Transcript Document
Go-Back-N ARQ (Sliding Window Protocol) Go-Back-N is an instance of the Automatic Repeat-request (ARQ) Protocol, in which the sending process continues to send a number of frames specified by a window size even without receiving an ACK packet from the receiver. The receiver process keeps track of the sequence number of the next frame it expects, and sends it with every ACK it sends. If a frame from the sender does not reach the receiver, the receiver will stop acknowledging received frames. The sender after sending all of the frames will detect that all of the frames since the first lost frame are outstanding, and will go back to sequence number of the last ACK it received and fill its window starting with that frame and continue the process over again. Course Name: Networking Author(s) : Amitendu Panja, Veedhi Desai Mentor: Aruna Adil Level(UG/PG): UG *The contents in this ppt are licensed under Creative Commons Attribution-NonCommercial-ShareAlike 2.5 India license Learning Objectives After interacting with this Learning Object, the learner will be able to explain how Go-Back-N ARQ works. Understanding Go-Back-N ARQ: 1 2 Go-Back-N ARQ is a specific instance of automatic repeat request(ARQ) protocol which is an error recovery mechanism to provide reliability. objects with a span smaller than the structuring element. It is a special type of sliding window protocol. In this the sending process continues to send a number of frames specified by a window size even without receiving an acknowledgement (ACK) packet from the receiver. removes perimeter pixels from larger image objects. 3 In Go-Back-N ARQ, the size of the sender window must be less than N and the size of the receiver window is always 1. When the frame is damaged the sender goes back and sends a set of frames starting from the last one ACKn‘d. 4 The number of retransmitted frames is N. 1 Constraints taken while using Go-Back-N ARQ: 1. Sequence Number 2. Sender and Receiver Sliding Window 3. Acknowledgment 4. Resending Frames 2 Sender's Action: Once the sender has sent all of the frames in its window, it will detect that all of the frames since the first lost frame are outstanding, and will go back to sequence number of the last ACK it received from the receiver process and fill its window starting with that frame and continue the process over again. 3 Receiver's Action: 1.The receiver process keeps track of the sequence number of the next frame it expects to receive, and sends that number with every ACK it sends. 2.The receiver will ignore any frame that does not have the exact sequence number it expects – whether that frame is a 'past' duplicate of a frame it has already ACK'ed or whether that frame is a 'future' frame past the last packet it is waiting for. 4 Choosing a Window size (N): There are a few things to keep in mind when choosing a value for N. 1. The sender must not transmit too fast. N should be bounded by the receiver’s ability to process packets. 2. N must be smaller than the number of sequence numbers (if they are numbered from zero to N) to verify transmission in cases of any packet (any data or ACK packet) being dropped. 3. Given the bounds presented in (1) and (2), choose N to be the largest number possible. Master Layout 1 1 2 3 4 5 *Click on 'Start' to initiate animation *Click on 'Change Settings' to animation settings *Click on 'Help' for more information Step 1: 1 When user clicks “start” button 2 3 4 Instruction for the animator Text to be displayed in the working area (DT) • When the user clicks start, the sender starts transmitting the data. * Click on 'Stop' to stop animation • And the receiver starts giving acknowledgment to the sender. --------------------------- * Click on 'Pause' to pause animation - Case: Ideal 5 - Frames Sent:10 - Frame Lost: 0 1 Step 2: To change settings 2 3 4 Instruction for the animator 5 Text to be displayed in the working area (DT) • When the user selects ‘Change Settings’ , show a square box at that particular position. * Click on 'Start' to initiate animation • The text in DT of previous slide should be shown * Click on 'Help' for more information * Click on 'Change Settings' to animation settings 1 Step 3: To Stop the demo. 2 3 4 5 Instruction for the animator Text to be displayed in the working area (DT) • When the user clicks start button, there is a option to click “stop” button to stop the demo. * Click on 'Stop' to stop animation • The text in DT should appear in parallel to the figures --------------------------- * Click on 'Pause' to pause animation - Case: Lost Frames - Frames Sent:10 - Frames Lost: Every 3 frame 1 Step 4: When user clicks “Help” button 2 3 4 Instruction for the animator 5 Text to be displayed in the working area (DT) • When the user clicks “Help” button show a window shown above. * Click on 'Start' to initiate animation • The text in DT should appear in parallel to the figures * Click on 'Help' for more information * Click on 'Change Settings' to animation settings 1 Step 5: To Resume the Demo 2 3 4 Instruction for the animator • When the user clicks “Pause” there is an option for resuming the demo. • The text in DT should appear in parallel to the figures 5 Text to be displayed in the working area (DT) * Click on 'Stop' to stop animation * Click on 'Resume' to resume animation --------------------------- Case: Lost Frames - Frames Sent:10 - Frames Lost: Every 2 frame Electrical Engineering Slide 1 Slide 3 Introduction Definitions Slide 17 Slide 13-15 Analogy Want to know more… Test your understanding Lets Sum up (summary) (Further Reading) (questionnaire) Interactivity: In this LO demo itself is interactive session. Try it yourself 11 Credits Questionnaire 1 2 1.What is the size of the receiver window in Go-Back-N ARQ? Answers: a) 3 6 b) Changes Dynamically c) 1 d) Same as that of the sender's window 4 5 Questionnaire 1 2 2. From the given scenario which Frame is lost and from which frame the outstanding frames has to be retransmitted? Answers: a) Frame 0 3 b) Frame 1 c) Frame 2 4 5 d) Frame 3 Questionnaire 1 2 3. In a Go-Back-N ARQ, if the window size is 63, what is the range of sequence numbers? Answers: a) 0 to 63 3 b) 0 to 64 c) 1 to 63 4 5 d) 1 to 64 Links for further reading Reference websites: http://www.slideshare.net/soumya604159/ch-11-1751132 http://www.ee.iitb.ac.in/~sarva/EE706/EE706_Spring2012.html http://en.wikipedia.org/wiki/Go-Back-N_ARQ Books: 1. Communication Systems by Simon Haykin, fourth Edition. 2. Data and Computer Communications by William Stallings, eighth Edition.