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ECE 4450:427/527 - Computer Networks Spring 2014

ECE 4450:427/527 - Computer Networks Spring 2014. Dr. Nghi Tran Department of Electrical & Computer Engineering. Lecture 4: Network Performance Metrics. Some Discussions. Up to now, we have discussed on the functional aspects of network

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ECE 4450:427/527 - Computer Networks Spring 2014

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  1. ECE 4450:427/527 - Computer NetworksSpring 2014 Dr. Nghi Tran Department of Electrical & Computer Engineering Lecture 4: Network Performance Metrics ECE 4450:427/527

  2. Some Discussions • Up to now, we have discussed on the functional aspects of network • Certainly, when considering a network, we also need to evaluate how it performs: Important to understand various factors that impact network performance • Today, our focus will be on Network Performance Metrics ECE 4450:427/527

  3. Outline • Bandwidth/Throughput • Latency or Delay • High-speed Network • Application Performance Needs • Network Jitter ECE 4450:427/527

  4. Outline • Bandwidth/Throughput • Latency or Delay • High-speed Network • Application Performance Needs • Network Jitter ECE 4450:427/527

  5. Bandwidth/Throughput • In Electrical Engineering, what is Bandwidth? • In networking • Bandwidth is an amount of data transmitted per unit of time; per link, or end-to-end • 1Mbps = 106 bits per sec • It is sometimes useful to think of bandwidth in terms of how long it takes to transmit each bit of data: On 10-Mbs network, it takes 0.1 microsecond to transmit each bit Bits transmitted at a particular bandwidth can be regarded as having some width (a) 1Mbs- each bit is 1 microsecond wide (b) 2Mbs- 0.5 Smaller the width more will be transmission per unit time. ECE 4450:427/527

  6. Bandwidth/Throughput • What is throughput then? • Maximum data rate available? • Number of bits per second we actually can transmit? • Throughput: The measured performance of a system • Example: For a link with bandwidth 10Mbs, due to some impairments, we can only achieve a throughput of 2Mbs. ECE 4450:427/527

  7. Units of Networking Definition of • Mega • Kilo What are: • MB • Mbps • KB • kbps ECE 4450:427/527

  8. Outline • Bandwidth/Throughput • Latency or Delay • High-speed Network • Application Performance Needs • Network Jitter ECE 4450:427/527

  9. Delay/Latency • Time for sending data from one host to another (in sec, msec, or μsec) • Per link or end-to-end • Usually consists of • Tt: Transmission delay • Tp: Propagation delay • Tq: Queuing delay • Round Trip Time (RTT) : time to send a message a host to another and back • Important for flow control mechanisms ECE 4450:427/527

  10. Delay Calculation Tt: Transmission Delay: Tp: Propagation Delay: time needed for signal to travel the medium, Tq: Queuing Delay: time waiting in router’s buffer ECE 4450:427/527

  11. Example Transfer 1,5 MB file, assuming RTT of 80 ms, a packet size of 1-KB and an initial “handshake” of 2xRTT Bandwidth is 10 Mbps and data packets can be sent continuously A B request RTT reply confirm Ack Tt Tp . . . t ECE 4450:427/527

  12. Example Transfer 1,5 MB file, assuming RTT of 80 ms, a packet size of 1-KB and an initial “handshake” of 2xRTT After sending each packet must wait one RTT A B request RTT reply confirm Ack Tt RTT . . . t ECE 4450:427/527

  13. Example Suppose a 128-kbps point-to-point link is set up between the Earth and a rover on Mars. The distance from the Earth to Mars (when they are closest together) is approximately 55 Gm, and data travels over the link at the speed of light—3×10^8m/s. What is the minimum RTT for the link? A camera on the rover takes pictures of its surroundings and sends these to Earth. How quickly after a picture is taken can it reach Mission Control on Earth? Assume that each image is 5MB in size. ECE 4450:427/527

  14. Example Transfer 1,5 MB file, assuming RTT of 80 ms, a packet size of 1-KB and an initial “handshake” of 2xRTT Only 20 packets can be send per RTT, but infinitely fast B A request RTT reply confirm Ack RTT . . . t ECE 4450:427/527

  15. Example Transfer 1,5 MB file, assuming RTT of 80 ms, a packet size of 1-KB and an initial “handshake” of 2xRTT 1st RTT one packet, 2nd RTT two packets, Infinite transmission rate A B request RTT reply confirm Ack RTT . . . t ECE 4450:427/527

  16. Delay x Bandwidth • We think the channel between a pair of processes as a hollow pipe • Latency (delay) length of the pipe and bandwidth the width of the pipe • Delay of 50 ms and bandwidth of 45 Mbps • 50 x 10-3 seconds x 45 x 106 bits/second • 2.25 x 106 bits = 280 KB data: Amount of data channel can hold. Network as a pipe ECE 4450:427/527

  17. Delay x Bandwidth • How many bits the sender must transmit before the first bit arrives at the receiver if the sender keeps the pipe full • Takes another one-way latency to receive a response from the receiver: Usually, delay means RTT scenario • If the sender does not fill the pipe—send a whole delay × bandwidth product’s worth of data before it stops to wait for a signal—the sender will not fully utilize the network ECE 4450:427/527

