1. ECE 4450:427/527 - Computer Networks Spring 2015
Dr. Nghi Tran
Department of Electrical & Computer Engineering
Lecture 4: Network Performance Metrics
Dr. Nghi Tran (ECE-University of Akron)
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
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

3. Outline Bandwidth/Throughput Latency or Delay High-speed Network
Application Performance Needs
Network Jitter
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

4. Outline Bandwidth/Throughput Latency or Delay High-speed Network
Application Performance Needs
Network Jitter
Dr. Nghi Tran (ECE-University of Akron)
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.
Dr. Nghi Tran (ECE-University of Akron)
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.
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

7. Units of Networking Definition of Mega Kilo What are: MB Mbps KB kbps
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

8. Outline Bandwidth/Throughput Latency or Delay High-speed Network
Application Performance Needs
Network Jitter
Dr. Nghi Tran (ECE-University of Akron)
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
Dr. Nghi Tran (ECE-University of Akron)
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
Dr. Nghi Tran (ECE-University of Akron)
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
Dr. Nghi Tran (ECE-University of Akron)
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
Dr. Nghi Tran (ECE-University of Akron)
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—3×10^8 m/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.
Dr. Nghi Tran (ECE-University of Akron)
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
Dr. Nghi Tran (ECE-University of Akron)
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
Dr. Nghi Tran (ECE-University of Akron)
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
Dr. Nghi Tran (ECE-University of Akron)
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
Dr. Nghi Tran (ECE-University of Akron)
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)
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

19. Examples Link Type Bandwidth Distance RTT Delay x BW Dial-up 56 kbps
10 km
87 μs
5 bits
Wireless LAN
54 Mbps
50 m
0.33 μs
18 bits
Satellite link
45 Mbps
35,000 km
230 ms
10 Mb
Cross-country fiber
10 Gbps
4,000 km
40 ms
400 Mb
Dr. Nghi Tran (ECE-University of Akron)
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.
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

21. Outline Bandwidth/Throughput Latency or Delay High-speed Networks
Application Performance Needs
Network Jitter
Dr. Nghi Tran (ECE-University of Akron)
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
Dr. Nghi Tran (ECE-University of Akron)
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
Dr. Nghi Tran (ECE-University of Akron)
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?
Dr. Nghi Tran (ECE-University of Akron)
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
Dr. Nghi Tran (ECE-University of Akron)
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
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

27. Outline Bandwidth/Throughput Latency or Delay High-speed Networks
Application Performance Needs
Network Jitter
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

28. Some Discussions
Up to now, 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
Dr. Nghi Tran (ECE-University of Akron)
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?
Dr. Nghi Tran (ECE-University of Akron)
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
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

31. Outline Bandwidth/Throughput Latency or Delay High-speed Networks
Application Performance Needs
Network Jitter
Dr. Nghi Tran (ECE-University of Akron)
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
Dr. Nghi Tran (ECE-University of Akron)
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?
Dr. Nghi Tran (ECE-University of Akron)
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?
Dr. Nghi Tran (ECE-University of Akron)
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
Dr. Nghi Tran (ECE-University of Akron)
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
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527