ECE 4450:427/527 - Computer Networks Spring 2014

ECE 4450:427/527 - Computer NetworksSpring 2015

Dr. Nghi TranDepartment of Electrical & Computer Engineering

Lecture 4: Network Performance Metrics

Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527 Computer Networks 1

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

Outline


• Bandwidth/Throughput

• Latency or Delay

• High-speed Network

• Application Performance Needs

• Network Jitter

Outline






• Network Jitter

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-end1Mbps = 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.

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.

Units of Networking


What are:• MB• Mbps • KB• kbps

Definition of• Mega• Kilo

Outline






• Network Jitter

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

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

Example


Transfer 1,5 MB file, assuming RTT of 80 ms, a packet size of 1-KB and an initial “handshake” of 2xRTTBandwidth is 10 Mbps and data packets can be sent continuously

RTT

. . .

Tt

Tp

request

replyconfirmAck

A B

t

Example


Transfer 1,5 MB file, assuming RTT of 80 ms, a packet size of 1-KB and an initial “handshake” of 2xRTTAfter sending each packet must wait one RTT

RTT

. . .

Tt

request

replyconfirmAck

A B

t

RTT

ExampleSuppose 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.


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

RTT

. . .

request

replyconfirmAck

A B

t

RTT

Example


Transfer 1,5 MB file, assuming RTT of 80 ms, a packet size of 1-KB and an initial “handshake” of 2xRTT1st RTT one packet, 2nd RTT two packets, Infinite transmission rate

RTT

. . .

request

replyconfirmAck

A B

t

RTT

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 MbpsÞ 50 x 10-3 seconds x 45 x 106 bits/secondÞ 2.25 x 106 bits = 280 KB data: Amount of data channel can

hold.

Network as a pipe

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

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)

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

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.

Outline




• High-speed Networks


• Network Jitter

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

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

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?

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

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

Outline






• Network Jitter

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

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?

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

Outline






• Network Jitter

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

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?

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?

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

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 - Computer Networks Spring 2014

Documents

Transcript of ECE 4450:427/527 - Computer Networks Spring 2014