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Unformatted text preview: system may use a periodic“idle”
signal to maintain reference sampling time. Then the start of a frame can be defined by a start
delimiter pattern that violates the period for a
standard time, usually 8 bit times: In other physical systems, no signal occurs
unless a frame is to be transmitted.
In such cases a “preamble” waveform - initial
periodic, then start of frame delimiter. M ethods for start and end of frame
(Excluding delimiter pattern):
1. Starting and ending flags, with bit
2. Flag bytes with byte stuffing.
3. Byte count or length field.
Most common flag:
Usually start and end flag, bit stuffing after 5
ones. Error Control
A frame can be lost.
An error in a start flag can make the start of
the frame unrecognizable.
An error in the data field can cause a false end
flag, loss of part of the frame.
Error control tools:
Added redundant bits for error detection, error
retransmission of frames with detected errors.
Error control can be exercised on each link, on
noisy links only, and/or end-to-end at the
transport layer. The basic principles for reliable communication
1. Include enough redundant bits for error
detection so that the chance of undetected
error is negligible.
2. Use acknowledgments, and if there is any
doubt that some data unit was not received
correctly, retransmit it.
3. Sequence number the data. ERROR DETECTION
A frame of data usually includes check bits for
k = number of "data" bits in the frame.
c = number of checking bits.
n = k+c = total frame length, excluding flags.
So the fraction of all n-tuples that check perfectly is
just Error detection and correction - receiver space With N correction points per decision
region, the fraction of points in decision
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This note was uploaded on 04/03/2014 for the course EE 362 taught by Professor Laporta during the Fall '08 term at Penn State.
- Fall '08