to a set of procedures used to restrict the amount of data flow between sending
and receiving stations. It tells the sender how much data it can transmit
before it must wait for an acknowledgement from the receiver.
two methods are used. They are,
1. stop and
STOP AND WAIT:
method the sender waits for acknowledgment after every frame it sends. Only
after an acknowledgment has been received, then the sender sends the next
advantage is simplicity. The disadvantage is inefficiency.
method, the sender can transmit several frames before needing an
acknowledgment. The receiver acknowledges only some of the frames, using a
single ACK to confirm the receipt of multiple data frames.
sliding window refers to imaginary boxes at both the sender and receiver. This
window provides the upper limit on the number of frames that can be transmitted
before requiring an acknowledgement. To identify each frame the sliding window
scheme introduces the sequence number. The frames are numbered as 0 to n-1. And
the size of the window is n-1. Here the size of the window is 7 and the frames
are numbered as 0,1,2,3,4,5,6,7.
beginning the sender‟s window
contains n-1 frames. As frames are sent out the left boundary of the window
moves inward, shrinking the size of the window. Once an ACK receives the window
expands at the right side boundary to allow in a number of new frames equal to
number of frames acknowledged by that ACK.
control is implemented in such a way that every time an error is detected, a
negative acknowledgement is returned and the specified frame is retransmitted.
This process is called automatic repeat
control is implemented with the flow control mechanism. So there are two types
in error control. They are,
1. stop and
STOP AND WAIT ARQ:
It is a
form of stop and wait flow control, extended to include retransmission of data
in case of lost or damaged frames.
frame is discovered by the receiver to contain an error, it returns a NAK frame
and the sender retransmits the last frame.
LOST DATA FRAME:
sender is equipped with a timer that starts every time a data frame is
transmitted. If the frame lost in transmission the receiver can never
acknowledge it. The sending device waits for an ACK or NAK frame until its
timer goes off, then it tries again. It retransmits the last data frame.
frame was received by the receiver but the acknowledgement was lost in
transmission. The sender waits until the timer goes off, then it retransmits
the data frame. The receiver gets a duplicated copy of the data frame. So it
knows the acknowledgement was lost so it discards the second copy.
SLIDING WINDOW ARQ
used to send multiple frames per time. The number of frame is according to the
window size. The sliding window is an imaginary box which is reside on both
sender and receiver side.
two types. They are,
method, if one frame is lost or damaged, all frames sent since the last frame
acknowledged or retransmitted.
SELECTIVE REPEAT ARQ
repeat ARQ re transmits only the damaged or lost frames instead of sending
multiple frames. The selective transmission increases the efficiency of
transmission and is more suitable for noisy link. The receiver should have