Stop and Wait Automatic Repeat Request

Stop and Wait Automatic Repeat Request
This is a simple data link layer protocol. It is implemented using acknowledgement for error free frame and retransmission on timeout. it is further optimized using sequence numbers in frames and acknowledgements. Course Name: Communication Networks Authors Amita Shinde Mentor Prof. Sarvanan

Learning Objectives After interacting with this Learning Object, the learner will be able to; Describe simple Automatic Repeat Request Stop and Wait Protocol Identify various problems occurring during transmission while implementing Stop and Wait ARQ and mechanisms to solve them. Estimating timeout period for optimum transmission.

Theory button content :Transmitter Flowchart
Start N=0 Transmit Nth Frame + Seq. No. N Start Timer Received Ack.? No No Yes Error Free ACK? Timeout? No Yes Yes Is ACK Seq. No. = N+1 No Yes N= N+1 End of Frames? No Yes Note for animator: Colours/ positions/design etc. are suggestive and will be finalized by designer Stop

Theory button content :Receiver Flowchart
Start Next Frame Expected No. = N Received Frame.? No Yes Is Frame Error Free ? No Yes Reject Frame Is Frame Seq. No. =N? No Yes Accept Frame ‘N’ Transmit Acknowledgement + Seq. No. N+1 N = N+1 Note for animator: Colours/ positions/design etc. are suggestive and will be finalized by designer

Master Layout 1 Transmitter Receiver Error Check Error Check 01 Source
Destination C R C1 C2 ACK C R 1 Frame path Error Check C R Frame channel C3 C4 Ack channel Error Check Ack path Transmitter ACK C R 1 Receiver Time 01 Time-line

Elements of Master Layout 1
Transmitter time line Arrow a1 Arrow a2 Arrow a3 Arrow a4 Timeline 1 Timeline 2 Timeline 3 Timeline 4 Arrow f1 Arrow f2 Arrow f3 Arrow f 0 Receiver time line C R H Frame C R C R Frame 0 Packet 1 C R C R Frame 1 H Header ACK C R 1 Ack with sequence no. C R 2 C R Frame 2 C R CRC 1 Ack Sequence No. ACK C R H Ack 3 C R C R Frame 3 Frame Sequence no. ACK Ack Packet C R Frame with sequence no. Note for animator: Colours/ positions/design etc. are suggestive and will be finalized by designer

Elements of Master Layout 1
P1 P2 Image i/p Image o/p P3 P4 Instructions to illustrator: Image i/p & o/p should be same size. P1,p2,p3,p4 are parts of i/p image…all of equal size & 1/4th of i/p image i/p image & p1,p2,p3,p4 are same for all screens. Only, o/p image may vary as shown. P1,p2,p3,p4 make o/p image Note for animator: Colours/ positions/design etc. are suggestive and will be finalized by designer

Master Layout 2 Transmitter Receiver Error Check Expected 1 Ack. No
Source Destination Duplicate ack Duplicate frame status message : indicates duplicate ackin ack error, frame & ack seq. no. early timeout status message : indicates duplicate frames in ack error, early timeout etc. Noise image ( like lightening) In frame error, ack error , early timeout & next frame error C R C1 Next Frame Expected C2 ACK C R 1 Frame path Error Check C R 1 Expected Ack. No Frame channel C3 C4 Ack channel Error Check Ack path Transmitter ACK C R 1 Receiver Time 01 Timeout status message : indicates timeout event In frame error onwards Time-line Note for animator: Colours/ positions/design etc. are suggestive and will be finalized by designer

Elements of master layout 2 in addition to the master layout 1
Duplicate frame Noise Duplicate ack or Timeout Noise image ( like lightening) In frame error, ack error , early timeout & next frame error status message : indicates duplicate ackin ack error, frame & ack seq. no. early timeout status message : indicates duplicate frames in ack error, early timeout etc. status message : indicates timeout event In frame error onwards Transparent color used to show that frame is passing ‘through’. A rectangle or hollow cube can be show. Compare Consecutive Error Check No. indicating next frame expected. Start with 0 increment by 1 up to 3 as every frame passes through it. The number digit in frame should coincide with this digit while passing. No. indicating next ack expected. Start with 1. increment as every ack passes through it up to 3 The number digit in frame should coincide with this digit while passing. Next Frame Expected 1 Expected Ack. No Note for animator: Colours/ positions/design etc. are suggestive and will be finalized by designer. Refer to these elements from step 4 onwards. All elements are not used at every step. Elements are added at every step as shown in the step itself.

