CS485 Sylabus
Intro: Error
Detection
and
Correction
at the Physical Layer
Errors at the Physcial Layer:
Fact:
A
transmitted signal
can be
received
incorrectly
Example:
A
"1" signal
can
be
received
(and
decoded
) as a
"0" signal
Common causes
of
transmission errors:
Background
noise
:
Radiation
(from space) can cause
single
bit errors
in a
stream of transmitted bits
Weather related
(i.e.,
lightning
):
Lightning
can cause
multiple consecutive
bit errors
in a
stream of transmitted bits
Typical error rates
Typical
error rates
:
Error rate
in
twisted pairs (copper wires)
:
1 bit error
in every few
1000 bits
transmitted.
Error rate
in
optical fibers
:
1 bit error
in several
1,000,000 bits
transmitted
Why
detect errors in the
Physical
layer
Detecting
transmission
errors
:
Error detection
is a
part
of the function to
provide
reliable communication
The
layer
(see
OSI model
:
click here
) that is
responsible
for providing
reliable communication
is:
The
data link
layer
(And
not
the
physical
layer
)
Technically
, it is
not
necessary
to
detect errors
at the
Physical Layer
:
Why
detect errors at the
physical
layer
:
Early
error detection
in the
Physical layer
will result in
faster
response
It is
also
more
efficient
:
The
sooner
we detect an
error
(and
stop
the
processing
of a
message
), the
less computation resources
we will
waste