1. Understanding Networked Applications: A First Course Chapter 20 by David G. Messerschmitt

2. Understanding Networked Applications A First Course 2 Outline Industry structure Communication link characteristics Mitigating the impact of a link: –Compression –Caching –Mobile code

3. Understanding Networked Applications A First Course 3 Industry Structure At the link layer, bits are bits –Voice over IP –IP over voice New technologies for access links Deregulation Turbulent times ahead

4. Understanding Networked Applications A First Course 4 Gateway Public telephone network Internet Computer w/modem    IP over Voice Computer w/modem

5. Understanding Networked Applications A First Course 5 Gateway Public telephone network Internet IP telephone Plain old telephone    Voice over IP

6. Understanding Networked Applications: A First Course Communication Link by David G. Messerschmitt

7. Understanding Networked Applications A First Course 7 Simplest link: fiber optics Pulse of light = “1” Absence of pulse = “0” Speed of light

8. Understanding Networked Applications A First Course 8 Key Concepts Bitrate –bits accepted per unit of time Transmission time –Time to get all bits transmitted on link Propagation delay –Time for each bit to reach destination Message latency –transmission time + propagation delay

9. Understanding Networked Applications A First Course 9 Conveyer belt Bits waiting to be sent Bits that have already been received Conveyer belt analogy = “0” = “1” Constant speed

10. Understanding Networked Applications A First Course 10 Conveyer belt Conveyer belt parameters Speed = predetermined by Einstein Length = predetermined by distance Size of block Bits communicated per block Where we have control

11. Understanding Networked Applications A First Course 11 Determinants of Propagation Time Speed of propagation –speed of conveyor belt –limited by medium Distance –length of conveyor belt –closer is faster Remember: this is one link –Routers/switches introduce extra delay

12. Understanding Networked Applications A First Course 12 Determinants of Bitrate Bandwidth –size of blocks on conveyor belt –how rapidly the signal changes Spectral efficiency –number of bits written on each block –how many distinguishable signal levels –depends on s/n ratio of medium Popular usage: Bandwidth = bitrate

13. Understanding Networked Applications A First Course 13 Ways to increase bitrate Conveyer belt Make blocks smaller and put them on belt faster Use blocks with more shades of gray (more bits per block)

14. Understanding Networked Applications A First Course 14 Wavelength division multiplexing (WDM) Conveyer belt Stack blocks with different colors Today in commercial use: 40 wavelengths, each at 10 Gbps

15. Understanding Networked Applications A First Course 15 Numerical parameters Conveyer belt B = Bitrate = rate bits put on belt  = Propagation delay = distance/speed  B = Bitrate x delay = # bits in transit

16. Understanding Networked Applications A First Course 16 Transmit time Time Message latency Propagation latency Message in transit Distance The Graphical Version

17. Understanding Networked Applications A First Course 17 Message latency Message latency = Time to transmit message (m/B) + Time for last bit to reach destination (  ) These two components are equal when Message length = bitrate x delay (m =  B)

18. Understanding Networked Applications A First Course 18 Bitrate-limited case Propagation delay-limited delay

19. Understanding Networked Applications A First Course 19 Time m <<  B m >>  B Propagation delay-limited Bitrate-limited Entire message in transit most of the time Only part of message in transit at any time Distance 

20. Understanding Networked Applications A First Course 20 Two situations Bitrate limited –Increasing bit rate decreases message latency –True for long messages Propagation delay limited –Increasing bit rate has little impact on message latency –True to short messages Over time, with technology advances, we become delay limited!

21. Understanding Networked Applications A First Course 21 Assumption: v = c/2 = 1.5 x 10 8 meters/sec) Across chip One meter Across U.S.A. Halfway around world 28.8 kb/s <<1 b 891 b 4460 b 1.5 Mb/s <<1 b 4780 b 239 kb 622 Mb/s <<1 5 19 Mb 96 Mb B  = number of bits in transit Large! Bitrate (b/s) Distance

22. Understanding Networked Applications A First Course 22 Physical Media Wired –fiber optics –coax –twisted pair (copper) Wireless –Terrestrial radio –Satellite (long propagation delays)

23. Understanding Networked Applications A First Course 23 Fundamental limitation Over time –Bitrates increase with electronics and fiber optics advances –Message latencies become delay-limited –Further technological advances have little impact on application performance Exception: wireless

24. Understanding Networked Applications A First Course 24 Mitigating communication bottlenecks Bitrate-limited regime –Compression Delay-limited regime –Caching –Mobile code –In the future, these techniques will be very important

25. Understanding Networked Applications A First Course 25 Compression Lossless –Typically 2x or 3x Lossy (but imperceptible) –Easily 10x for audio and 100x for video –Discards perceptually unimportant information Important for today’s bitrate-limited regime

26. Understanding Networked Applications A First Course 26 Remote data being accessed Local cache Communication bottleneck Future references (where possible) Caching Predictive caching becomes very important in the delay-limited regime

27. Understanding Networked Applications A First Course 27 Communication bottleneck Execute locally Send code Mobile code Mobile code is especially useful to enable faster interaction in the delay limited regime

28. Understanding Networked Applications A First Course 28 1110100010110 1 1 101000 1 0 1 1 0 Bitrate-limited case Propagation delay-limited delay

29. Understanding Networked Applications: A First Course Supplements by David G. Messerschmitt

30. Understanding Networked Applications A First Course 30 Period T Frequency f = 1/T Time

31. Understanding Networked Applications A First Course 31 10 11 01 00 10 01 11 Time 10 11 01 00 10 01 11 Time Symbol interval

32. Understanding Networked Applications A First Course 32 Ethernet Hosts Switch Bus Broadcast Tree

33. Understanding Networked Applications A First Course 33 ProxyFilter Web server HTML or XML WML Wireless access protocol

34. Understanding Networked Applications A First Course 34 1 S C Hit rate H Access time 2C S-2C S-C S = 100  C  H = 0.989