List of animations: Step 3 Ideal Case Step 4 Frame Error Step 5
Acknowledgement Error Step 6 Early Timeout Step 8 Comparison of consecutive content! Step 9 Frame Sequence Number Step 10 Frame Sequence Number: Acknowledgement Error Step 11 Frame Sequence Number: Early Timeout Frame Sequence Number: Early Timeout & Next Frame Error Step 12 Step 13 Frame & Acknowledgement Sequence Number Step 14 Frame & Acknowledgement Sequence Number: Early Timeout & Next Frame Error Step 15 1 bit sequence number Note for animator: This is for animator’s reference. Can be used in drop-down menu.

Step 1 Introduction The basic principle behind automatic repeat request (ARQ) is to initiate frame retransmissions when errors are detected in the received frame. Retransmissions are initiated using a combination of timeouts and acknowledgements. Stop-and-wait ARQ (SW ARQ) is the simplest ARQ protocol. When a source node sends a frame, it starts a timer and waits for an acknowledgement (ACK) from the destination. If the destination node receives a frame in which no errors are detected, it sends an ACK frame back to the sender informing it of error-free frame reception. Once an ACK for a frame arrives at the source, the next frame is transmitted. If errors are detected in the frame, then the destination does nothing. If the timer at the source exceeds a predetermined threshold, a timeout event is declared and the frame is resent. This simple process further imposes some problems, you can learn about these problems and the mechanisms used to solve them, step by step. Instructions to animator: This will be the very first introduction screen as the user opens this animation.

Step 2 Transmitter Receiver
The image is to be accurately transmitted from source to destination using Stop and Wait ARQ. The image is divided in four parts & transmitted as four consecutive frames. Observe how Stop and Wait ARQ protocol achieves accurate transmission of this image to the destination. Error Check Error Check Transmitter Receiver Transmitter Receiver

Step 3 Ideal Case H H H H H H H H Transmitter Receiver Error Check
ACK C R ACK C R ACK C R ACK C R Transmitter Receiver Transmitter Receiver

Step 3 Sequence Description of Action/Interactivity
Ideal Case Sequence Description of Action/Interactivity Instructions to animator 1 Continue from step 2 Refer to master layout1 for images etc 2 In i/p image , show P1 selected (rectangle) 3 Show P1 appear down at the shown position 4 P1 transforms into packet with animation 5 Header rectangle appears 6 CRC rectangle appears Now Packet, Header & CRC should become one object i.e. frame 7 Start moving frame to right 8 As soon as it touches C1( its rightmost boundary touches channel’s left boundary); Move frame through channel Show arrow f0 starting from top left Timeline 1 starts X –coordinates of timeline 1 & arrow f0 should be same at every instance 9 Frame reaches C2 Arrow f0 completes 10 Frame moves further into receiver

Step 3 Seq. Description of Action/Interactivity
Ideal case Seq. Description of Action/Interactivity Instructions to animator 11 Frame passes from ‘Error Check’ block CRC rectangle disappears It should look like passing ‘though’ 12 Header & packet reach at shown position. They also disappear P1 appears at shown position in receiver 13 Ack appears at shown position in receiver 14 Ack starts moving to left 15 Ack touches C4, Arrow a1 starts from bottom left Now, X –coordinates of timeline 1 & arrow a1 should be same at every instance 16 Ack crosses point C3 completely Arrow a1 completes. 17 Ack passes through ‘Error check’ block in transmitter CRC disappears 18 Timeline 1 completes Ack disappears 19 Repeat sequence 2 to 18 for image P2, packet, header, frame, arrow f1, timeline 2, arrow a2, ack X –coordinates of timeline 2 & arrow a2 or f1 should be same at every instance 20 Repeat sequence 2 to 18 for image P3, packet, header, frame, arrow f2, timeline 3, arrow a3, ack 21 Repeat sequence 2 to 18 for image P4, packet, header, frame, arrow f3, timeline 4, arrow a4, ack 22 Show text’ Image reached accurately’ Follow the style of animation for all next cases

