Understanding Networked Applications: A First Course Chapter 20 by David G. Messerschmitt

Understanding Networked Applications:A First Course

Chapter 20

by

David G. Messerschmitt

Understanding Networked Applications A First Course2

Outline

• Industry structure

• Communication link characteristics

• Mitigating the impact of a link:– Compression– Caching– Mobile code


Industry Structure

• At the link layer, bits are bits– Voice over IP– IP over voice

• New technologies for access links

• Deregulation

• Turbulent times ahead


Gateway

Public telephone network

Internet

Computer w/modem

IP over Voice

Computer w/modem


Gateway

Public telephone network

Internet

IP telephone

Plain oldtelephone

Voice over IP


Communication Link

by



Simplest link: fiber optics

Pulse of light = “1” Absence of pulse = “0”

Speed of light


Key Concepts

• Bitrate – bits accepted per unit of time

• Transmission time– Time to get all bits transmitted on link

• Propagation delay– Time for each bit to reach destination

• Message latency – transmission time + propagation delay


Conveyer belt

Bits waitingto be sent

Bits thathave alreadybeen received

Conveyer belt analogy

= “0”

= “1”

Constant speed


Conveyer belt

Conveyer belt parameters

Speed = predetermined by EinsteinLength = predetermined by distanceSize of blockBits communicated per block

Where we havecontrol


Determinants of Propagation Time

• Speed of propagation– speed of conveyor belt– limited by medium

• Distance– length of conveyor belt– closer is faster

• Remember: this is one link– Routers/switches introduce extra delay


Determinants of Bitrate

• Bandwidth– size of blocks on conveyor belt– how rapidly the signal changes

• Spectral efficiency– number of bits written on each block– how many distinguishable signal levels– depends on s/n ratio of medium

• Popular usage: Bandwidth = bitrate


Ways to increase bitrate

Conveyer belt

Conveyer belt

Make blocks smaller and put them on belt faster

Use blocks with more shades of gray (more bits per block)


Wavelength division multiplexing (WDM)

Conveyer belt

Stack blocks with different colors

Today in commercial use: 40 wavelengths, each at 10 Gbps


Numerical parameters

Conveyer beltB = Bitrate = rate bits put on belt

= Propagation delay = distance/speed

B = Bitrate x delay = # bits in transit


Transmit time

Time

Messagelatency

Propagation latency

Message in transit

Distance

The Graphical Version


Message latency

Message latency =

Time to transmit message (m/B)

+

Time for last bit to reach destination ()

These two components are equal when

Message length = bitrate x delay (m = B)


Bitrate-limited case

Propagation delay-limited delay


Time

m << B

m >>B

Propagation delay-limited

Bitrate-limited

Entire message in transitmost of the time

Only part of message in transit at any time

Distance


Two situations

• Bitrate limited– Increasing bit rate decreases message latency

– True for long messages

• Propagation delay limited– Increasing bit rate has little impact on message latency

– True to short messages

• Over time, with technology advances, we become delay limited!


Assumption: v = c/2 = 1.5 x 108 meters/sec)

Across chip

One meter

Across U.S.A.

Halfway around world

28.8 kb/s

<<1 b

<<1 b

891 b

4460 b

1.5 Mb/s

<<1 b

<<1 b

4780 b

239 kb

622 Mb/s

<<1

5

19 Mb

96 Mb

B = number of bits in transit

Large!

Bitrate (b/s)

Distance


Physical Media

• Wired– fiber optics– coax– twisted pair (copper)

• Wireless– Terrestrial radio– Satellite (long propagation delays)


Fundamental limitation

• Over time– Bitrates increase with electronics and fiber

optics advances– Message latencies become delay-limited– Further technological advances have little

impact on application performance

• Exception: wireless


Mitigating communication bottlenecks

• Bitrate-limited regime– Compression

• Delay-limited regime– Caching– Mobile code– In the future, these techniques will be very

important


Compression

• Lossless– Typically 2x or 3x

• Lossy (but imperceptible)– Easily 10x for audio and 100x for video– Discards perceptually unimportant information

• Important for today’s bitrate-limited regime


Remote databeing accessed

Local cache

Communicationbottleneck

Futurereferences(wherepossible)

Caching

Predictive caching becomes very important in the delay-limited regime


Communicationbottleneck

Executelocally

Send code

Mobile code

Mobile code is especially useful to enable faster interaction in the delay limited regime


1110100010110

11

1 0 1 0 0 0

1

0

1

10

Bitrate-limited case

Propagation delay-limited delay


Supplements

by



Series2

Period T

Frequency f = 1/T

Time


10

11

01

00

101010

01

1111

Time

10

11

01

00

101010

01

1111

Time

Symbol interval


Ethernet

Hosts Switch

Bus

Broadcast

Tree


ProxyFilterWeb

serverHTMLorXML

WML

Wirelessaccessprotocol


1

S

CHit rate H

Access time

2C

S-2CS-C

S = 100C H = 0.989

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Understanding Networked Applications: A First Course Chapter 20 by David G. Messerschmitt