EE450-U1-PerformanceMeasures-Nazarian-Spring10

EE450-U1-PerformanceMeasures-Nazarian-Spring10 - University...

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University of Southern California Viterbi School of Engineering EE450 omputer Networks Computer Networks etwork Performance and Latency Measures Network Performance and Latency Measures Shahin Nazarian Spring 2010
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Network Performance Measures Two most important measures are delay or Latency (in seconds) and throughput (in bps or packets per sec) stems with lower elay elay nd igher A systems with lower delay delay and higher throughput throughput has a higher performance Shahin Nazarian/EE450/Spring 2010 2
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Four Sources of Packet Delay 1. Nodal processing delay (d proc ) To check bit errors 2. Queuing delay (d queue ) Time waiting at output To determine output link transmission link for transmission Depends on congestion level of router A propagation B nodal processing queueing Both d proc and d queue are random parameters There is no deterministic equation for them, therefore they Shahin Nazarian/EE450/Spring 2010 3 are studied statistically
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Delay in packet-switched networks 3. Transmission delay (d trans ) R = link bandwidth (bps) = packet ngth (bits) 4. Propagation delay (d prop ) d = length of physical link = propagation speed in L = packet length (bits) time to send bits into link = d trans = L/R s = propagation speed in medium (2.10 8 to 3.10 8 m/sec) propagation delay = d prop = d/s A propagation transmission Note: s and R are very different quantities! B odal nodal processing queueing Shahin Nazarian/EE450/Spring 2010 4 Both d trans and d proc are deterministic parameters
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How Do Loss and Delay Occur? Packets queue in router buffers Packet arrival rate to link exceeds output link capacity ckets ueue wait for turn Packets queue, wait for turn packet being transmitted (delay) A B packets queueing (delay) Shahin Nazarian/EE450/Spring 2010 5 free (available) buffers: arriving packets dropped ( loss ) if no free buffers
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Transmission Delay, d trans Transmission delay , denoted by d trans , is the time it takes to push into the link (transmit) all the message bits: L / R d trans = L / R where L (bits) is the message length and R (bps) is the bit rate d trans can be significant for low-speed links Note: d trans is only about transmitting the message (send the message to the link.) it does not include the time it takes for message to be received by the other end of the link) A B 0 message d trans g Shahin Nazarian/EE450/Spring 2010 6 Time (sec)
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Transmission Delay (Cont.) Example: Logic level 10 1 1 0 0 Bit Duration 1/R What is the bit rate, R? Time (sec) 1 R = 6bits/1sec = 6bps What is the transmission delay for a message of length 128bits? d trans = 128bits/6bps = 21.33 sec What is the transmission delay for one bit? rans bit = 1/6bps = 0 167 sec Shahin Nazarian/EE450/Spring 2010 7 d trans-1bit 1/6bps 0.167 sec Note the transmission delay for one bit, is also referred to as the bit duration. Bit duration is desired to be low
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Transmission Delay (Cont.) Example: Host H1, is downloading the season finale of “Heroes” from its peer, H2. Is the download time the same as the transmission delay of H2?
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This note was uploaded on 09/04/2010 for the course EE 450 at USC.

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EE450-U1-PerformanceMeasures-Nazarian-Spring10 - University...

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