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7/27/2019 3. Computer Networks Unit-II Part 1
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Character count.Flag bytes with byte stuffing.
Starting and ending flags, with bit stuffing.
Physical layer coding violations.
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1. The network layer on A gives packet 1 to its data link layer. The
packet is correctly received at B and passed to the network layer on
B. B sends an acknowledgement frame back to A.
2. The acknowledgement frame gets lost completely. It just never arrives
at all.
3. The data link layer on A eventually times out. Not having received an,
lost or damaged and sends the frame containing packet 1 again.
4. The duplicate frame also arrives at the data link layer on B perfectly
and is unwittingly passed to the network layer there. If A is sending a
file to B, part of the file will be duplicated (i.e., the copy of the file
made by B will be incorrect and the error will not have been detected).In other words, the protocol will fail.
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In the previous protocols, data frames were transmitted in one directiononly.
In most practical situations, there is a need for transmitting data in both
directions. One way of achieving full-duplex data transmission is to
have two separate communication channels and use each one for
simplex data traffic.
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TheAddress fieldis primarily of importance on lines with multiple terminals,
where it is used to identify one of the terminals.
The Control fieldis used for sequence numbers, acknowledgements, and
other purposesThe Data fieldmay contain any information
The Checksum fieldis a cyclic redundancy code
The minimum frame contains three fields and totals 32 bits
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A supervisory frame.
Type 0 is an acknowledgement frame (RECEIVE READY) used toindicate the next frame expected
Type 1 is a negative acknowledgement frame (REJECT). It is used to
indicate that a transmission error has been detected
Type 2 is RECEIVE NOT READY. It acknowledges all frames up to but
not including Next, just as RECEIVE READY does, but it tells the sender to
stop sendingType 3 is the SELECTIVE REJECT. It calls for retransmission of only the
frame specified
An Unnumbered frame
It is sometimes used for control purposes but can also carry data when
unreliable connectionless service is called for.
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