session -7 - Sliding Window Protocol 1- N oisy Channels.ppt
- 1. 10. Data Link Control and Protocols
- 2. PROTOCOLS Now let us see how the data link layer can combine flow control , and error control to achieve the delivery of data from one node to another . The protocols are normally implemented in software by using one of the common programming languages. Stop-and-Wait ARQ Go-Back-N ARQ Selective Repeat ARQ PROTOCOLS
- 3. It is the simplest flow and error control mechanism . A transmitter sends a frame then stops and waits for an acknowledgment. Stop-and-Wait ARQ has the following features: The sending device keeps a copy of the sent frame transmitted until it receives an acknowledgment( ACK) The sender starts a timer when it sends a frame. If an ACK is not received within an allocated time period, the sender resends it Both frames and acknowledgment (ACK) are numbered alternately 0 and 1( two sequence number only) This numbering allows for identification of frames in case of duplicate transmission Stop-and-Wait ARQ
- 4. The acknowledgment number defines the number of next expected frame. (frame 0 received ACK 1 is sent) If the receiver detects an error in the received frame, or receives a frame out of order it simply discards the frame The receiver send only positive ACK for frames received safe; it is silent about the frames damage or lost. The sender has a control variable S that holds the number of most recently sent frame (0 or 1). The receiver has control variable R, that holds the number of the next frame expected (0,or 1) Stop-and-Wait ARQ
- 5. Cases of Operations: 1. Normal operation 2. The frame is lost 3. The Acknowledgment (ACK) is lost 4. The Ack is delayed Stop-and-Wait ARQ
- 6. Normal operation The sender will not send the next frame until it is sure that the current one is correctly receive sequence number is necessary to check for duplicated frames Stop-and-Wait ARQ
- 7. 1. Stop and Wait ARQ 2. Lost or damaged frame A damage or lost frame treated by the same manner by the receiver. No NACK when frame is corrupted / duplicate
- 8. 3. Lost ACK frame Importance of frame numbering Stop-and-Wait ARQ
- 9. 4. Delayed ACK and lost frame Importance of frame numbering Stop-and-Wait ARQ
- 10. After each frame sent the host must wait for an ACK inefficient use of bandwidth To improve efficiency ACK should be sent after multiple frames Alternatives: Sliding Window protocol Go-back-N ARQ Selective Repeat ARQ Stop-and-Wait ARQ
- 11. Pipelining Pipelining: A task is begun before the previous task has ended There is no pipelining in stop and wait ARQ because we need to wait for a frame to reach the destination and be acknowledged before the next frame can be sent Pipelining improves the efficiency of the transmission
- 12. Sliding window protocol Sliding window protocols apply Pipelining : Go-Back-N ARQ Selective Repeat ARQ Sliding window protocols improve the efficiency multiple frames should be in transition while waiting for ACK. Let more than one frame to be outstanding. Outstanding frames: frames sent but not acknowledged We can send up to W frames and keep a copy of these frames(outstanding) until the ACKs arrive. This procedures requires additional feature to be added :sliding window
- 13. Sliding window
- 14. Go_Back _N ARQ Sender sliding window If m = 3; sequence numbers = 8 and window size =7 Acknowledged frames
- 15. Receiver sliding window The receive window is an abstract concept defining an imaginary box of size 1 with one single variable Rn. The window slides when a correct frame has arrived; sliding occurs one slot at a time. Go_Back _N ARQ
- 16. control variables S: hold the sequence number of the recently sent frame SF: holds sequence number of the first frame in the window SL: holds the sequence number of the last frame R: sequence number of the frame expected to received Outstanding frames: frames sent but not acknowledged Go-Back-N ARQ
- 17. Go-Back-N ARQ
- 18. In Go-Back-N ARQ we use one timer for the first outstanding frame The receiver sends a positive ACK if a frame has arrived safe and in order. if a frame is damaged or out of order ,the receiver is silent and will discard all subsequent frames When the timer of an unacknowledged frame at the sender site is expired , the sender goes back and resend all frames , beginning with the one with expired timer.( that is why the protocol is called Go-Back-N ARQ) The receiver doesn't have to acknowledge each frame received . It can send cumulative Ack for several frame Go-Back-N ARQ
- 19. Go-Back-N ARQ Example: The sender has sent frame 6 , and timer expires for frame 3( frame 3 has not been acknowledge); the sender goes back and resends frames 3, 4,5 and 6
- 20. Go-Back-N ARQ Normal operation How many frame scan be transmitted Without acknowledgment? ACK1 is not necessary if ACK2 is sent: Cumulative ACK
- 21. Go-Back-N ARQ Damage or Lost Frame Correctly received out of order packets are not Buffered What is the disadvantage of this?
- 22. 10. In Selective Repeat ARQ, the size of In the sender and receiver window must be at most one-half of 2m. Note
- 23. Selective Repeat ARQ Go-Back-N ARQ is inefficient of a noisy link. In a noisy link frames have higher probability of damage , which means the resending of multiple frames. this resending consumes the bandwidth and slow down the transmission . Solution: Selective Repeat ARQ protocol : resent only the damage frame It defines a negative Acknolgment (NAK) that report the sequence number of a damaged frame before the timer expires It is more efficient for noisy link, but the processing at the receiver is more complex
- 25. Selective Repeat ARQ m=3 Sequences no=2m =8 : 0,1,2 ,3,4,5,6,7Window size =2m/2= 8/2=4 Lost Frame