Lecture 2

Performance Metrics

Performance Metrics

• Bandwidth

• Delay

• Bandwidth-delay product

• Latency

• Throughput

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 s

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

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

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)

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

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

BA S

Transmission length=

Latency with one switch

Queuing Delay=

Total Delay=

Latency=

BA S

Latency with three switches

BA S S S

Queuing Delay=

Total Delay=

Latency=

Latency with 200 bit “cut-through” feature

BA S S S

Queuing Delay=

Total Delay=

Latency=

Link versus end-to-end(effective) bandwidth

BA S S S

•10 Mbps Ethernet •Store and forward switches – immediate resend•Bandwidth=?

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=?

BA S S S

Link versus end-to-end(effective) bandwidth

•Latency for packet=•Latency for Ack=•End-to-end Bandwidth=?

BA S S S

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”

Throughput cont.

• Throughput:Transfer size/ {delay+transfer size/bandwidth}

1/{1/bandwidth+1/(transfer size/delay)}

• {bandwidth transfer size/delay)

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

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

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

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

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