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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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