  18. Delay x Bandwidth • Relative importance of bandwidth and latency depends on application • For large file transfer, bandwidth is critical • For small messages (HTTP, etc.), latency is critical • Variance in latency (jitter) can also affect some applications (e.g., audio/video conferencing) ECE 4450:427/527

  19. Examples ECE 4450:427/527

  20. Exercises • Calculate the delay x bandwidth using one-way delay, measured from first bit sent to last bit received: • 100-Mbps Ethernet with a delay of 10 micro second • 100-Mbps Ethernet with a single store-and-forward switch in the path and a packet size of 12,000 bits, 10 micro second per link propagation delay. It is also assumed the switch begins retransmitting immediately after it has finished receiving packet. ECE 4450:427/527

  21. Outline • Bandwidth/Throughput • Latency or Delay • High-speed Networks • Application Performance Needs • Network Jitter ECE 4450:427/527

  22. High-Speed Networks • Bandwidth available on today’s networks are dramatically increasing • In the following, we shall discuss: • What does this mean by high-speed • A better way to understand the relationship between throughput and latency ECE 4450:427/527

  23. High-Speed Networks • Of course, higher bandwidth usually means higher speed • But high speed does not mean latency can be improved at the same rate as bandwidth: • Why? Look at The transcontinental link • Speed of light: You cannot change the laws of physics ECE 4450:427/527

  24. Significance of High-Speed • We now consider an example to appreciate the significance of high-speed for a fixed latency • Considering to transfer 1-MB file over • 1Mbs link • 1Gbs link • The same RTT of 100ms • How many RTTs we need? ECE 4450:427/527

  25. Significance of High-Speed • 1-MB file looks like a stream of data over a 1-Mbs network, while it looks like a small package (1/12) on 1-Gbs lin • The point: 1-MB file to 1-Gbps link looks like a 1-KB packet to 1-Mbps link ECE 4450:427/527

  26. Effective End-to-End Throughput • We can have some fairer measurement when comparing networks: Effective end-to-end throughput • Throughput=TransferSize/TransferTime • TransferTime=RTT+1/Bandwidth x TransferSize • Example: 1MB file across 1Gbps line with 100ms RTT, Throughput is ? • Clearly, with high bandwidth, we need to • Transfer a larger file • RTT also dominates ECE 4450:427/527

  27. Outline • Bandwidth/Throughput • Latency or Delay • High-speed Networks • Application Performance Needs • Network Jitter ECE 4450:427/527

  28. Some Discussions • Up tonow, we have discussed the performance in terms of what a link/channel can support: • It is related to capacity of the channel • Users want as much bandwidth as the network can provide • Give me an example? • There are, however, different scenarios: • Applications are able to state an upper limit on how much bandwidth they need • Simple example? • The ability of network providing more bandwidth is of no interest ECE 4450:427/527

  29. Calculating Application Bandwidth • We consider a video stream application with one quarter size of standard TV screen, e.g., resolution of 352x240 pixels • Usually, how many bits needed to represent each pixel? • Then how many bits in each frame? • With 30 frames/second, what is the needed throughput? ECE 4450:427/527

  30. Further Discussions • The calculated bandwidth: An average • In reality, video is transmitted in a different way: Usually, compressed version is transmitted • Do you know how we can compress and transmit video? • Therefore, the instantaneous rate for each frame is different • Bandwidth needs may vary • Considering an average is usually not good enough (average over what?) • Another technique is specify upper limit (only what’s needed) • Establish a burst an application is likely to transmit • Example: Video on demand • We shall get in to detail of bursty traffic later ECE 4450:427/527

  31. Outline • Bandwidth/Throughput • Latency or Delay • High-speed Networks • Application Performance Needs • Network Jitter ECE 4450:427/527

  32. What is Network Jitter? • For Bandwidth: An application’s bandwidth needs can be something other than “all it can get” • Application’s delay requirement: More complex than simply “as little as possible” • Some cases, it does not matter so much whether the latency is 100 ms or 500 ms • What is of more interest: How much latency varies from packet to packet: The variation in latency is called JITTER ECE 4450:427/527

  33. Network-Induced Jitter • The spacing between when packets arrive at the destination: Inter-packet gap – Usually variable • It means delay experienced by sequence of packets: variable: We say network has introduced jitter in to the packet stream • Where does variation come from? Physical link? ECE 4450:427/527

  34. Network-Induced Jitter Video-on-demand application: If jitter is known, application can decide how much buffering is needed Example: jitter is 50ms per frame and 10s video at 30fps must be transmitted. How many frames needed to be bufferred? ECE 4450:427/527

  35. Recap • We defined CONNECTIVITY in a Network: • Packet switching with statistical multiplexing • We looked at NETWORK ARCHITECTURE • Layering • Protocols • Internet Architecture • Protocol Encapsulation Application Transport Network Link Physical ECE 4450:427/527

  36. Recap • We considered Network Performance Metrics • Bandwidth and Delay • Bandwidth x Delay • Bandwidth requirement varies from packet to packet • Delay can also varies from packet to packet • Now we move further to a very important part • Layer: Layering and Protocols • Our main focus: Internet • Approach: Bottom-up ECE 4450:427/527

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