spike,
is a noncontinuous noise and one of the most difficult errors to detect because
it can occur randomly. Difficulty comes in separating the noise from the signal
c. Crosstalk – unwanted coupling between
two different signal paths
d. Echo – reflective feedback of a
transmitted signal as the signal moves through a medium
e. Jitter – the result of small timing
irregularities that become magnified during the transmission of digital signals
as the signals are passed from one device to another
f. Attenuation – continuous loss of signal’s
strength as it travels through a medium
B) Error
Prevention
C) Error
Detection
a. Parity
Checks
i. Simple parity (vertical redundancy check)
– easiest error-detection method to incorporate into a transmission system,
comes in two forms, even parity and odd parity
1. Even parity – the 0 or 1 added to the
string produces an even number of binary 1s
2. Odd parity – the 0 or 1 added to the
string produces an odd number of binary 1s
a. Problems arise
when two bits are wrong
ii. Longitudinal parity
(longitudinal redundancy check or horizontal parity) – provide extra level of
protection and uses additional parity check bits
1. Possible for
errors still if errors happen in two different rows. Also, too many added
parity bits
b. Arithmetic checksum –
characters to be transmitted are “summed” together and the sum is then added to
the end of the message
c. Cyclic redundancy checksum (cyclic
checksum) – method that adds 8 to 32 check bits to potentially large data
packets and yields an error-detection capability approaching 100 percent, works
with polynomials
i. Generating polynomial –
industry-approved bit string used to create the cyclic checksum remainder
D) Error Control – action that the receiver
takes when error is detected
a. Toss the
frame/packet – frame is simply discarded
i. Low level
of errors in fiber-optics plus the transport layer will keep track of the
frames and just ask for the frame to be transmitted again
b. Return a
message
i. Stop-and-wait error control –
technique usually associated with the stop-and-wait flow control protocol
1. Stops and
waits for acknowledgement that packet has been received
a. Timeout – waiting a certain amount of time
before transmitting again
ii. Sliding window error control – flow control
scheme that allows a station to transmit a number of data packets at one time
before receiving some form of acknowledgement
1. Piggybacking – if a receiver just
received some data and wishes to send data back to the sender, then the
receiver should include an ACK with the data it is about to send
c. Correct the
error
i. Forward error correction – receiver
fixes the error, redundant information must be present so that the receiver
knows which bit or bits are in error and what their original values were
1. Hamming code – specifically designed
code in which special check bits have been added to data bits such that, if an
error occurs during transmission, the receiver might be able to correct the
error using the included check and data bits
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