Performance Metrics - Computer Networks - Lecture Slides, Slides of Computer Networks

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Performance Metrics

• bandwidth (throughput) • latency (delay) • Bandwidth –^ – single physical link^ – logical process to channel • Definition of bandwidth: Number of bitstransmitted/second

Width of Bit and Bandwidth

1 secEach bit – 1

μ s wide 1 secEach bit – 0.

s wide^ μ

⇒^ Large Bandwidth

Performance Metrics

-^ Amount of time to transmit a unit of data^ – Network Bandwidth^ – Size of Packet •^ Queuing delays •^

(storing and forwarding at switches)

-^ Latency = propagation delay + transmit time +queue •^ Propagation delay = distance / speed of light •^ Bandwidth + latency = performancecharacteristics of a network

Performance Characteristics • channel could be 1 Mbps / 100 Mbps • Application behave different – across the continent – across the room • Round trip time: – 1 Mbps - 100ms – 100 Mbps - 1ms

Performance Characteristics • RTT = 100

ms , 1^

ms

• Latency = 100+10 × 10

-^

ms

• Latency = 1+10× 10

-^

ms

• Latency dominated by RTT.

Performance Metrics

• Large files^ – Image of size 25x

6 x8 bits

• Channel Capacity 10x

6 bits/s

• Time taken to transmit image 20 s – Suppose RTT = 1 ms^ • Latency = 20.001 sec – Suppose RTT = 100 ms^ • Latency = 20.1 sec – Bandwidth dominates latency

Performance Metrics

• 4000 instr / mile is lost^ – Is it worth going across network?^ – Bandwidth wasted^ – Solution^ • Treat the channel as pipe

Network as Pipe

• The pipe holds data

delay BW

Throughput

• Throughput:^ – Transfer Size / Transfer Time • Transfer Time^ – RTT + (Transfer Size / BW) • If RTT large, increase in BW does notreduce transfer time

Throughput

• Example:^ – Latency: 100ms^ – Channel Capacity: 1 Mbps, 1 Gbps^ – Data: 10 MB^ – On 1 Mbps channel^ • Time taken = 80.1s^ – On 1 Gbps channel^ • Time taken = 0.180s

Throughput

• Stream of packets – 1 Mbps channel

src^

dest 1 Mbps 1 Mb1 Mb 1 Mb 1 Mb

Throughput

• Single packet - 1 Gbps channel

src^

dest 1 Gbps 80 MB