Lecture 2

1. Lecture 2 Performance Metrics

2. Performance Metrics • Bandwidth • Delay • Bandwidth-delay product • Latency • Throughput

3. Bandwidth • Average rate that data is transmitted • notation • KB = 210 bytes=1,024 bytes • MB=220 bytes=1,048,576 bytes • Mbps = 106 bits per second • 16,000 bytes/sec=16/1.024 KB/sec=15.6 KB/sec • Time to transmit 1 bit=1/bandwidth • At 10 Mbps: time to transmit 1 bit=100 ns • Time to transmit “X bits”=X-bits/bandwidth • At 10 Mbps: time to transmit 1000 bit=100 ms

4. Propagation Delay • How long does is take for a signal to reach the moon? • One way delay= 3.85 108 m/3.0 108 m/s=1.3 sec • Round trip time=2.6 seconds

5. Queuing Delay • Time a bit spends waiting at a switch before retransmission Total Delay • Time interval between first bit sent to first bit received • Delay= Propagation delay + Queuing delay • Round trip versus 1-way

6. Delay-bandwidth product • How many bits can be sent to the moon at a sending rate (i.e. bandwidth) of 100 Mbps before an acknowledgement could be received? 2.6 seconds/10-8seconds/bit=2.6 108 • Does this cause any potential problems? • Bandwidth-Delay product=number of bits that can be sent before an acknowledgement is received-before you can know something is wrong. • It’s the number of bits “in flight” or “in the pipe” (The Archer analogy)

7. Latency • Time interval between first bit sent to last bit received • Latency=Delay+Duration of the bits in flight (Transmission time) • Transmission time=Transmission size/Bandwidth • Earth Moon example: How long to send a 10 MB picture from moon to earth at 100Mbps • Transmission time=1.048*8*107 b/100Mbps=0.84 s • Latency=0.8+1.3 s=2.1 sec

8. More Examples • 10 Mbps Ethernet • Store and forward switch-retransmits after receiving full packet • Packet size=5000 bits • Propagation delay=10 ms for each link Transmission length=

9. Latency with one switch Queuing Delay= Total Delay= Latency=

10. Latency with three switches Queuing Delay= Total Delay= Latency=

11. Latency with 200 bit “cut-through” feature Queuing Delay= Total Delay= Latency=

12. Link versus end-to-end(effective) bandwidth • 10 Mbps Ethernet • Store and forward switches – immediate resend • Bandwidth=?

13. Link versus end-to-end(effective) bandwidth • 10 Mbps Ethernet -10 ms link delay • Store and forward switches – 35 msec queuing delay • Packet size=5 kbits • End-to-end flow control-50 byte Ack(nowledgement) required before next packet can be transmitted • End-to-end Bandwidth=?

14. Link versus end-to-end(effective) bandwidth • Latency for packet= • Latency for Ack= • End-to-end Bandwidth=?

15. Throughput • Throughput=transfer (or message) size/transfer (or delivery) time • Transfer time=1(or 2)-way latency • Limiting cases: • Very short messages, long delays: • limited by delivery time • throughput=transfer size/delay • said to be “delay limited” – • Very long messages, short delays: • limited by sending rate • throughput= bandwidth • said to be “bandwidth limited”

16. Throughput cont. • Throughput: Transfer size/ {delay+transfer size/bandwidth} 1/{1/bandwidth+1/(transfer size/delay)} • {bandwidth úú (transfer size/delay)

17. Throughput cont. The limiting cases: A.Bandwidth>>transfer size/delay or delay*bandwidth>>transfer size: Throughput=transfer size/delay B.Bandwidth<<transfer size/delay or delay*bandwidth<<transfer size Throughput=Bandwidth

18. Bandwidth versus Delay: Effect on Throughput • Examples: Effective throughput= • 1 ms, 8 bits, 1 Mbps: Transfer size/delay=8 Kbps: Throughput=8Kbps • 100 ms, 8000 bits, 100 Mbps: Transfer size/delay=80 Mbps: Throughput=44Mbps • 1 ms, 25 MB, 10 Mbps: Transfer size/delay=210 Gbps: Throughput=10 Mbps

19. Summary • Bandwidth=Max. Rate that the pipe can be filled • Link Bandwidth • End-to-end bandwidth<Link Bandwidth if sending is restricted e.g. flow control • Delay=time interval between time that first bit is sent and first bit is received • Delay-Bandwidth Product=Data in flight or stored on the channel • Latency=time interval between time that first bit is sent and last bit is received

20. Summary cont. • Throughput: =Transfer size/ latency = bandwidth úú (transfer size/delay • Delay-limited if bandwidth is high and/or message is short • Bandwidth-limited if delay is short and/or message is long

21. Other performance issues • Video traffic is bursty: peak bandwidth versus average bandwidth-smoothing with a buffer • Delay • Jitter-variation of latency-interpacket gap • Buffering to smooth out jitter