Step 4 Frame Error Error Check Error Check Receiver Transmitter Noise
Don’t stack the Frames. Make them disappear after reaching here and make p1 through p4 appear ACKs don’t exist here from beginning. They appear 1 by 1 & move ACK ACK C C ACK Don’t stack the ACK. Make them disappear after reaching here C ACK C C C Noise Error Check Error Check Receiver Transmitter Timeout

Step 4 Description of Action/Interactivity Text to be displayed
Frame Error Description of Action/Interactivity Text to be displayed For all frames that cross the ‘error check’ block & reach their position in receiver block, Make the frame disappear & corresponding part of o/p image (frame 0=p1,frame 1=p2, frame 2=p3,frame3=p4) appear When the retransmission occurs, Show T1 when noise strikes Show T2 when frame rejected Then show T3 before & during timeout Show T4after timeout with retransmission Show ‘Next’ button after retransmit ack received Show T5 when destination image complete Show T6. words ‘acknowledgement error’ are highlighted as they indicate another animation menu from drop down list. T1: Frame is corrupted T2:…hence rejected T3: No acknowledgement! T4: Frame retransmitted T5: Image received accurately! T6: The transmitter retransmits a frame when ‘Timeout’ occurs. This mechanism is useful to handle a frame error . Is it equally useful for an acknowledgement error?

Step 5 Acknowledgement Error Find Solution Error Check Error Check
Don’t stack the Frames. Make them disappear after reaching here Step 5 Acknowledgement Error Duplicate frame ACK ACK C C C ACK ACK C ACK C C Don’t stack the ACK. Make them disappear after reaching here C Error Check Error Check Noise Receiver Transmitter Timeout Find Solution

Step 5 Acknowledgement Error Description of Action/Interactivity
Text to be displayed For all frames, When the frames reach their last point in receiver, make then disappear and make corresponding part of o/p image p1,p2,p3,p4)appear in receiver. When the retransmission occurs, Show T1 when 1st frame received. Make the frame disappear and make P1 appear Show T2 when noise strikes Show T3 when frame rejected Then show T3 before & during timeout Show T4 after timeout with retransmission Show T5 Show T6 at end of animation & ‘Find Solution’ action button T1: Frame received correctly T2: Noise strikes the acknowledgement T3:…hence acknowledgement rejected T4: Frame retransmitted T5: Duplicate frame received! T6: A duplicate fames was received twice. How to solve this problem? Interactivity Boundary Limit Result Instructions for learner action button ‘Find Solution Activate after T5 Go to step 7 when clicked Click here to find solution to the problem of duplicate frames

Step 6 Early Timeout Find Solution Duplicate frame Error Check
ACK C C C C C Error Check Error Check Receiver Transmitter Timeout Find Solution

Step 6 Description of Action/Interactivity Text to be displayed
Early Timeout Description of Action/Interactivity Text to be displayed For all frames that cross the ‘error check’ block & reach their position in receiver block, Make the frame disappear & part of o/p image (frame 0=p1, frame 1=p2, frame 2=p3, frame 3=p4) appear. Only 1st frame & its retransmission is shown here. But also show 2nd, 3rd, frame like step 5 When the retransmission occurs, Show T1 when 1st frame received Show T2 after timeout with retransmission Show ‘Next’ button after retransmit ack received Show T3 Show T4 after entire image received.] Show T5 and action button ‘Find Solution ‘ T1: Frame received correctly T2: Frame retransmitted T3: Duplicate frame received! T4: Inaccurate image reached destination! T5: A duplicate fames was received twice. How to solve this problem? Interactivity Boundary Limit Result Instruction to learner (optional) Clickable action button ‘Find Solution Show after T5 If clicked Go to step 7 Click here to find solution to the problem of duplicate frames

Add a sequence no. to each frame & verify the same at the receiver..
Step 7 Can you suggest a solution to solve this problem? Compare content of consecutive frames at receiver & discard if it is same. Add a sequence no. to each frame & verify the same at the receiver.. Interactivity Boundary Limit Result Instructions to animator Instructions to learner Two selectable options: Action buttons Only one selected at a time. 2nd option is correct. 1st selected: Show comparison (step 8) 2nd selected: Show sequence numbers (step 9) Don’t show –ve feedback. Show only positive feedback for 2nd option Click on the correct option

