1. ELEN 602 Lecture 5 Review of last lecture –Error Detection -- parity/CRC etc. Automatic Retransmission Request (ARQ) Reading: Chap 5.1-5.2

2. 3 2 1 1 2 2 1 3 2 1 1 2 2 1 2 1 Medium 1 2 Physical layer entity Data link layer entity 3 Network layer entity Physical Layer Data link Layer Physical Layer Data link Layer A B A B Packets Frames (a) (b) Peer-to-peer protocol across a single hop

3. Physical Layer Data link Layer Physical Layer Data link Layer End system  Network Layer Network Layer Physical Layer Data link Layer Network Layer Physical Layer Data link Layer Network Layer Transport Layer Transport Layer Messages Segments End system  Network Peer-to-peer across a network -- Protocol stack View

4. 1 2 3 4 5 Data ACK/NAK Data 1 2 3 4 5 ACK/ NAK End-to-end Hop-by-hop End-to-end versus Hop-by-hop Approaches

5. Packet sequence Error-free packet sequence Information frames Control frames TransmitterReceiver CRC Information packet Header Station A Station B Information Frame Control frame CRC Header Basic elements of ARQ

6. (a) Frame 1 lost A B frame 0 frame 1 ACK frame 1 ACK time Time-out frame 2 (b) ACK lost A B frame 0 frame 1 ACK frame 1 ACK time Time-out frame 2 ACK In parts (a) and (b) transmitting station A acts the same way, but part (b) receiving station B accepts frame 1 twice. Possibilities with Unnumbered Frames

7. A B frame 0 frame 0 ACK frame 1 ACK time time-out frame 2 Transmitting station A misinterprets duplicate ACKs Unnumbered Acks

8. Transmitter Receiver S last R next 0 1 (0,0) (0,1) (1,0) (1,1) Timer Global State: (S last, R next ) Error-free frame 0 arrives at receiver ACK for frame 0 arrives at transmitter ACK for frame 1 arrives at transmitter Error-free frame 1 arrives at receiver Station A Station B R next S last System State Information in Stop-and-Wait ARQ

9. (a) Frame 1 lost A B frame 0 frame 1 ACK 1 ENQ ACK 1 time Time-out frame 1 (b) ACK for frame 1 lost A B frame 0 frame 1 ACK 1 ENQ ACK 0 time Time-out frame 0 ACK 0 Stop-and-wait ARQ Enquiry Frame

10. A B fr 0 time fr 1 fr 2 fr 3 fr 4 fr 5 fr 6 fr 3 ACK1ACK1 error Out-of-sequence frames Go-Back-4: 4 frames are outstanding; so go back 4 fr 5 fr 6 fr 4 fr 7 fr 8 fr 9 ACK2ACK2 ACK3ACK3 ACK4ACK4 ACK5ACK5 ACK6ACK6 ACK7ACK7 ACK8ACK8 ACK9ACK9 Basic Go-back-N ARQ

11. A B fr 0 time fr 1 fr 2 fr 3 fr 0 error Out-of-sequence frames 4 frames are outstanding; so go back 4 fr 2 fr 3 fr 1 fr 4 fr 5 fr 6 A B time fr 0 fr 0 error Time-out expires fr 1 ACK1ACK1 Stop-and-Wait Go-Back-N ACK1ACK1 ACK2ACK2 ACK3ACK3 ACK4ACK4 ACK5ACK5 ACK6ACK6 Go-back-N ARQ vs. Stop-and-wait ARQ

12. Timer S last S last +1 S recent S last +W s -1 Timer Transmitter Receiver... Receive Window Buffers S last S last +W s -1... Send Window S recent Frames transmitted and ACKed R next frames received The receiver will only accept a frame that is error-free and that has sequence number R next... Go-Back-N ARQ

13. A B fr 0 time fr 1 fr 2 fr 3 fr 0 fr 1 fr 2 fr 3 ACK1ACK1 M =2 2 = 4, Go-Back - 4: ACK4ACK4 ACK2ACK2 ACK3ACK3 Transmitter goes back 4 Receiver has R next =0, but it does not know whether its ACK for frame 0 was received, so it does not know whether this is the old frame 0 or a new frame 0 A B fr 0 time fr 1 fr 2 fr 0 fr 1 fr 2 ACK1ACK1 M=2 2 =4, Go-Back-3: ACK2ACK2 ACK3ACK3 Transmitter goes back 3 Receiver has R next =3, so it rejects the old frame 0 Window with fixed number of bits

14. A B fr 0 time fr 1 fr 2 fr 3 fr 4 fr 5 fr 1 fr 2 ACK1ACK1 error Out-of-sequence frames Go-Back-7: fr 4 fr 5 fr 3 fr 6 fr 7 fr 0 NAK1NAK1 ACK3ACK3 ACK4ACK4 ACK5ACK5 ACK6ACK6 ACK7ACK7 ACK2ACK2 Transmitter goes back to frame 1 NAK Error Recovery

15. Transmitter Receiver Transmitter Receiver ACKs are piggybacked in headers Station A Station B R A next “A” Receive Window R B next “B” Receive Window S A last S A last +1 S A recent S A last +W A s -1 Timer S A last +W A s -1... Buffers “A” Send Window... S B last S B last +1 S B recent S B last +W B s -1... Buffers “B” Send Window... S A recent R A next S B recent R B next Timer

16. TfTf TfTf T proc T prop T out How large a Timeout?

17. Transmitter Receiver Buffers S last S last +W s -1... Send Window S recent Frames transmitted and ACKed Timer S last S last +1 S recent S last +W s -1 Timer... Receive Window R next Frames received R next +W r -1 R next +1 R next +2 R next +W r -1... Buffers Selective Repeat ARQ

18. A B fr 0 time fr 1 fr 2 fr 3 fr 4 fr 5 fr 6 fr 2 ACK1ACK1 error fr 8 fr 9 fr 7 fr 10 fr 11 fr 12 ACK2ACK2 NAK2NAK2 ACK7ACK7 ACK8ACK8 ACK9ACK9 ACK10ACK10 ACK11ACK11 ACK12ACK12 ACK2ACK2 ACK2ACK2 ACK2ACK2 Error Recovery in Selective Repeat ARQ

19. A B fr 0 time fr 1 fr 2 fr 0 ACK1ACK1 M=2 2 =4, Selective Repeat: Send Window = Receive Window = 3 ACK2ACK2 ACK3ACK3 Receive Window {3,0,1} Frame 0 resent A B fr 0 time fr 1 fr 0 ACK1ACK1 Send Window = Receive Window = 2 ACK2ACK2 Receive Window{2,3} Frame 0 resent frame 0 rejected Maximum Window size in Selective Repeat ARQ

20. frame t f time A B t prop t ack t proc t prop t proc t0t0 Delay Components in Stop-and-Wait ARQ

21. Selective Repeat Go-Back-N Stop & Wait p  Transmission Efficiency of ARQ Protocols

22. nfnf  Selective Repeat Go-Back-N Stop &Wait 62 32 17 10 6 3 3 3 Optimum Frame Size