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Sliding Windows Flow Control-Data Communication-Lecture Slides, Slides of Data Communication Systems and Computer Networks
This lecture is part of lecture series delivered by Dr. Siddanth Suri at Cochin University of Science and Technology for Data Communication course. Its main points are: Sliding, Windows, Flow, Control, Diagram, Enchancements, Piggybacking, Error, Automatic, Repeat, Request
Typology: Slides
2011/2012
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Sliding Windows Flow Control
• Allow multiple frames to be in transit
• Receiver has buffer W long
• Transmitter can send up to W frames without ACK
• Each frame is numbered
• ACK includes number of next frame expected
Multiple acknowledgement are possible
• Frames has a field for sequence number
• Sequence number bounded by size of field (k)
Frames are numbered modulo 2
k
Example Sliding Window
Sliding Window Enhancements
• Receiver can acknowledge frames without permitting
further transmission (Receive Not Ready,RNR)
• Must send a normal acknowledge to resume
• If full duplex, use piggybacking
If no data to send, use acknowledgement frame
If data but no acknowledgement to send, send last
acknowledgement number again
• Much efficient than stop and wait as
Stop and wait treats the link as a pipe with one frame at any time
Sliding Window treats the link as a pipe filled with frames
flow control techniques cont..
- Positive Acknowledgement Receiver informs successful reception of some frame i by sending ACK i
- Negative acknowledgement and retransmission Receiver detects a damaged frame and send a rejection REJ
- Retransmission after timeout Transmitter waits for an acknowledgement and retransmits after timer is expired
- Negative acknowledgement and retransmission Receiver is not ready to receive further frames but has successfully received previous frames RNR
Automatic Repeat Request (ARQ)
• Techniques used for controlling flow errors are
collectively termed as Automatic Repeat Request
(ARQ )
• Typical ARQs
Stop and wait
Go back N
Selective reject (selective retransmission)
stop and wait ARQ... Solution
- Each frame numbered 0 1 alternatively
- Use ACK0 and ACK
- If same frame is received twice, receiver detects it by looking at frame number and discards it.
Damaged Frame Damaged ACK
Go Back N ARQ
• The Scheme
Based on sliding window
No-Error
• reply with RRi acknowledgement with next frame expected e.g.
RR
Damaged Frame
• Reply with rejection ( REJi ) i indicates damaged frame
• Discard that frame and all future frames until error frame
received correctly
• Transmitter must go back and retransmit that frame and all
subsequent frames
go Back N cont..
- Lost Frame Case-I [ Transmitter has data to send] Frame i lost Transmitter sends next frame i.e. i+ Receiver gets frame i+1 out of sequence Receiver discards frame i+1 and sends rejection REJi ,all subsequent frames will also be discarded Transmitter goes back to frame i and retransmits frame i and all subsequent frames
go Back N ..
- Damaged Acknowledgement Receiver gets frame i and send acknowledgement ( i+1 ) which is lost Acknowledgements are cumulative, so next acknowledgement ( i+n ) may arrive before transmitter times out on frame i If transmitter times out, it sends acknowledgement with P bit set as before This can be repeated a number of times before a reset procedure is initiated
Sliding window Go Back N ARQ