Step 8 Comparison Transmitter Receiver H H H H Error Check Error Check
Buffer Duplicate Error Check H C R H C R H C R H C R Error Check ACK C H ACK C H ACK C H ACK C H

Step 8 Comparison Description of Action/Interactivity
Text to be displayed The animation is shown here considering parts of another image (chessboard). Show the same using parts of i/p image used (4 of diamond card)…p1,p2,p3,p4 When the retransmission occurs, Show T1 when 1st frame received Show T2 when 2nd frame received show T3 when 3rd frame rejected Show ‘Next’ button after duplicate content Show T4 when 4nd frame received Show T5 when destination image complete Show T6. Words ‘sequence numbers’ are highlighted as they indicate another animation menu from drop down list. T1 New content, hence accepted. Content buffered for further comparison! T2: New content, hence accepted. T3: : Duplicate content, hence rejected T4: New content, hence accepted. T5: Inaccurate image reached destination! T6: Now try to check if adding sequence numbers to the frames would solve this problem!

Step 9 Frames with Seq. No. Transmitter Receiver Error Check
Expected Fame No. =0 Expected Fame No. =1 Error Check C R C R Error Check ACK C R

Step 9 Frames with Seq. No. Description of Action/Interactivity
Text to be displayed Only first frame & corresponding image part is shown animated here. Similarly show remaining 3. Similar to step 3 (+ expected frame no. increment while every ACK is generated) Show T1 when frame is created & keep till it is received( Keep as long as frame’0’ remains on frame) Show T2 every time a frame is received (for all 4 frames) Show T3 at the end of animation T1: Frame with sequence number ‘0’ T2: Sequence no. matched! Frame accepted! T3: Will the frames with sequence number also be duplicated if there is an acknowledgement error or early timeout?

Step 10 Frame Sequence No. : Acknowledgement Error 1 2 3 Error Check
No match ACK ACK ACK C C C C C C ACK C ACK C C Next Frame Expected C 1 2 3 Error Check Error Check Noise Receiver Timeout Image reached destination successfully! Frame with sequence no. 1 retransmitted Frame sequence no. not matched Frame with Sequence no. 2 received Frame sequence number ‘0’ received successfully! Acknowledgement received successfully! Frame sequence number ‘1’ received successfully! Acknowledgement corrupted , hence rejected! Acknowledgement received successfully!

Step 10 Frame Sequence No. : Acknowledgement Error
Description of Action/Interactivity Text to be displayed For all frames that cross the ‘error check’ & ‘expected frame ‘ block & reach their position in receiver block, Make the frame disappear & corresponding part of o/p image (p1,p2,p3,p4) appear. Also, make the ‘ack’ disappear after they reach the last point in transmitter block. Show T1 when frame 0 is created & keep till it is received( Keep as long as frame’0’ remains on screen) Show T2 for ack1 Show T3 for frame 1 Show T4 for ack 2 (noise strikes) Show T5 when frame 1 sent again Show T6 when frame 1 thrown Show T7 for ack2 Show T8 at the end T1: Frame with sequence number ‘0’ received & accepted! T2: Ack received & accepted! T3: Frame with sequence number ‘1’ received & accepted! T4: Error in ACK! ACK rejected ! T5: Frame 1 retransmitted T6: Frame sequence no. not matched! Frame rejected T7: ACK received &accepted T8: Image reached destination successfully The text to be displayed is shown in animation. At every shown instance, overwrite old text with new text. (old text disappears & new text appears)

Step 11 1 2 3 Frames with Sequence No.: Early Timeout Error Check
No Match C C C C C C Next Frame Expected 1 2 3 C c Error Check Error Check ACK ACK ACK ACK ACK Receiver Timeout Timer

