Download - Lecture 2 Performance Metrics. Bandwidth Delay Bandwidth-delay product Latency Throughput
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