dB =10log P1 P2

! Packet-level error control mechanisms detect
and correct packet-level errors such as loss,
duplication, and reordering.
! We typically implement bit-level error control
at the datalink layer of the protocol stack.
! Packet-level error control is typically found at
the transport layer.
! Thus, bit level error control is usually hop-by-hop,
whereas packet-level error control is usually end-toend.
! Information is transmitted on a link by
varying the state of a signal.
! On a digital link, each signal state
corresponds to one or more 0's and 1's-a
signal that can take 2 n , states represents n
bits of information. To decipher a signal on a
digital link, the receiver compares the
received signal with a set of predefined
references.
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! Generally speaking, we prefer hop-by-hop
error control on links where the error rate is
high (so-called lossy links) and end-to-end
error control when entire path is more or less
error free.
! We usually measure the error probability on a
digital link in terms of the bit error rate or
BER
! the ratio of the mean number of errors in any given
interval to the total number of bits transmitted in
that interval.
! Typical fiber-optic links have a bit error ratio in the
range of 10 -18 - 10 -14 , but copper links can have a
substantially higher bit error ratio, depending on
shielding and the operating environment.
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