Step 11 Frame Sequence No. Early Timeout
Description of Action/Interactivity Text to be displayed For all frames that cross the ‘error check’ & ‘expected frame ‘ block & reach their position in receiver block, Make the frame disappear & corresponding part of o/p image (p1,p2,p3,p4) appear. Also, make the ‘ack’ disappear after they reach the last point in transmitter block. Complete animation sequence is shown . Include above instructions to the same. Timer (circle) is used for time synchronization. However it can be replaced with counter on the timing diagram. Show T1 when frame is received Show T2 for timeout Show T3 for frame 1transmission Show T4 for frame ‘0’ (duplicated) rejected! Show T5 when ACK 1 (duplicate) rejected Show T6 when frame 2 transmitted Show T7 at the end Show T8 T1: Frame sequence number ‘0’ received & accepted! T2: Timeout! Frame ‘0’ retransmitted! T3: ACK ‘1’ received! Frame 1 transmitted! T4: Duplicate frame ‘0’ rejected! T5: Duplicate ACK ‘1’ rejected! T6: ACK ‘2’ received! Frame ‘2’ transmitted! T7: The image accurately sent to destination! T8: Can we have accurate transmission using frame sequence number even when early timeout occurs followed by error in the next frame?

Step 12 1 Frames with Sequence No.: Early Timeout, Next Frame Error
No Match No Match ACK ACK C C C C c C C c Next Frame Expected 1 c C Error Check Error Check Transmitter Receiver Timeout Timeout Timer

Step 12 Frames with Sequence No.: Early Timeout, Next Frame Error
Description of Action/Interactivity Text to be displayed For all frames that cross the ‘error check’ & ‘expected frame ‘ block & reach their position in receiver block, Make the frame disappear & corresponding part of o/p image (p1,p2,p3,p4) appear. Also, make the ‘ack’ disappear after they reach the last point in transmitter block. Show T1 when frame is created & keep till it is received( Keep as long as frame’0’ remains on screen) Show T2 after timeout Show T3 for frame 0 rejected Show T4 for frame 1 at receiver Show T5 when frame 2 sent Show T6 when frame 2 thrown Show T7 T8 at the end T1: Frame with sequence number ‘0’ received & accepted! T2: Timeout! Frame ‘0’ retransmitted! T3: This is not expected frame! T4:Error in frame ‘1’ ! T5: Ack is assumed to be for frame ‘1’ and frame ‘2’ transmitted T6: This is not the expected frame T7: In this way, the system gets stuck in the same loop The image can not each destination accurately! T8: Observe if using sequence numbers in frames as well as acknowledgements can solve this problem!

Step 13 Frames and Acknowledgements with Sequence No. 3 4 1 2 1 4 3 2
Next Ack Expected Next Frame Expected 2 1 4 3 2 C Error Check Error Check Transmitter Receiver

Step 13 Frames and Acknowledgements with Sequence No
Description of Action/Interactivity Text to be displayed For all frames that cross the ‘error check’ & ‘expected frame ‘ block & reach their position in receiver block, Increment ‘Next Frame Expected’ number Make the frame disappear & corresponding part of o/p image (p1,p2,p3,p4) appear. For all ’ack’ that cross the ‘error check’ & ‘expected ack ‘ block & reach their position make the ‘ack’ disappear Increment .’expected ack no. Show T1 for frame 0 Show T2 for ack1 Show T3 for frame 1 Show T4 for ack 2 Show T5 for frame 2 Show T6 for ack 3 Show T7 for frame 3 Show T8 for ack 4 Show T9 and T10 at the end! T1: Frame sequence number ‘0’ received & accepted! T2: Ack received & accepted! T3: Frame sequence number ‘1’ received & accepted! T4: Ack received & accepted! T5: Frame sequence number ‘2’ received & accepted! T6: Ack received & accepted T7: Frame sequence number ‘3’ received & accepted! T8: Ack received & accepted T9: The image reached destination successfully! T10: Now, see if this mechanism is useful when there is early timeout with next frame in error!

