Problems with Stop and Wait protocols
1) Data may be lost (both sender and receiver may go into dead lock)
To solve this problem of dead lock we can add a timer
2) Acknowledgment lost ( here we can get duplicate packet problem)
To solve this duplicate packet problem we assign sequence numbers to
the packets
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3) Acknowledgment is delayed (we can get missing packet problem)
by assigning numbers to acknowledgments we can solve this problem
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Solution
SLIDING WINDOW PROTOCOLS
1) What is piggybacking? ( temporarily delaying out going
acknowledgments)
The technique of temporarily delaying outgoing acknowledgements so
that they can be hooked onto the next outgoing data frame is known as
piggybacking
2) What is a sender window? (a sender maintains a set of sequence
numbers corresponding to the frames it
has to send)
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3) What is a receiver window?( a receiver maintains a set of sequence
numbers corresponding to the set of
frames it is permitted to accept)
4) What the sequence number within sender’s window represent
(Frames that have been sent)
5) What a sender does when a new packet arrives?
1. It will give next highest sequence
Number
2.The upper edge of the window is
by one
6) What a sender do when an acknowledgment comes? (the lower
edge is advanced by one, In this way
the window continuously maintains a
list of unacknowledged frames.)
7) Diagram
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8) Types of sliding window protocols? (
1. A One-Bit Sliding Window
Protocol
2. A Protocol Using Go-Back-N
3. A Protocol Using Selective
Repeat
1. A One-Bit Sliding Window Protocol
1) Technique used here? (stop and wait )
A sliding window protocol with a window size of 1 uses stop-and-wait
since the sender transmits a frame and waits for its acknowledgement
before sending the next one
2) Assumption (Computer A tries to send its frame 0 to computer B
Computer B tries to send its frame 0 to Computer A)
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3) When the problem of duplicate frame arises? (when time out period
is too short )
4) Example (A may time out repeatedly, sending a series of identical
frames)
Diagram
In (a) each frame arrival brings a new packet for the network layer;
there are no duplicates
In (b) half of the frames contain duplicates, even though there are no
transmission errors.
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Sliding Window ProtocolSliding window protocol is a flow control protocol.
It allows the sender to send multiple frames before needing
the acknowledgements.
Sender slides its window on receiving the acknowledgements
for the sent frames.
This allows the sender to send more frames.
It is called so because it involves sliding of sender’s window.
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Implementations of Sliding Window Protocol
The two well-known implementations of sliding window protocol
are
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Go back N sliding window protocol
1) Sender window size is N
2) Receiver window size is 1
1) For what purpose this protocol is used?(for reliable and sequential
delivery of data frames)
2) What is sender window size and meaning of Go-Back-N?(
The senders window size is N
Go-Back-N means at a time sender can
send N frames without waiting for ack
from receiver
3) Which technique is used by it? (Protocol pipe lining for sending
multiple frames before receiving
acknowledgment for the first frame)
4) Max number of frames that can be sent? (depends on size of
the senders window size)
5) What happens if the acknowledgment is not received? (
all the frames starting from that frame
will be resent)
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Example
1) no of frames that can be sent by sender? (4 frames , 0,1,2,3)
2) Step 1;( sender sends all the 4 frames and waits for ack for 0th frame)
Step 2 (sender receives acknowledgement for the 0th frame)
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Step 3(sender send’s the next frame i.e frame 4 and window slides
containing 4 frames 1,2,3,4)
Step 4( receiver sends the acknowledgment for frame 1 and sender sends
the next frame i.e frame 5 now , sender window will be slided to
hold the frames 2,3,4,5)
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Step 5( assume that frame 2 is lost or ack of frame 2 is lost , then the
sender will goback(4) from the current window and retransmit all the
frames 2,3,4,5
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3. A Protocol Using Selective Repeat
Sender’s window size = N
Receivers window size = N
1) What type of protocol is it ?(data link protocol using sliding window
technique for reliable frame delivery )
2) What is implemented here? (Only erroneous or lost frames are
retransmitted in this case, while good
frames are received and buffered)
Step 1 (initial position)
Step 2(sender sends 4 frames)
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Step 3(receiver sends acknowledgement for frame 0)
Step 4(sender sends next frame i.e frame 4)
Step 5( receiver sends acknowledgment for frame 1)
Step 6(sender sends frame 5 , and frame 2 or ack of frame 2 is lost)
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Step7 (sender re transmit the frame 2 only , since it did not get the ack
with in time)
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