- Recall:
- A frame received with bit errors is is discarded
Reason:
- Since we do not know
which part of the
frame (message) contains
error(s),
it would be foolish to
try to
use some
part of the
frame
So:
- the entire frame (message) is discarded
- Note that:
- Both
- data frames and
- ACK frames
can be lost (= received in error)
- Both
- Consider the following
basic protocol to
achieve
reliable transmission
(that I called
the basic protocol):
- Sender:
Start: Transmit frame; Set a timeout; while ( not timeout && ACK not received ) { Wait for ACK frame; } if ( timeout expired ) { go to Start; } Done; (ACK was received !)
- Receiver:
Receive frame; if ( CRC check is OK ) { Send ACK frame to sender; Deliver frame; } else { Discard frame; }
- Sender:
- A communication protocol must
be correct under these
circumstances:
- Error-free
- With Errors
- Types of
errors you need to
consider in
a protocol analysis:
- Data frame is lost (bit error(s))
- ACK frame is lost (bit error(s))
- ACK frame is late
Note:
- The duplicate receiption and
out-of-order receiption
errors are
caused by:
- Frame loss !!!
Specifically:
- Duplicate frame receiption is
caused by
lost of
ACK frame:
click here
- Out-of-order frame receiption is caused by lost of data frame: click here
- Example:
error-free operation
According to the basic protocol:
- Sender transmits data frame
containing xxx
- Sender sets a timeout for frame
- Receiver receives the
data frame
- Receiver transmits a ACK frame
- Receiver delivers the data frame
- Sender receives the ACK
frame
before
timeout expires
- Done !!!
- Sender transmits data frame
containing xxx
- Scenario 1 :
transmission error in the
data frame
According to the basic protocol:
- The sender transmits
data frame
- Sender sets a timeout for frame
- Data frame is
lost (bit error(s))
- Receiver will not transmit an ACK frame !!!
- The sender will
time out:
- Sender will retransmit the same frame
- The time, the receiver recived it
correctly and
transmits
a ACK frame
- The sender transmits
data frame
- Conclusion:
- The simple protocol can
recover:
- Data frame transmission errors !!!
- The simple protocol can
recover:
- Scenario 2 :
transmission error in the
acknowledgement frame
According to the basic protocol:
- The sender transmits the
data frame
- The receiver
receives the data frame
(correctly)
- The receiver transmits an ACK frame
- The receiver will deliver the data frame
- The ACK frame
is lost.....
- The sender will time out !!!
Consequently:
- The sender will assume that the data frame was lost !!!
- The sender will
re-transmit the
same data frame
- The receiver receives a (duplicate) frame
- The receiver will deliver the data frame
Error:
- A data frame is delivered multiple times !!!!
Recall that:
-
Reliable communication means:
- All messages received exactly once
- (And in the same order as they were transmitted)
- The sender transmits the
data frame
- Right now,
we cannot
correct
the protocol error
because:
- The receiver
is unable to
distinguish:
- A re-transmission (= old) of a data frame and
- A new transmission of a data frame
-
Graphically explained:
- Case 1:
re-transmission of
an old frame
Remember, the receiver will only "see" the following events:
- Case 2:
Remember, the receiver will only "see" the following events:
The receiver do not have enough information to distinguish these 2 different cases from one another !!!
- Case 1:
re-transmission of
an old frame
- The receiver
is unable to
distinguish:
- The fix
(add more
information !!!):
- The sender
adds
a send sequence number
to identify
each data frame
uniquely
- The receiver will
use the
send sequence number to
check whether a
data frame is:
- a new transmission or
- a re-transmission
- The sender
adds
a send sequence number
to identify
each data frame
uniquely