Step 14 Frames and Acknowledgements with Sequence No. : Early Timeout, NFE No Match No Match ACK 1 ACK 1 C C C C C C 1 2 Next ACK Expected Next Frame Expected 1 C C Error Check Error Check Receiver Timeout Timeout Timeout

Step 14 Frames and Acknowledgements with Sequence No. : Early Timeout, NFE Description of Action/Interactivity Text to be displayed (numbered T1,T2 etc.) For all frames that cross the ‘error check’ & ‘expected frame ‘ block & reach their position in receiver block, Make the frame disappear & corresponding part of o/p image (p1,p2,p3,p4) appear. Also, make the ‘ack’ disappear after they reach the last point in transmitter block. Show T1 as frame’0’ received Show T2 for timeout & 0frame 0 retransmit Show T3 for frame 0 (duplicate) rejected Show T4 for frame 1 Show T5 when ack 0 (duplicate) thrown Show T6 for frame1 thrown from error check block Show T7 from frame 1 retransmit Halt here or Repeat early timeout case for frame 1, 2,3. but don’t repeat the frame error case Show T8 at the end! Show T9 T1: Frame sequence number ‘0’ received & accepted! T2: Timeout! Frame. ‘0’ retransmitted T3: This is not the expected frame! Discarded! ACK sent! T4: Frame 1 transmitted T5: This is not the expected ACK! Discarded! T6: Error detected in frame ‘1’! Rejected! T7: Timeout! Frame ‘1’ retransmitted! T8 : In spite of early timeout and also next frame being in error, the image is finally received at the destination accurately! T9: The sequence numbers can be reduced to 1 bit!

Step 15 Frames and Acknowledgements with 1 bit Sequence No. 1 1
C C C ACK 1 c ACK C 1 Next Ack Expected Next Frame Expected 1 C Error Check Error Check Transmitter Receiver

Step 15 Frames and Acknowledgements with 1 bit Sequence No
Description of Action/Interactivity Text to be displayed For all frames that cross the ‘error check’ & ‘expected frame ‘ block & reach their position in receiver block, toggle ‘Next Frame Expected’ number (0 to 1 or 1 to 0) Make the frame disappear & corresponding part of o/p image (p1,p2,p3,p4) appear. For all ’ack’ that cross the ‘error check’ & ‘expected ack ‘ block & reach their position make the ‘ack’ disappear toggle.’expected ack no. Show T1 as (frame’0’) received Show T2 as frame1 received Show T3 as frame 2 received Show T4 as frame 3 received Show T5 after animation over T1: Frame sequence number ‘0’ received & accepted! T2: Frame with sequence no. ‘1’ received and accepted T3: Frame sequence number ‘0’ received & accepted! T4: Frame with sequence no. ‘1’ received and accepted T5: Image reached destination accurately

Interactivity Step 1 .Observe the detailed timing diagram of frame transmission. Tf+Tp+2τ Error Check t=0 ACK C R H Tp Tf τ τ Ta Frame Ack Error Check Ta+Tf+Tp+2τ Tf+τ C R H C R C R H Tf+Tp+τ τ Ta+Tf+Tp+τ Receiver Transmitter RTT = 2τ + Tf + Ta + Tp Glossary

Interactivity Step 2 Seq Description of Action/Interactivity
Boundary Limit Result Instructions to learner Link to ‘Glossary’ Selectable link (clickable) When selected, link to ‘Glossary’ Refer to glossary for meanings of important terms Seq Description of Action/Interactivity Instructions to animator 1 Header rectangle appears 2 CRC rectangle appears Now Packet, Header & CRC should become one object i.e. frame 3 Start moving frame to right 4 As soon as frame it touches transmitter block’s boundary ( its rightmost boundary touches channel’s left boundary); Move frame through channel Show left most arrow starting from top left Timeline starts Start all other arrows one by one. X –coordinates of timeline & arrows should be same at every instance 5 Frame reaches C2 1st arrow completes X –coordinates of timeline & arrow should be same at every instance 6 Frame moves further into receiver Other arrows complete one by one

Interactivity Step 2 Seq Description of Action/Interactivity
Instructions to animator 7 Frame reaches last position. All frame arrows complete. 8 ACK starts moving to left. 9 As soon as ACK touches receiver block’s boundary ( its left boundary touches channel’s right boundary); Move ACK through channel Show left most ‘ACK’ arrow starting from down left Start all other arrows one by one. X –coordinates of timeline & arrows should be same at every instance 10 ‘ACK’ reaches transmitter boundary 1st arrow completes X –coordinates of timeline & arrow should be same at every instance 11 ‘ACK’ moves further into transmitter Other arrows complete one by one 12 ‘ACK’ reaches last position. All frame arrows complete.

