1. Lecture 2 Performance Metrics

2. Bandwidth Delay Bandwidth-delay product Latency Throughput

3. Bandwidth –Average rate that data is transmitted –notation KB = 2 10 bytes=1,024 bytes MB=2 20 bytes=1,048,576 bytes Mbps = 10 6 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  s

4. Propagation Delay How long does is take for a signal to reach the moon? One way delay= 3.85 10 8 m/3.0 10 8 m/s=1.3 sec Round trip time=2.6 seconds

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

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 -8 seconds/bit=2.6 10 8 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*10 7 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  s 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  s link delay Store and forward switches – 35  sec 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   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  s, 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   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