1. COMT 4291 Performance of Data Communications Protocols General Protocol Concepts

2. COMT 4292 OSI Model for Networks 7Application Layer 6Presentation Layer 5Session Layer 4Transport Layer 3Network Layer 2Data Link Layer 1Physical Layer

3. COMT 4293 Flow Control Synchronize devices or routines at the sender and the receiver Basis for error recovery May exist in every protocol layer –Layer 2 for device to device –Layer 4 for user to user

4. COMT 4294 Flow Control Layers Transport Network Data Link Physical Data Link Physical Transport Network Data Link Physical

5. COMT 4295 Stop-and-Wait SenderReceiver A B C D H G I F J E

6. COMT 4296 Transmit SenderReceiver A B C D H G I F J E

7. COMT 4297 Propagate SenderReceiver A B C D H G I F J E

8. COMT 4298 Ack-Transmit SenderReceiver B C D H G I F J E Ack

9. COMT 4299 Ack-Propagate SenderReceiver B C D H G I F J E Ack

10. COMT 42910 Transmission Time Definition: the time required for the transmitter to create the electrical signals needed to completely represent the message Depends on the length (in bits) of the message and the circuit speed (bits/sec). (Transmission Time) = (Message Length)/(Circuit Speed)

11. COMT 42911 Propagation Time Data Link Layer Definition: the time required for the electrical signals to travel from sender to receiver Depends on the length of the circuit and the signal speed. (Propagation Time) = (Circuit Length)/(Signal Speed) Speed of Light –In Vacuum: 300,000 km/sec –In a typical wire: 200,000 km/sec

12. COMT 42912 Propagation Time Transport Layer Definition: the total time required for the packet to travel from sender to receiver Depends on all intermediate Data Link Layer propagation times as well as all queue and buffer delays. Also includes all transmission times except for the first one (from the sender to the first network device).

13. COMT 42913 Short Message or High Bit Rate SenderReceiver

14. COMT 42914 Long Message or Low Bit Rate SenderReceiver

15. COMT 42915 Stop and Wait Time Line Time Packet ready length=W bits Last bit transmitted Transmission Time t I Last bit received Ack returned Propagation Time t p Note: We ignore the length of the acknowledgement Next packet ready

16. COMT 42916 What is the transmission rate? In the diagram, “W” bits were transmitted (Message Length) The elapsed time was t i +2*t p Define B eff as the effective bit rate, B as the circuit speed, and Eff as the efficiency

17. COMT 42917 How do we get high efficiency? Note that t i is related to W and B

18. COMT 42918 Why are very large packets impractical? Intermediate routers may not be able to store the packet Sharing a circuit is impractical because long packets tie up the circuit for long times Error checks are performed on a per-packet basis; we may need to re-transmit the entire (large) packet after an error

19. COMT 42919 An aside … The time in the denominator is the total time elapsed from the start of transmission to the receipt of a “reaction” from the other side This time is more generally refered to as the Round Trip Time (RTT); for Transport Layer protocols this includes queue, buffer, and transmisson times as well.

20. COMT 42920 What is the answer? On fast circuits (or on circuits with high RTT) I need a large amount of data “in flight” We don’t want large packets Therefore, we have to allow transmission of additional packets before the first packet is acknowledged

21. COMT 42921 Sliding Window SenderReceiver A B C D H G I F J E 0 2 1 1 0 6 7 4 5 3

22. COMT 42922 Sliding Window SenderReceiver A B C D H G I F J E 0 2 1 1 0 6 7 4 5 3

23. COMT 42923 Sliding Window SenderReceiver A B C D H G I F J E 0 2 1 1 0 6 7 4 5 3

24. COMT 42924 Sliding Window SenderReceiver A B C D H G I F J E 0 2 1 1 0 6 7 4 5 3

25. COMT 42925 Sliding Window SenderReceiver B C D H G I F J E 0 2 1 1 0 6 7 4 5 3 Ack

26. COMT 42926 Sliding Window SenderReceiver B C D H G I F J E 0 2 1 1 0 6 7 4 5 3 Ack

27. COMT 42927 Sliding Window SenderReceiver C D H G I F J E 0 2 1 1 0 6 7 4 5 3 Ack