Interactivity Step 2 Timeout duration 80 100 120
When this image is transmitted using simple stop and wait ARQ, an error is detected in the 2nd frame. The round trip time (RTT) for each frame is 100. Three options of timeout duration (80,100,120) are available. Observe the transmission using all 3 values one by one. Does this value affect the image reaching destination? According to you, which value gives maximum throughput with accurate transmission? Error Check Timeout duration 80 100 120 Error Check Transmitter Receiver Transmitter Receiver

Interactivity Step 2 Interactivity Boundary Limit Result
Instructions to learner Selection of timeout value List of values User selects a value Select a value for timeout duration. Play button Activate when values selected The animation will start . It will take the time value depending upon user entered values Click on play to view animation 80 Show animation as early timeout/ early timeout with next frame error case 100 Show animation like ideal case/ frame error 120 Show animation like ideal case/ frame error with late retransmission

Glossary button content
(Propagation Delay): Time taken by a single bit to reach from one node to the other, say from source to destination τ Tf (Transmit Duration of the frame): Time required to put all frame bits on the channel. Ta (Transmit duration of the ACK): Time required to put all ACK bits on the channel. Tp (Processing delay): Delay between the reception of the frame at the destination and the start of the ACK transmission. RTT (Round Trip Time): Time taken to transmit a frame and receive an acknowledgement when no errors are introduced by the channel. Round-trip time (RTT)= Propagation delays + Transmit duration + Processing delays RTT = 2τ + Tf + Ta + Tp

Questionnaire Q 1 A timer is started at the beginning of frame transmission. What should be the duration for timeout? Timeout duration > Round Trip Time (RTT) Timeout duration < Round Trip Time (RTT) Instructions to animator: 1st answer (in green) is correct. Show positive feedback if correct answer is selected. Give negative feedback for wrong answer selected. Use same interactivity for all questions ( either the suggested animation or radio button etc.)

Questionnaire Q 2 Destination image 4 packets transmitted Source image
Observe the images. The source image is transmitted in 4 consecutive packets without sequence numbers. However, the image generated at destination is shown here. It doesn’t match with the source image exactly. Which sequence of events would have caused this? 4 packets transmitted Destination image Source image Error detected in ACK and next frame Early timeout and error detected in retransmitted frame Error detected in ACK Early timeout and error detected in next frame Instructions to animator: 4th is correct answer. Show positive feedback if correct answer is selected. Give negative feedback for wrong answer selected.

Questionnaire Q 3 When frames and acknowledgements are sent on a noiseless channel with sequence numbers and timeout duration is less than half of RTT; Every frame will be transmitted only once. Every frame will be transmitted at least thrice. 1st frame will be transmitted twice. 1st frame will be transmitted at least thrice and next frames only once.. Instructions to animator: 2nd answer (in green) is correct. Show positive feedback if correct answer is selected. Give negative feedback for wrong answer selected. Use same interactivity for all questions ( either the suggested animation or radio button etc.)

Questionnaire Q 4 The following image is transmitted in four parts as shown; using a mechanism for comparing consecutive frame content. How would received image look? a b c d Instructions to animator: ‘d’ is correct answer. Show positive feedback if correct answer is selected. Give negative feedback for wrong answer selected. Use same interactivity for all questions ( either the suggested animation in Q1 or radio button etc.)

Questionnaire Q 5 Match the problem with appropriate mechanism used to solve the problem. 1. Frame received with error A. Frame sequence number 2. A frame duplicated and next frame lost B. Retransmit frame 3. Duplicate frames received C. Frame and ACK sequence number 1-A, 2-B, 3-C Instructions to animator: ‘3rd option (in green) is correct answer. Show positive feedback if correct answer is selected. Give negative feedback for wrong answer selected. Use same interactivity for all questions ( either the suggested animation in Q1 or radio button etc.) 1-C, 2-B, 3-A 1-B, 2-C, 3-A 1-B, 2-A, 3-C

References Alberto Leon-Garcia, Indra Widjaja, Communication Networks: Fundamental Concepts and Key architectures, Tata McGraw-Hill, 2nd edition, 2004. Andrew S. Tanenbaum, Computer networks, Pearson Prentice Hall, 4th edition,2003.

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Stop and Wait Automatic Repeat Request

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