Understanding Networked Applications:
A First Course
Chapter 20
by
David G. Messerschmitt
Outline
• Industry structure
• Communication link characteristics
• Mitigating the impact of a link:
– Compression
– Caching
– Mobile code
Understanding Networked Applications
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A First Course
Industry Structure
• At the link layer, bits are bits
– Voice over IP
– IP over voice
• New technologies for access links
• Deregulation
• Turbulent times ahead
Understanding Networked Applications
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A First Course
IP over Voice
Public telephone network

Computer
w/modem
Computer
w/modem
Gateway


Internet
Understanding Networked Applications
4
A First Course
Voice over IP
Public telephone network

Plain old
telephone
Gateway
IP telephone


Internet
Understanding Networked Applications
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A First Course
Understanding Networked Applications:
A First Course
Communication Link
by
David G. Messerschmitt
Simplest link: fiber optics
Speed of light
Pulse of light = “1”
Understanding Networked Applications
Absence of pulse = “0”
7
A First Course
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
Understanding Networked Applications
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A First Course
Conveyer belt analogy
Bits waiting
to be sent
= “0”
= “1”
Conveyer belt
Bits that
have already
been received
Constant speed
Understanding Networked Applications
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A First Course
Conveyer belt parameters
Conveyer belt
Speed = predetermined by Einstein
Length = predetermined by distance
Size of block
Where we have
Bits communicated per block
control
Understanding Networked Applications
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A First Course
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
Understanding Networked Applications
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A First Course
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
Understanding Networked Applications
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A First Course
Ways to increase bitrate
Make blocks
smaller and put
them on belt faster
Conveyer belt
Use blocks with more
shades of gray (more
bits per block)
Conveyer belt
Understanding Networked Applications
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A First Course
Wavelength division
multiplexing (WDM)
Stack blocks with
different colors
Conveyer belt
Today in commercial use: 40 wavelengths, each at
10 Gbps
Understanding Networked Applications
14
A First Course
Numerical parameters
B = Bitrate = rate
bits put on belt
Conveyer belt
 = Propagation delay = distance/speed
B = Bitrate x delay = # bits in transit
Understanding Networked Applications
15
A First Course
The Graphical Version
Transmit
time
Message
latency
Message in transit
Propagation
latency
Distance
Time
Understanding Networked Applications
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A First Course
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)
Understanding Networked Applications
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A First Course
Bitrate-limited case
Propagation delay-limited delay
Understanding Networked Applications
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A First Course
m << B
Entire message in transit
most of the time
m >>B
Only part of message
in transit at any time
Propagation delay-limited
Bitrate-limited
Time
Understanding Networked Applications
Distance
19
A First Course

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!
Understanding Networked Applications
20
A First Course
B = number of bits in transit
Bitrate (b/s)
28.8 kb/s
Distance
1.5 Mb/s
622 Mb/s
Across chip
<<1 b
<<1 b
<<1
One meter
<<1 b
<<1 b
5
Across U.S.A.
891 b
4780 b
19 Mb
Halfway
around world
4460 b
239 kb
96 Mb
Large!
Assumption: v = c/2 = 1.5 x 108 meters/sec)
Understanding Networked Applications
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A First Course
Physical Media
• Wired
– fiber optics
– coax
– twisted pair (copper)
• Wireless
– Terrestrial radio
– Satellite (long propagation delays)
Understanding Networked Applications
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A First Course
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
Understanding Networked Applications
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A First Course
Mitigating communication
bottlenecks
• Bitrate-limited regime
– Compression
• Delay-limited regime
– Caching
– Mobile code
– In the future, these techniques will be very
important
Understanding Networked Applications
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A First Course
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
Understanding Networked Applications
25
A First Course
Caching
Communication
bottleneck
Remote data
being accessed
Future
references
(where
Local cache possible)
Predictive caching becomes very important in the
delay-limited regime
Understanding Networked Applications
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A First Course
Mobile code
Send code
Communication
bottleneck
Execute
locally
Mobile code is especially useful to enable faster
interaction in the delay limited regime
Understanding Networked Applications
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A First Course
1
0
1
1
Bitrate-limited case
1
0
1
0
0
0
1
0
1
Propagation delay-limited delay
1011101000101
Understanding Networked Applications
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A First Course
Understanding Networked Applications:
A First Course
Supplements
by
David G. Messerschmitt
Series2
Time
Period T
Frequency f = 1/T
Understanding Networked Applications
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A First Course
Symbol interval
11
10
10
11
10
10
11
Time
01
01
00
11
10
11
10
10
11
10
Time
01
01
00
Understanding Networked Applications
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A First Course
Ethernet
Hosts
Bus
Switch
Tree
Broadcast
Understanding Networked Applications
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A First Course
Web
server
Filter
HTML
or
XML
Wireless
access
protocol
Proxy
WML
Understanding Networked Applications
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A First Course
Access time
S
2C
C
Hit rate H
S-2C 1
S-C
S = 100C  H = 0.989
Understanding Networked Applications
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A First Course