1. COMT 625 1 Performance of Data Communications Protocols General Protocol Concepts

2. COMT 625 2 Outline General protocol principles for flow control and error handling, including: Examples of protocols using these schemes Principles of interconnecting networks Bridges, Switches, Routers, Gateways

3. COMT 625 3 OSI Model for Networks 7Application Layer 6Presentation Layer 5Session Layer 4Transport Layer 3Network Layer 2Data Link Layer 1Physical Layer

4. COMT 625 4 OSI Model Implementation? Layer N Layer N-1 RequestRespond ConfirmIndicate

5. COMT 625 5 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

6. COMT 625 6 Flow Control Layers Transport Network Data Link Physical Data Link Physical Transport Network Data Link Physical

7. COMT 625 7 Stop-and-Wait SenderReceiver A B C D H G I F J E

8. COMT 625 8 Transmit SenderReceiver A B C D H G I F J E

9. COMT 625 9 Propagate SenderReceiver A B C D H G I F J E

10. COMT 625 10 Ack-Transmit SenderReceiver B C D H G I F J E Ack

11. COMT 625 11 Ack-Propagate SenderReceiver B C D H G I F J E Ack

12. COMT 625 12 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)

13. COMT 625 13 Propagation Time 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

14. COMT 625 14 Short Message or High Bit Rate SenderReceiver

15. COMT 625 15 Long Message or Low Bit Rate Sender Receiver

16. COMT 625 16 Examples

17. COMT 625 17 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

18. COMT 625 18 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

19. COMT 625 19 How do we get high efficiency? Note that t i is related to W and B

20. COMT 625 20 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

21. COMT 625 21 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 referred to as the Round Trip Time (RTT), and may include queue delays as well as multiple transmission and propagation times

22. COMT 625 22 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

23. COMT 625 23 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 625 24 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 625 25 Sliding Window SenderReceiver A B C D H G I F J E 0 2 1 1 0 6 7 4 5 3

26. COMT 625 26 Sliding Window SenderReceiver A B C D H G I F J E 0 2 1 1 0 6 7 4 5 3

27. COMT 625 27 Sliding Window SenderReceiver B C D H G I F J E 0 2 1 1 0 6 7 4 5 3 Ack

28. COMT 625 28 Sliding Window SenderReceiver B C D H G I F J E 0 2 1 1 0 6 7 4 5 3 Ack

29. COMT 625 29 Sliding Window SenderReceiver C D H G I F J E 0 2 1 1 0 6 7 4 5 3 Ack